# Classical Control Systems Lectures

**URL:** <http://discourse.iapct.org/t/classical-control-systems-lectures/2570>\
**Category:** CSG2013\
**Created:** [December 19, 2013, 12:40am UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570 "2013-12-19T00:40:08Z")\
**Posts on this page:** 20\
**Page:** 2

<div class="post-metadata">

**Author:** ![rsmarken](http://discourse.iapct.org/user_avatar/discourse.iapct.org/rsmarken/32/3553_2.png) [@rsmarken](http://discourse.iapct.org/u/rsmarken)\
**Post date:** [December 20, 2013, 4:12pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/21 "2013-12-20T16:12:54Z")

</div>

[From Rick Marken (2013.12.20.0810)]

> Bruce Abbott (2013.12.19.1400 EST)–
> 
> BA: Excellent point! But we need to be sure that stability and control are not
> 
> seen as belonging to different kinds of systems.

RM: Stability and control are two different _phenomena_, something that it is now clear to me that Mr. Douglas is completely unaware of. These two different phenomena are produced by two different kinds of systems; stability is a kind of behavior exhibited by certain “open-loop” or what I call “causal” systems. Control is produced _only_ by negative feedback control systems. So I couldn’t disagree with you more when you say “we need to be sure that stability and control are not seen as belonging to different kinds of systems”. In fact, we need to be VERY sure that we understand that stability and control “belong” to two very different systems: open-loop, causal systems for the former and closed loop negative feedback systems for the latter.

Best

Rick

> **···**
>
> > The system represented by the ball-in-a-bowl example is an equilibrium
> > 
> > system. It is stable against a transient disturbance because the energy
> > 
> > imparted by the transient disturbance becomes potential energy that converts
> > 
> > to a restorative force. In the case of the ball, the force that moves the
> > 
> > ball uphill against gravity creates a potential energy that is converted by
> > 
> > gravitational acceleration to a force that pushes the ball back to the
> > 
> > bottom. Similarly, compressing a spring stores energy in the spring that is
> > 
> > released when the spring is released, restoring the spring to its initial
> > 
> > length. The restoring energy comes from the disturbance.
> > 
> > A control system (if properly designed and tuned and operating within its
> > 
> > design environment) also exhibits stability, but as you note, it does so
> > 
> > even against a continuing disturbance. The restorative force comes, not
> > 
> > from the disturbance, but from the control system’s own energy supply.
> > 
> > Consequently it can oppose a disturbance that acts continuously on the
> > 
> > controlled variable. (An example is keeping the biceps on your arm
> > 
> > contracted to keep your forearm held level.)
> > 
> > Both equilibrium systems and control systems can exhibit other properties
> > 
> > besides stability. A ball balanced on the crest a hill exhibits
> > 
> > instability: the slightest disturbance will send it rolling downhill. A
> > 
> > control system with too much gain, or too much lag, can also exhibit
> > 
> > instability. It may go into oscillation and even run away to infinity if
> > 
> > its actions get sufficiently out of phase with changes in the controlled
> > 
> > variable so as to convert negative feedback into positive feedback.
> > 
> > Bruce
> > 
> > Rick Marken (2013.12.19.1020) –
> > 
> > On Thu, Dec 19, 2013 at 4:45 AM, Warren Mansell [wmansell@gmail.com](mailto:wmansell@gmail.com) wrote:
> > 
> > > 
> > 
> > > Hi Bruce, I switched off after the first minute. Surely, from a PCT
> > 
> > perspective, stability cannot be defined as the capacity to restore to an
> > 
> > output of zero?
> > 
> > I see Bruce already answered you but let me put in my own 2 cents. I liked
> > 
> > the videos (actually I only saw the first video) that Bruce sent because I
> > 
> > thought it made a nice distinction between stability and control. These are
> > 
> > two different phenomena and confusion between the two has caused enormous
> > 
> > problems for PCT.
> > 
> > Stability is the characteristic of a variable that _returns_ to it’s initial
> > 
> > state after a _transient_ disturbance. Thus, a ball in a bowl is stable
> > 
> > because after a transient disturbance the ball returns to its initial
> > 
> > position. Same is true of a pendulum or a mass on a spring.
> > 
> > Control is a characteristic of a variable that _remains_ in a predetermined
> > 
> > state in the face of a \_continuously varying \_ disturbance.
> > 
> > The superficial similarity between these two phenomena is that both seem to
> > 
> > involve “resistance to disturbance”. But the nature of this “resistance” is
> > 
> > quite different in the two cases. In stability, there really is no
> > 
> > resistance; the same forces that moved the variable away from the initial
> > 
> > position return it to that position once the disturbance force is removed.
> > 
> > In control, the resistance is active; the resistive forces are generated by
> > 
> > the system that is actively acting to keep the variable in a particular
> > 
> > state.
> > 
> > The problem for PCT is that psychologists who have an allergic reaction to
> > 
> > anything that smacks of purpose (as control does) have assumed that the
> > 
> > obvious controlling done by organisms is actually an example of stability so
> > 
> > that disturbance resistance can be explained in terms of non-control models
> > 
> > of stable systems. Thus, we have “mass-spring” models of limb stability;
> > 
> > “coordinative structure”
> > 
> > models of limb movement; “dynamic attractor” models of many different
> > 
> > activities.
> > 
> > I’ve done research to try to show that stability models cannot account for
> > 
> > control phenomena (Marken, R. S. (1991) Degrees of Freedom in Behavior.
> > 
> > Psychological Science, 2, 92 - 100). But I think part of the problem is that
> > 
> > psychologists don’t understand (or possibly don’t want to understand) the
> > 
> > difference between the _phenomenon_ of stability and the _phenomenon_ of
> > 
> > control. That’s why I think the first Douglas lecture can be useful; it can
> > 
> > help people understand the difference between these two phenomena and,
> > 
> > hopefully, see that what people do is control, not just come to rest in
> > 
> > stable states after disturbances.
> > 
> > Best
> > 
> > Rick
> > 
> > > 
> > 
> > > 
> > 
> > > On Thu, Dec 19, 2013 at 12:40 AM, Bruce Abbott [bbabbott@frontier.com](mailto:bbabbott@frontier.com) \> \> wrote:
> > 
> > > >
> > 
> > > > From Bruce Abbott (2014.12.18 1940 EST)
> > 
> > > >
> > 
> > > >
> > 
> > > >
> > 
> > > > For those of you who would like to learn more about the classical
> > 
> > approach to control systems, I highly recommend a series of YouTube videos
> > 
> > presented by control systems engineer Brian Douglas
> > 
> > ([http://www.youtube.com/user/ControlLectures/videos](http://www.youtube.com/user/ControlLectures/videos) ). The lectures range
> > 
> > from basic concepts to sophisticated techniques used in control system
> > 
> > design, analysis, and tuning, so whether you are a control systems neophyte
> > 
> > or an engineer who needs a bit of brushing up on these techniques, you will
> > 
> > find something worthwhile to view.
> > 
> > > >
> > 
> > > >
> > 
> > > >
> > 
> > > > Douglas is an engaging and talented teacher who is able to communicate
> > 
> > complex ideas clearly. A nice place to start is with Douglas’ lecture,
> > 
> > “introduction to system stability and control”
> > 
> > ([Introduction to System Stability and Control](http://www.youtube.com/watch?v=uqjKG32AkC4)). Another is the lecture
> > 
> > "examples of PID control ([Simple Examples of PID Control](http://www.youtube.com/watch?v=XfAt6hNV8XM)). Once
> > 
> > you have watched a given video, simply click on the link “41 videos” next to
> > 
> > Brian Douglas’ name just below the video, on the left to return to the page
> > 
> > on which all the videos are displayed.
> > 
> > > >
> > 
> > > >
> > 
> > > >
> > 
> > > > Bruce A.
> > 
> > > 
> > 
> > > 
> > 
> > > 
> > 
> > > 
> > 
> > > –
> > 
> > > Dr Warren Mansell
> > 
> > > Reader in Psychology
> > 
> > > Cognitive Behavioural Therapist & Charte-----
> > 
> > No virus found in this message.
> > 
> > Checked by AVG - [www.avg.com](http://www.avg.com)
> > 
> > Version: 2014.0.4259 / Virus Database: 3658/6933 - Release Date: 12/19/13

---

<div class="post-metadata">

**Author:** ![JohnK](http://discourse.iapct.org/user_avatar/discourse.iapct.org/johnk/32/3043_2.png) [@JohnK](http://discourse.iapct.org/u/JohnK)\
**Post date:** [December 20, 2013, 7:21pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/22 "2013-12-20T19:21:36Z")

</div>

[John Kirkland 20131221]

Side bar: Solstice, again. OK the golden orb is soon to be tossed back to those of you in the other hemisphere.

Comment and question: The previous discussions and video-lecture helped resolve a small problem for me that’s related to page 236 where Bill presents ‘…the crucial factor in applying The Test’ (para 2). Rick (and anybody else) - is that paragraph once again affirming the open versus closed loop distinction Douglas is chatting about with his delightful examples?

Checking back onto a previous thread, my personal definition of a genius is anyone who can reveal what was staring me in the face. Thus far Bill takes line honours though several others in the PCT community are included as well. In a seasonal pun: they are complements of the reasoning.

Thanks in advance. And, Season’s Greetings to one and all whatever your creed.

JohnK.

> **···**
>
> On Sat, Dec 21, 2013 at 5:12 AM, Richard Marken [rsmarken@gmail.com](mailto:rsmarken@gmail.com) wrote:
> 
> > [From Rick Marken (2013.12.20.0810)]
> > 
> > > Bruce Abbott (2013.12.19.1400 EST)–
> > > 
> > > BA: Excellent point! But we need to be sure that stability and control are not
> > > 
> > > seen as belonging to different kinds of systems.
> 
> > 
> 
> > RM: Stability and control are two different _phenomena_, something that it is now clear to me that Mr. Douglas is completely unaware of. These two different phenomena are produced by two different kinds of systems; stability is a kind of behavior exhibited by certain “open-loop” or what I call “causal” systems. Control is produced _only_ by negative feedback control systems. So I couldn’t disagree with you more when you say “we need to be sure that stability and control are not seen as belonging to different kinds of systems”. In fact, we need to be VERY sure that we understand that stability and control “belong” to two very different systems: open-loop, causal systems for the former and closed loop negative feedback systems for the latter.
> 
> > Best
> 
> > Rick
> 
> > 
> 
> > > The system represented by the ball-in-a-bowl example is an equilibrium
> > > 
> > > system. It is stable against a transient disturbance because the energy
> > > 
> > > imparted by the transient disturbance becomes potential energy that converts
> > > 
> > > to a restorative force. In the case of the ball, the force that moves the
> > > 
> > > ball uphill against gravity creates a potential energy that is converted by
> > > 
> > > gravitational acceleration to a force that pushes the ball back to the
> > > 
> > > bottom. Similarly, compressing a spring stores energy in the spring that is
> > > 
> > > released when the spring is released, restoring the spring to its initial
> > > 
> > > length. The restoring energy comes from the disturbance.
> > > 
> > > A control system (if properly designed and tuned and operating within its
> > > 
> > > design environment) also exhibits stability, but as you note, it does so
> > > 
> > > even against a continuing disturbance. The restorative force comes, not
> > > 
> > > from the disturbance, but from the control system’s own energy supply.
> > > 
> > > Consequently it can oppose a disturbance that acts continuously on the
> > > 
> > > controlled variable. (An example is keeping the biceps on your arm
> > > 
> > > contracted to keep your forearm held level.)
> > > 
> > > Both equilibrium systems and control systems can exhibit other properties
> > > 
> > > besides stability. A ball balanced on the crest a hill exhibits
> > > 
> > > instability: the slightest disturbance will send it rolling downhill. A
> > > 
> > > control system with too much gain, or too much lag, can also exhibit
> > > 
> > > instability. It may go into oscillation and even run away to infinity if
> > > 
> > > its actions get sufficiently out of phase with changes in the controlled
> > > 
> > > variable so as to convert negative feedback into positive feedback.
> > > 
> > > Bruce
> > > 
> > > Rick Marken (2013.12.19.1020) –
> > > 
> > > On Thu, Dec 19, 2013 at 4:45 AM, Warren Mansell [wmansell@gmail.com](mailto:wmansell@gmail.com) wrote:
> > > 
> > > > 
> > > 
> > > > Hi Bruce, I switched off after the first minute. Surely, from a PCT
> > > 
> > > perspective, stability cannot be defined as the capacity to restore to an
> > > 
> > > output of zero?
> > > 
> > > I see Bruce already answered you but let me put in my own 2 cents. I liked
> > > 
> > > the videos (actually I only saw the first video) that Bruce sent because I
> > > 
> > > thought it made a nice distinction between stability and control. These are
> > > 
> > > two different phenomena and confusion between the two has caused enormous
> > > 
> > > problems for PCT.
> > > 
> > > Stability is the characteristic of a variable that _returns_ to it’s initial
> > > 
> > > state after a _transient_ disturbance. Thus, a ball in a bowl is stable
> > > 
> > > because after a transient disturbance the ball returns to its initial
> > > 
> > > position. Same is true of a pendulum or a mass on a spring.
> > > 
> > > Control is a characteristic of a variable that _remains_ in a predetermined
> > > 
> > > state in the face of a \_continuously varying \_ disturbance.
> > > 
> > > The superficial similarity between these two phenomena is that both seem to
> > > 
> > > involve “resistance to disturbance”. But the nature of this “resistance” is
> > > 
> > > quite different in the two cases. In stability, there really is no
> > > 
> > > resistance; the same forces that moved the variable away from the initial
> > > 
> > > position return it to that position once the disturbance force is removed.
> > > 
> > > In control, the resistance is active; the resistive forces are generated by
> > > 
> > > the system that is actively acting to keep the variable in a particular
> > > 
> > > state.
> > > 
> > > The problem for PCT is that psychologists who have an allergic reaction to
> > > 
> > > anything that smacks of purpose (as control does) have assumed that the
> > > 
> > > obvious controlling done by organisms is actually an example of stability so
> > > 
> > > that disturbance resistance can be explained in terms of non-control models
> > > 
> > > of stable systems. Thus, we have “mass-spring” models of limb stability;
> > > 
> > > “coordinative structure”
> > > 
> > > models of limb movement; “dynamic attractor” models of many different
> > > 
> > > activities.
> > > 
> > > I’ve done research to try to show that stability models cannot account for
> > > 
> > > control phenomena (Marken, R. S. (1991) Degrees of Freedom in Behavior.
> > > 
> > > Psychological Science, 2, 92 - 100). But I think part of the problem is that
> > > 
> > > psychologists don’t understand (or possibly don’t want to understand) the
> > > 
> > > difference between the _phenomenon_ of stability and the _phenomenon_ of
> > > 
> > > control. That’s why I think the first Douglas lecture can be useful; it can
> > > 
> > > help people understand the difference between these two phenomena and,
> > > 
> > > hopefully, see that what people do is control, not just come to rest in
> > > 
> > > stable states after disturbances.
> > > 
> > > Best
> > > 
> > > Rick
> > > 
> > > > 
> > > 
> > > > 
> > > 
> > > > On Thu, Dec 19, 2013 at 12:40 AM, Bruce Abbott [bbabbott@frontier.com](mailto:bbabbott@frontier.com)
> > > 
> > > wrote:
> > > 
> > > > >
> > > 
> > > > > From Bruce Abbott (2014.12.18 1940 EST)
> > > 
> > > > >
> > > 
> > > > >
> > > 
> > > > >
> > > 
> > > > > For those of you who would like to learn more about the classical
> > > 
> > > approach to control systems, I highly recommend a series of YouTube videos
> > > 
> > > presented by control systems engineer Brian Douglas
> > > 
> > > ([http://www.youtube.com/user/ControlLectures/videos](http://www.youtube.com/user/ControlLectures/videos) ). The lectures range
> > > 
> > > from basic concepts to sophisticated techniques used in control system
> > > 
> > > design, analysis, and tuning, so whether you are a control systems neophyte
> > > 
> > > or an engineer who needs a bit of brushing up on these techniques, you will
> > > 
> > > find something worthwhile to view.
> > > 
> > > > >
> > > 
> > > > >
> > > 
> > > > >
> > > 
> > > > > Douglas is an engaging and talented teacher who is able to communicate
> > > 
> > > complex ideas clearly. A nice place to start is with Douglas’ lecture,
> > > 
> > > “introduction to system stability and control”
> 
> > > ([Introduction to System Stability and Control](http://www.youtube.com/watch?v=uqjKG32AkC4)). Another is the lecture
> > > 
> > > "examples of PID control ([Simple Examples of PID Control](http://www.youtube.com/watch?v=XfAt6hNV8XM)). Once
> > > 
> > > you have watched a given video, simply click on the link “41 videos” next to
> > > 
> > > Brian Douglas’ name just below the video, on the left to return to the page
> > > 
> > > on which all the videos are displayed.
> > > 
> > > > >
> > > 
> > > > >
> > > 
> > > > >
> > > 
> > > > > Bruce A.
> > > 
> > > > 
> > > 
> > > > 
> > > 
> > > > 
> > > 
> > > > 
> > > 
> > > > –
> > > 
> > > > Dr Warren Mansell
> > > 
> > > > Reader in Psychology
> 
> > > > Cognitive Behavioural Therapist & Charte-----
> > > 
> > > No virus found in this message.
> > > 
> > > Checked by AVG - [www.avg.com](http://www.avg.com)
> > > 
> > > Version: 2014.0.4259 / Virus Database: 3658/6933 - Release Date: 12/19/13

---

<div class="post-metadata">

**Author:** ![MartinT](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/m/ee7513/32.png) [@MartinT](http://discourse.iapct.org/u/MartinT)\
**Post date:** [December 20, 2013, 9:21pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/23 "2013-12-20T21:21:45Z")

</div>

[Martin Taylor 2013.12.20.11.47]

```
I still haven't looked at the videos, but it strikes me that you are

```

doing the old philosophers’ trick of taking a word that has a  
variety of meanings, and using a meaning appropriate in one context  
as though that were the meaning appropriate to a different context.  
Yes, “stability” and “control” are indeed different phenomena, in  
the same way “food” and “vegetable” are different concepts. That  
doesn’t stop a vegetable from being a food, nor does the fact that a  
particular dynamical system is a control loop prevent it from having  
stability.  
All feedback loops, in fact all dynamical systems, whether control  
systems or not, have stability criteria. Either they are stable or  
not. Some are more stable than others. Some are metastable, meaning  
they will maintain their current values until something momentarily  
disturbs one of their signal values. Some of those will continue to  
diverge from the original metastable value after the disturbance,  
some will just maintain the disturbed set of signal values without  
further change. Some are absolutely stable, meaning that after any  
kind of momentary disturbance they will return their values to their  
original levels. Most real systems don’t do that, and are stable  
only if the momentary disturbance doesn’t exceed some limit.  
The key concept is the “orbit”. All systems that can be described by  
a vector of variables have a state. Their state is the vector of  
current variable values together with the rates of change of the  
current variables. That includes control systems, ball-in-a-bowl  
systems, the synapse strengths of networks of millions of neurons,  
etc. etc. If the system is at some location in the state space and  
is not further disturbed from outside, it will follow some track  
through the state space. That track is an orbit, and there is only  
one orbit through any point in the state space.  
A stable system is one for which the orbit will converge to some  
track that is the same for all the initial locations in the state  
space. That track is called an “attractor”. The attractor may be a  
fixed point, a closed path (which represents a stable oscillator) or  
a “strange attractor” (which I won’t explain now). An unstable  
system is one for which the orbits diverge. Here are a couple of  
examples of attractors, or at least the projections of them into two  
dimensions, because even in 2-D, the orbit is actually in a space of  
four dimensions, two for location and two for velocity. I have  
omitted the velocity coordinates in these examples, and in the  
fixed-point example it is the velocity that distinguishes the orbits  
where two of them cross in the figure. In the 4-D state space, only  
one orbit passes through any particular point.  
A control system is one for which the attractor converges in at  
least one dimension (the perception-value dimension), but that’s not  
the main criterion for differentiating the “ball-in-the-bowl” from a  
trivial control loop. Bruce Abbott put his finger on it when he  
pointed out that the ball-in-the-bowl uses the energy supplied by  
the disturbance to return the ball to its fixed point, whereas the  
control loop uses an independent energy supply to oppose the effect  
of the disturbance on one (and only one) of the variables in the  
state space of the loop. The manner in which control is established  
is irrelevant. It so happens that the PCT definition of control is  
the maintenance of one particular value among the many different  
signal values in a negative feedback loop, so Rick’s comment  
“Control is produced _only_ by negative feedback control systems” is  
a tautology.  
To which I can only say that there are several applicable proverbs  
along the lines that one is better advised to listen and learn  
rather than to guess and pontificate. Bruce is quite right to say  
“we need to be sure that stability and control are not seen as  
belonging to different kinds of systems”. To contradict Bruce is to  
say something as nonsensical as “we need to be sure that leafiness  
and trees are not seen as belonging to different kinds of objects”.  
“Stability” applies to all kinds of dynamical systems, which control  
systems are.  
Martin

![Attractors.jpg](http://discourse.iapct.org/uploads/default/original/1X/f95899be1fc710df351ee2dac6d13c29c52c1a91.jpeg)

> **···**
>
> > [From Rick Marken (2013.12.20.0810)]
> > 
> > > ```
> > > Bruce Abbott
> > > 
> > > ```
> > > 
> > > (2013.12.19.1400 EST)–
> > > 
> > > ```
> > > BA: Excellent point! But we need to be sure that stability
> > > 
> > > ```
> > > 
> > > and control are not
> > > 
> > > ```
> > > seen as belonging to different kinds of systems.
> > > 
> > > ```
> 
> > ```
> > RM: Stability and control are two different _phenomena_,
> > 
> > ```
> > 
> > something that it is now clear to me that Mr. Douglas is  
> > completely unaware of. These two different phenomena are  
> > produced by two different kinds of systems; stability is a  
> > kind of behavior exhibited by certain “open-loop” or what I  
> > call “causal” systems.
> 
> > ```
> > Control is produced _only_ by negative feedback control
> > 
> > ```
> > 
> > systems. So I couldn’t disagree with you more when you say  
> > “we need to be sure that stability and control are not seen  
> > as belonging to different kinds of systems”. In fact, we  
> > need to be VERY sure that we understand that stability and  
> > control “belong” to two very different systems: open-loop,  
> > causal systems for the former and closed loop negative  
> > feedback systems for the latter. [MT: “you” here is Bruce  
> > Abbott.]
> 
> >

---

<div class="post-metadata">

**Author:** ![rsmarken](http://discourse.iapct.org/user_avatar/discourse.iapct.org/rsmarken/32/3553_2.png) [@rsmarken](http://discourse.iapct.org/u/rsmarken)\
**Post date:** [December 21, 2013, 12:03am UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/24 "2013-12-21T00:03:12Z")

</div>

[From Rick Marken (2013.12.20.1600)]

> John Kirkland (20131221)--
> 
> JK: Side bar: Solstice, again. OK the golden orb is soon to be tossed back to  
> those of you in the other hemisphere.

RM: I was just explaining it to my 2 1/2 month old granddaughter this  
morning (a day before the solstice for us). I wanted to prepare her  
for the dizzying increase in the length of the daylight.

> JK: Comment and question: The previous discussions and video-lecture helped  
> resolve a small problem for me that's related to page 236 where Bill  
> presents '...the crucial factor in applying The Test' (para 2). Rick (and  
> anybody else) - is that paragraph once again affirming the open versus  
> closed loop distinction Douglas is chatting about with his delightful  
> examples?

RM: Yes, exactly. The "crucial factor in applying The Test" for those  
not reading this with a copy of B:CP at their side, is this: lack of  
the expected effect of a disturbance on the hypothetical controlled  
variable. So the position of a ball lying at the bottom of a bowl  
would be exposed as \_not\_ being a controlled variable because applying  
a force to the ball has exactly the expected effect; the ball moves up  
the side. If the force is maintained the ball remains up the side; if  
the force is removed the ball falls back and eventually settles again  
at the bottom. If the position of the ball were actually under  
control, the position of the ball would be virtually unaffected by the  
force disturbances (of course, one would have to show that this was  
happening without the the ball being anchored to the bottom of the  
bowl).

So The Test is, indeed, a way of discriminating a controlled variable  
-- a variable kept in a predetermined state, protected from  
disturbance, by a closed loop control system -- and an uncontrolled  
variable, one that behaves according to the laws of open-loop  
causality (ie. physics).

My example should make it clear that variables that have "stabilty" --  
that return to an equilibrium state after a disturbance -- are \_not\_  
controlled variables; they are just plain old physical variables  
behaving according to the laws of physics.

Best

Rick

> **···**
>
> > Checking back onto a previous thread, my personal definition of a genius is  
> > anyone who can reveal what was staring me in the face. Thus far Bill takes  
> > line honours though several others in the PCT community are included as  
> > well. In a seasonal pun: they are complements of the reasoning.
> > 
> > Thanks in advance. And, Season's Greetings to one and all whatever your  
> > creed.
> > 
> > JohnK.
> > 
> > On Sat, Dec 21, 2013 at 5:12 AM, Richard Marken \<rsmarken@gmail.com\> wrote:
> > 
> > > [From Rick Marken (2013.12.20.0810)]
> > > 
> > > > Bruce Abbott (2013.12.19.1400 EST)--
> > > > 
> > > > BA: Excellent point! But we need to be sure that stability and control  
> > > > are not
> > > > 
> > > > seen as belonging to different kinds of systems.
> > > 
> > > RM: Stability and control are two different \_phenomena\_, something that it  
> > > is now clear to me that Mr. Douglas is completely unaware of. These two  
> > > different phenomena are produced by two different kinds of systems;  
> > > stability is a kind of behavior exhibited by certain "open-loop" or what I  
> > > call "causal" systems. Control is produced \_only\_ by negative feedback  
> > > control systems. So I couldn't disagree with you more when you say "we need  
> > > to be sure that stability and control are not seen as belonging to different  
> > > kinds of systems". In fact, we need to be VERY sure that we understand that  
> > > stability and control "belong" to two very different systems: open-loop,  
> > > causal systems for the former and closed loop negative feedback systems for  
> > > the latter.
> > > 
> > > Best
> > > 
> > > Rick
> > > 
> > > > The system represented by the ball-in-a-bowl example is an equilibrium  
> > > > system. It is stable against a transient disturbance because the energy  
> > > > imparted by the transient disturbance becomes potential energy that  
> > > > converts  
> > > > to a restorative force. In the case of the ball, the force that moves  
> > > > the  
> > > > ball uphill against gravity creates a potential energy that is converted  
> > > > by  
> > > > gravitational acceleration to a force that pushes the ball back to the  
> > > > bottom. Similarly, compressing a spring stores energy in the spring that  
> > > > is  
> > > > released when the spring is released, restoring the spring to its initial  
> > > > length. The restoring energy comes from the disturbance.
> > > > 
> > > > A control system (if properly designed and tuned and operating within its  
> > > > design environment) also exhibits stability, but as you note, it does so  
> > > > even against a continuing disturbance. The restorative force comes, not  
> > > > from the disturbance, but from the control system's own energy supply.  
> > > > Consequently it can oppose a disturbance that acts continuously on the  
> > > > controlled variable. (An example is keeping the biceps on your arm  
> > > > contracted to keep your forearm held level.)
> > > > 
> > > > Both equilibrium systems and control systems can exhibit other properties  
> > > > besides stability. A ball balanced on the crest a hill exhibits  
> > > > instability: the slightest disturbance will send it rolling downhill. A  
> > > > control system with too much gain, or too much lag, can also exhibit  
> > > > instability. It may go into oscillation and even run away to infinity if  
> > > > its actions get sufficiently out of phase with changes in the controlled  
> > > > variable so as to convert negative feedback into positive feedback.
> > > > 
> > > > Bruce
> > > > 
> > > > Rick Marken (2013.12.19.1020) --
> > > > 
> > > > On Thu, Dec 19, 2013 at 4:45 AM, Warren Mansell \<wmansell@gmail.com\> \>\>\> wrote:  
> > > > \>  
> > > > \> Hi Bruce, I switched off after the first minute. Surely, from a PCT  
> > > > perspective, stability cannot be defined as the capacity to restore to an  
> > > > output of zero?
> > > > 
> > > > I see Bruce already answered you but let me put in my own 2 cents. I  
> > > > liked  
> > > > the videos (actually I only saw the first video) that Bruce sent because  
> > > > I  
> > > > thought it made a nice distinction between stability and control. These  
> > > > are  
> > > > two different phenomena and confusion between the two has caused enormous  
> > > > problems for PCT.
> > > > 
> > > > Stability is the characteristic of a variable that \_returns\_ to it's  
> > > > initial  
> > > > state after a \_transient\_ disturbance. Thus, a ball in a bowl is stable  
> > > > because after a transient disturbance the ball returns to its initial  
> > > > position. Same is true of a pendulum or a mass on a spring.
> > > > 
> > > > Control is a characteristic of a variable that \_remains\_ in a  
> > > > predetermined  
> > > > state in the face of a \_continuously varying \_ disturbance.
> > > > 
> > > > The superficial similarity between these two phenomena is that both seem  
> > > > to  
> > > > involve "resistance to disturbance". But the nature of this "resistance"  
> > > > is  
> > > > quite different in the two cases. In stability, there really is no  
> > > > resistance; the same forces that moved the variable away from the initial  
> > > > position return it to that position once the disturbance force is  
> > > > removed.  
> > > > In control, the resistance is active; the resistive forces are generated  
> > > > by  
> > > > the system that is actively acting to keep the variable in a particular  
> > > > state.
> > > > 
> > > > The problem for PCT is that psychologists who have an allergic reaction  
> > > > to  
> > > > anything that smacks of purpose (as control does) have assumed that the  
> > > > obvious controlling done by organisms is actually an example of stability  
> > > > so  
> > > > that disturbance resistance can be explained in terms of non-control  
> > > > models  
> > > > of stable systems. Thus, we have "mass-spring" models of limb stability;  
> > > > "coordinative structure"  
> > > > models of limb movement; "dynamic attractor" models of many different  
> > > > activities.
> > > > 
> > > > I've done research to try to show that stability models cannot account  
> > > > for  
> > > > control phenomena (Marken, R. S. (1991) Degrees of Freedom in Behavior.  
> > > > Psychological Science, 2, 92 - 100). But I think part of the problem is  
> > > > that  
> > > > psychologists don't understand (or possibly don't want to understand) the  
> > > > difference between the \_phenomenon\_ of stability and the \_phenomenon\_ of  
> > > > control. That's why I think the first Douglas lecture can be useful; it  
> > > > can  
> > > > help people understand the difference between these two phenomena and,  
> > > > hopefully, see that what people do is control, not just come to rest in  
> > > > stable states after disturbances.
> > > > 
> > > > Best
> > > > 
> > > > Rick
> > > > 
> > > > \>  
> > > > \>  
> > > > \> On Thu, Dec 19, 2013 at 12:40 AM, Bruce Abbott \<bbabbott@frontier.com\> \>\>\> wrote:  
> > > > \>\>  
> > > > \>\> From Bruce Abbott (2014.12.18 1940 EST)  
> > > > \>\>  
> > > > \>\>  
> > > > \>\>  
> > > > \>\> For those of you who would like to learn more about the classical  
> > > > approach to control systems, I highly recommend a series of YouTube  
> > > > videos  
> > > > presented by control systems engineer Brian Douglas  
> > > > ([http://www.youtube.com/user/ControlLectures/videos](http://www.youtube.com/user/ControlLectures/videos) ). The lectures  
> > > > range  
> > > > from basic concepts to sophisticated techniques used in control system  
> > > > design, analysis, and tuning, so whether you are a control systems  
> > > > neophyte  
> > > > or an engineer who needs a bit of brushing up on these techniques, you  
> > > > will  
> > > > find something worthwhile to view.  
> > > > \>\>  
> > > > \>\>  
> > > > \>\>  
> > > > \>\> Douglas is an engaging and talented teacher who is able to communicate  
> > > > complex ideas clearly. A nice place to start is with Douglas' lecture,  
> > > > "introduction to system stability and control"  
> > > > (Introduction to System Stability and Control). Another is the lecture  
> > > > "examples of PID control (Simple Examples of PID Control). Once
> > > > 
> > > > you have watched a given video, simply click on the link "41 videos" next  
> > > > to  
> > > > Brian Douglas' name just below the video, on the left to return to the  
> > > > page  
> > > > on which all the videos are displayed.  
> > > > \>\>  
> > > > \>\>  
> > > > \>\>  
> > > > \>\> Bruce A.  
> > > > \>  
> > > > \>  
> > > > \>  
> > > > \>  
> > > > \> --  
> > > > \> Dr Warren Mansell  
> > > > \> Reader in Psychology  
> > > > \> Cognitive Behavioural Therapist & Charte-----
> > > > 
> > > > No virus found in this message.  
> > > > Checked by AVG - www.avg.com  
> > > > Version: 2014.0.4259 / Virus Database: 3658/6933 - Release Date: 12/19/13
> 
> --  
> Richard S. Marken PhD  
> www.mindreadings.com
> 
> The only thing that will redeem mankind is cooperation.  
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-- Bertrand Russell

---

<div class="post-metadata">

**Author:** ![rsmarken](http://discourse.iapct.org/user_avatar/discourse.iapct.org/rsmarken/32/3553_2.png) [@rsmarken](http://discourse.iapct.org/u/rsmarken)\
**Post date:** [December 21, 2013, 1:37am UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/25 "2013-12-21T01:37:49Z")

</div>

[From Rick Marken (2013.12.20.1740)]

![Attractors.jpg](http://discourse.iapct.org/uploads/default/original/1X/f95899be1fc710df351ee2dac6d13c29c52c1a91.jpeg)

> **···**
>
> > Martin Taylor (2013.12.20.11.47)–
> > 
> > > ```
> > > RM: Stability and control are two different _phenomena_,
> > > 
> > > ```
> > > 
> > > something that it is now clear to me that Mr. Douglas is  
> > > completely unaware of. These two different phenomena are  
> > > produced by two different kinds of systems; stability is a  
> > > kind of behavior exhibited by certain “open-loop” or what I  
> > > call “causal” systems.
> 
> > 
> 
> > ```
> > I still haven't looked at the videos, but it strikes me that you are
> > 
> > ```
> > 
> > doing the old philosophers’ trick of taking a word that has a  
> > variety of meanings, and using a meaning appropriate in one context  
> > as though that were the meaning appropriate to a different context.  
> > Yes, “stability” and “control” are indeed different phenomena, in  
> > the same way “food” and “vegetable” are different concepts.
> 
> RM: No,“stability” and “control”, as described by Douglas in the very first control lecture, are different phenomena in the way “food” and “poison” are different. A stable system (according to the lecture) is one that returns to its original (“equilibrium”) state after a transient disturbance; a control system is is one that remains in a reference state during continuous disturbance. Douglas should have talked about variables rather than systems but you get the idea.
> 
> > ```
> > MT: All feedback loops, in fact all dynamical systems, whether control
> > 
> > ```
> > 
> > systems or not, have stability criteria. Either they are stable or  
> > not. Some are more stable than others.
> 
> RM: Yes, they do. Indeed, I measure control in terms of stability (observed/expected variance of the variable). In this case “stability” is simply referring to a measure of the observed variations in a variable and it can be used to measure the variability of a controlled variable or an uncontrolled variable (like the variable position of the ball in the bowl that Douglas refers to as a stable system).
> 
> > ```
> > MT: Some are metastable, meaning
> > 
> > ```
> > 
> > they will maintain their current values until something momentarily  
> > disturbs one of their signal values. Some of those will continue to  
> > diverge from the original metastable value after the disturbance,  
> > some will just maintain the disturbed set of signal values without  
> > further change. Some are absolutely stable, meaning that after any  
> > kind of momentary disturbance they will return their values to their  
> > original levels. Most real systems don’t do that, and are stable  
> > only if the momentary disturbance doesn’t exceed some limit.
> > 
> > ```
> > The key concept is the "orbit". All systems that can be described by
> > 
> > ```
> > 
> > a vector of variables have a state. Their state is the vector of  
> > current variable values together with the rates of change of the  
> > current variables. That includes control systems, ball-in-a-bowl  
> > systems, the synapse strengths of networks of millions of neurons,  
> > etc. etc. If the system is at some location in the state space and  
> > is not further disturbed from outside, it will follow some track  
> > through the state space. That track is an orbit, and there is only  
> > one orbit through any point in the state space.
> > 
> > ```
> > A stable system is one for which the orbit will converge to some
> > 
> > ```
> > 
> > track that is the same for all the initial locations in the state  
> > space. That track is called an “attractor”. The attractor may be a  
> > fixed point, a closed path (which represents a stable oscillator) or  
> > a “strange attractor” (which I won’t explain now). An unstable  
> > system is one for which the orbits diverge. Here are a couple of  
> > examples of attractors, or at least the projections of them into two  
> > dimensions, because even in 2-D, the orbit is actually in a space of  
> > four dimensions, two for location and two for velocity. I have  
> > omitted the velocity coordinates in these examples, and in the  
> > fixed-point example it is the velocity that distinguishes the orbits  
> > where two of them cross in the figure. In the 4-D state space, only  
> > one orbit passes through any particular point.
> 
> RM: All of this simply describes the observed behavior of a variable. Nothing about the shape of these orbits can tell you whether the variable is controlled or not.
> 
> > ```
> > MT: A control system is one for which the attractor converges in at
> > 
> > ```
> > 
> > least one dimension (the perception-value dimension), but that’s not  
> > the main criterion for differentiating the “ball-in-the-bowl” from a  
> > trivial control loop.
> 
> RM: That’s not only is not a “main” criterion; it’s not a criterion at all. The only criterion for distinguishing the “ball-in-the-bowl” from a controlled ball in the bowl is the criterion John Kirkland just mentioned: The criterion of The Test for the Controlled Variable, which is whether there is less of an effect of a disturbance on the controlled variable than expected. You simply cannot tell, by looking at just the observed behavior of the “ball-in-the-bowl” (like the “fixed point” and “stable oscillator” orbits pictured above) whether you are observing the behavior of a controlled or uncontrolled variable. The orbits plotted above could be the behavior of a controlled or uncontrolled ball. This is exactly analogous to the situation in my mindreading demo ([http://www.mindreadings.com/ControlDemo/Mindread.html](http://www.mindreadings.com/ControlDemo/Mindread.html)). When you move one avatar around the screen in a controlled manner, the other two move as well; you can’t tell from the movements (orbits) of the avatars, which is controlled and which are not. In order to determine control you have to disturb the position of the avatars and see which avatar is affected _least_ by the disturbance.
> 
> > ```
> > MT: Bruce Abbott put his finger on it when he
> > 
> > ```
> > 
> > pointed out that the ball-in-the-bowl uses the energy supplied by  
> > the disturbance to return the ball to its fixed point, whereas the  
> > control loop uses an independent energy supply to oppose the effect  
> > of the disturbance on one (and only one) of the variables in the  
> > state space of the loop. The manner in which control is established  
> > is irrelevant.
> 
> RM: This is a description of models that produce the observed behaviors: the open loop physics model for the “ball-in-the-bowl”; closed-loop control for the controlled ball. The manner in which control is established may be irrelevant (I have no idea what that means actually; the only way I know of to establish that control is happening is by using the Test) but one has to have established that control is going on in one case and that it’s not going on in the other in order to apply the correct explanations (models) to each case.
> 
> > ```
> > MT: It so happens that the PCT definition of control is
> > 
> > ```
> > 
> > the maintenance of one particular value among the many different  
> > signal values in a negative feedback loop, so Rick’s comment  
> > “Control is produced _only_ by negative feedback control systems” is  
> > a tautology.
> 
> RM: Actually, that’s not the PCT definition of control. The definition of control is “maintenance of a variable in a pre-selected state, protected from disturbance”. A negative feedback loop is a model of how control works. It’s not a tautology.
> 
> > > ```
> > > MT: Control is produced _only_ by negative feedback control
> > > 
> > > ```
> > > 
> > > systems. So I couldn’t disagree with you more when you say  
> > > “we need to be sure that stability and control are not seen  
> > > as belonging to different kinds of systems”. In fact, we  
> > > need to be VERY sure that we understand that stability and  
> > > control “belong” to two very different systems: open-loop,  
> > > causal systems for the former and closed loop negative  
> > > feedback systems for the latter. [MT: “you” here is Bruce  
> > > Abbott.]
> 
> RM: So let me get this straight. Are you saying that the “stability” of the behavior of the “ball-in-the-bowl” is the same as the “stability” of the behavior of, say, the water level in Ktesibios’ water clock?
> 
> Best
> 
> Rick
> 
> > >
> 
> > >
> 
> > ```
> > To which I can only say that there are several applicable proverbs
> > 
> > ```
> > 
> > along the lines that one is better advised to listen and learn  
> > rather than to guess and pontificate. Bruce is quite right to say  
> > “we need to be sure that stability and control are not seen as  
> > belonging to different kinds of systems”. To contradict Bruce is to  
> > say something as nonsensical as “we need to be sure that leafiness  
> > and trees are not seen as belonging to different kinds of objects”.  
> > “Stability” applies to all kinds of dynamical systems, which control  
> > systems are.
> > 
> > ```
> > Martin
> > 
> > ```
> 
> –  
> Richard S. Marken PhD  
> [www.mindreadings.com](http://www.mindreadings.com)  
> The only thing that will redeem mankind is cooperation.
> 
> ```
> -- Bertrand Russell
> 
> ```

---

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**Author:** ![MartinT](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/m/ee7513/32.png) [@MartinT](http://discourse.iapct.org/u/MartinT)\
**Post date:** [December 21, 2013, 4:04am UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/26 "2013-12-21T04:04:51Z")

</div>

[Martin Taylor 2013.12.20.22.58]

```
That depends which meaning of the term "stability" you want to use.

```

In the technical dynamical sense, yes, they are the same. I know you  
commented on many of the paragraphs of my message in which I  
explained this, but did you actually read them? I don’t understand  
how you could ask the question if you did.  
And to answer a point you did make, an irrelevant point, but a  
point: Yes, the orbits are “just” descriptions of behaviour, and do  
not suggest how this behaviour comes about. I included them to show  
how the concept of stability is independent of the mechanism of  
stability. The ball in the bowl, a single scalar control loop, a  
brain, and the weather of the world are all dynamical systems, but  
only one of these is a scalar control loop.  
Martin

> **···**
>
> > [From Rick Marken (2013.12.20.1740)]
> > 
> > ```
> > RM: So let me get this straight. 
> > 
> > ```
> > 
> > Are you saying that the “stability” of the behavior of the  
> > “ball-in-the-bowl” is the same as the “stability” of the  
> > behavior of, say, the water level in Ktesibios’ water  
> > clock?

---

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**Author:** ![wmansell](http://discourse.iapct.org/user_avatar/discourse.iapct.org/wmansell/32/36_2.png) [@wmansell](http://discourse.iapct.org/u/wmansell)\
**Post date:** [December 21, 2013, 12:24pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/27 "2013-12-21T12:24:03Z")

</div>

Hi Bruce, that’s not quite what I was thinking of. I was wondering whether there might be negative feedback loops within the joystick the ensure that it applies it’s received reference force at that moment despite disturbances within the joystick, such as electrical interference, positioning of components, etc. But maybe it doesn’t. If it didn’t though, I am assuming it would have to be regularly calibrated (like a printer) for it to know that it was any where near accurate in applying the force that the computer thinks it is applying…  
Warren

> **···**
>
> On Fri, Dec 20, 2013 at 2:04 PM, Bruce Abbott [bbabbott@frontier.com](mailto:bbabbott@frontier.com) wrote:
> 
> > [From Bruce Abbott (2013.12.20.0905 EST)]
> 
> > 
> 
> > Warren Mansell –
> 
> > 
> 
> > WM: Maybe we need to look at the circuitry of our joystick to see if it used a negative feedback loop to check that it is applying the force that the computer tells it to? I can’t quite work it out, but it certain shows that for the joystick there are some very detailed mechanics going on beyond the point at which it receives its signal to elicit a force. This does make me think that we need to accept that the biological and physical dynamics of the body within our PCT model will be very important, but nonetheless still could involve a form of negative feedback?
> 
> > 
> 
> > [http://www.google.co.uk/patents?hl=en&lr=&vid=USPAT5742278&id=5D0jAAAAEBAJ&oi=fnd&dq=force+feedback+joystick+patent&printsec=abstract#v=onepage&q=force%20feedback%20joystick%20patent&f=false](http://www.google.co.uk/patents?hl=en&lr=&vid=USPAT5742278&id=5D0jAAAAEBAJ&oi=fnd&dq=force+feedback+joystick+patent&printsec=abstract#v=onepage&q=force%20feedback%20joystick%20patent&f=false)
> 
> > 
> 
> > I looked at the patent briefly and did not see any evidence that negative feedback control is employed by the joystick circuitry. Such control would treat the user’s attempts to move the joystick as disturbances and the system would apply whatever force is necessary (within the limits of its ability to generate force) to prevent the user’s actions from changing the controlled variable – which is not the way you’d want the joystick to behave. Instead, what you want is for the joystick to generate forces that summate with the forces that the user inputs to the stick. For example, the device can simulate a spring-loaded joystick in which a restoring force is generated that pushes the stick back toward the center position whenever the user moves the stick away from center. The strength of force would increase with the deviation from center. The device can also apply a damping force proportional to the rate at which the stick is moved. In this case the stick would feel like it was moving through a viscous liquid. The computer to which the stick connects sends instructions to the processor onboard the joystick telling it how to behave – what forces to generate relative to how and where the stick is moved; the onboard processor senses the stick’s position and from change in position per unit of time computes the velocity of movement, and then applies forces according to these values (depending on what kind of effect or effects have been selected) in open-loop fashion.
> 
> > You are correct that the biological and physical dynamics of the body will be very important – when an attempt is made to create a detailed and highly accurate model of these systems. In our current PCT simulations, more programming is devoted to creating a simulation of the physical environment with which the person will interact than to simulate the relevant control systems within the individual. Fortunately, we can model many control situations without having to represent those control systems in great detail. Most of how a control system behaves is due to the organization of its parts rather to the details of how those parts are constructed, and for many purposes we can just assume that the lower-level systems are doing their jobs and “hide” them inside the output functions of the systems being modeled.
> 
> > Bruce
> 
> –  
> Dr Warren Mansell  
> Reader in Psychology  
> Cognitive Behavioural Therapist & Chartered Clinical Psychologist  
> School of Psychological Sciences
> 
> Coupland I  
> University of Manchester  
> Oxford Road  
> Manchester M13 9PL  
> Email: [warren.mansell@manchester.ac.uk](mailto:warren.mansell@manchester.ac.uk)
> 
> Tel: +44 (0) 161 275 8589
> 
> Website: [http://www.psych-sci.manchester.ac.uk/staff/131406](http://www.psych-sci.manchester.ac.uk/staff/131406)
> 
> See [teamstrial.net](http://teamstrial.net) for further information on our trial of CBT for Bipolar Disorders in NW England
> 
> The highly acclaimed therapy manual on [A Transdiagnostic Approach to CBT using Method of Levels](http://www.amazon.co.uk/Transdiagnostic-Approach-Method-Levels-Therapy/dp/0415507642/ref=sr_1_1?ie=UTF8&qid=1351756948&sr=8-1) is available now.
> 
> Check [www.pctweb.org](http://www.pctweb.org) for further information on Perceptual Control Theory

---

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**Author:** ![rsmarken](http://discourse.iapct.org/user_avatar/discourse.iapct.org/rsmarken/32/3553_2.png) [@rsmarken](http://discourse.iapct.org/u/rsmarken)\
**Post date:** [December 21, 2013, 7:06pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/28 "2013-12-21T19:06:20Z")

</div>

[From Rick Marken (2013.12.21.1100)]

> Martin Taylor (2013.12.20.22.58)--
> 
> RM: So let me get this straight. Are you saying that the "stability" of the  
> behavior of the "ball-in-the-bowl" is the same as the "stability" of the  
> behavior of, say, the water level in Ktesibios' water clock?

> MT: And to answer a point you did make, an irrelevant point, but a point: Yes,  
> the orbits are "just" descriptions of behaviour, and do not suggest how this  
> behaviour comes about.

RM: How the behavior comes about is a theoretical question. The prior  
question must be "what kind of behavior is it --control or  
non-control", a question that can only be answered using a version of  
the Test for the Controlled Variable. What you can't tell by looking  
at the orbits (the visible behavior of a variable alone) is whether  
the variable doing the orbit is a controlled or uncontrolled variable.

I believe that talking about the orbits -- over behavior of a possibly  
controlled variable -- as a measure of "stability" leads to confusion  
because it implies that the observable behavior of the variable (the  
nature of the orbit) can be used to determine whether or not there is  
control going on. Apparently control is thought to be going on if the  
orbit returns to an "attractor" point after a transient disturbance.  
Of course, this is not the case. But this is clearly what Douglas  
thinks and describes in his lectures on control, making them (in my  
humble opinion) confusing at best and misleading at worst for PCT  
control theorists.

Clearly Douglas thinks of "stability", in the sense of a variable  
returning to an equilibrium or attractor state after a transient  
disturbance, as an example of control (ergo, the lecture on "open-loop  
control"). This misconception (shared by many control theorists) has  
created enormous problems for PCT. The problems exist in the form of  
cause-effect models of "stability" (such as the mass-spring and  
coordinative structure models of limb control) and the constant  
refrain from reviewers of my papers: "we already know that".

Douglas's lectures on control would be fine (from a PCT perspective)  
if he had done what I thought he was doing in the first lecture:  
defining the phenomenon of control and distinguishing it from  
superficially similar phenomena, such as the "stability" behavior of a  
ball pushed from the bottom of a bowl or a pendulum bob that is pushed  
from it's resting state.

I think one of Powers main contributions was to make clear that 1)  
control is a real phenomenon, different from causal phenomena (such as  
the ball in the bowl, bob on a string or mass on a spring, which  
return to an "attractor" point after transient disturbance) and 2)  
that the behavior of living systems \_is\_ control. I think Bill had  
these things in mind when he decided that the subtitle to his last  
book, LCS III, would be "The Fact of Control".

Best

Rick

> **···**
>
> --  
> Richard S. Marken PhD  
> www.mindreadings.com
> 
> The only thing that will redeem mankind is cooperation.  
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-- Bertrand Russell

---

<div class="post-metadata">

**Author:** ![MartinT](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/m/ee7513/32.png) [@MartinT](http://discourse.iapct.org/u/MartinT)\
**Post date:** [December 21, 2013, 8:05pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/29 "2013-12-21T20:05:49Z")

</div>

[Martin Taylor 2013.12.21.14.43]

> [From Rick Marken (2013.12.21.1100)]
> 
> > Martin Taylor (2013.12.20.22.58)--
> > 
> > RM: So let me get this straight. Are you saying that the "stability" of the  
> > behavior of the "ball-in-the-bowl" is the same as the "stability" of the  
> > behavior of, say, the water level in Ktesibios' water clock?  
> > MT: And to answer a point you did make, an irrelevant point, but a point: Yes,  
> > the orbits are "just" descriptions of behaviour, and do not suggest how this  
> > behaviour comes about.
> 
> RM: How the behavior comes about is a theoretical question. The prior  
> question must be "what kind of behavior is it --control or  
> non-control", a question that can only be answered using a version of  
> the Test for the Controlled Variable. What you can't tell by looking  
> at the orbits (the visible behavior of a variable alone) is whether  
> the variable doing the orbit is a controlled or uncontrolled variable.

Yes, you are getting the pointt.

> I believe that talking about the orbits -- over behavior of a possibly  
> controlled variable -- as a measure of "stability" leads to confusion  
> because it implies that the observable behavior of the variable (the  
> nature of the orbit) can be used to determine whether or not there is  
> control going on.

Oh, I thought from the previous paragraph that you were getting it. Clearly you aren't. So, how would one be able to do what you suggest?

> Apparently control is thought to be going on if the  
> orbit returns to an "attractor" point after a transient disturbance.

Why on earth would anyone assume that the state returning to an attractor implied control is going on?

> Of course, this is not the case.

No, it's not always the case. It probably isn't even usually the case. But it sometimes is the case. Sometimes a biological organism (or a robot) is indeed making the state return to an attractor. In that case, the attractor is defined by the reference values for the state described by the variables in the vector of perceptual values.

> But this is clearly what Douglas  
> thinks and describes in his lectures on control, making them (in my  
> humble opinion) confusing at best and misleading at worst for PCT  
> control theorists.

I guess I'll have to look at the lectures and see for myself, so I won't comment now. Clearly Bruce Abbott doesn't agree with you, so I don't expect I will. Bruce has recommended a particular sequence of viewing the videos, so I will follow his suggestions.

> I think one of Powers main contributions was to make clear that 1)  
> control is a real phenomenon, different from causal phenomena (such as  
> the ball in the bowl, bob on a string or mass on a spring, which  
> return to an "attractor" point after transient disturbance) and 2)  
> that the behavior of living systems \_is\_ control. I think Bill had  
> these things in mind when he decided that the subtitle to his last  
> book, LCS III, would be "The Fact of Control".

Control is a real phenomenon. In my view, when combined with the internal provision of the primary reference values, it is the defining characteristic of life, but if you keep saying that it is different from causal phenomena, you turn away from PCT all those who believe that even the biological world is subject to physical laws. To understand PCT, it is NOT necessary to believe in acausality (i.e. magic, divine intervention, or something like that). PCT is a product of causality, and cannot be understood if you invoke some distinction between control and causality, where analytically, experimentally, and philosophically there is none. Without causality there can be no control.

Martin

---

<div class="post-metadata">

**Author:** ![rsmarken](http://discourse.iapct.org/user_avatar/discourse.iapct.org/rsmarken/32/3553_2.png) [@rsmarken](http://discourse.iapct.org/u/rsmarken)\
**Post date:** [December 21, 2013, 10:45pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/30 "2013-12-21T22:45:03Z")

</div>

[From Rick Marken (2013.12.21.1445)]

> Martin Taylor (2013.12.21.14.43)--

> > RM: How the behavior comes about is a theoretical question. The prior  
> > question must be "what kind of behavior is it --control or  
> > non-control", a question that can only be answered using a version of  
> > the Test for the Controlled Variable. What you can't tell by looking  
> > at the orbits (the visible behavior of a variable alone) is whether  
> > the variable doing the orbit is a controlled or uncontrolled variable.
> 
> MT: Yes, you are getting the pointt.
> 
> > RM: I believe that talking about the orbits -- over behavior of a possibly  
> > controlled variable -- as a measure of "stability" leads to confusion  
> > because it implies that the observable behavior of the variable (the  
> > nature of the orbit) can be used to determine whether or not there is  
> > control going on.

> MT: Oh, I thought from the previous paragraph that you were getting it. Clearly  
> you aren't. So, how would one be able to do what you suggest?

RM: Using the test for the controlled variable (TCV). The orbits are  
one part of the TCV: the observed variations in the state of the  
hypothetical controlled variable. The other part of the TCV is the  
concomitant variations in a disturbance. If there is little  
relationship between temporal variations in the disturbance and the  
temporal variations in the hypotetical controlled variable (your  
orbits) then the variable is likely to be under control. If the  
variations in the hypothetical controlled variable are what are  
expected (on physical grounds) then the variable is \_not\_ under  
control.

> MT: Control is a real phenomenon. In my view, when combined with the internal  
> provision of the primary reference values, it is the defining characteristic  
> of life, but if you keep saying that it is different from causal phenomena,  
> you turn away from PCT all those who believe that even the biological world  
> is subject to physical laws.

RM: Sorry. I thought you understood that when I said "causal" I was  
talking about the "open-loop" or lineal causal model of the physical  
sciences (and, unfortunately, of the life sciences as well).

> MT: To understand PCT, it is NOT necessary to believe in acausality (i.e. magic, divine  
> intervention, or something like that).

RM: Right. You have to understand circular causality in order to understand PCT.

> MT: PCT is a product of causality, and cannot be understood if you invoke  
> some distinction between control and causality, where analytically,  
> experimentally, and philosophically there is none. Without causality there  
> can be no control.

RM: I don't believe I've ever said that there was a distinction  
between control and causality. But perhaps I was not clear. The  
distinction I make (and that Bill made, of course) is between control  
and non-control behavior. Non-control behavior is exemplified by the  
behavior of the ball in the bowl; no control is involved because there  
is no resistance to disturbance. Such behavior can be accounted for by  
causal models, by which I mean the lineal causal models of physics.  
Control behavior is exemplified by the behavior of the distance  
between target and cursor in a tracking task; control is involved  
because there is nearly perfect resistance to disturbance. Such  
behavior can be accounted for only by circular causal models --  
negative feedback control models.

The problem with the treatment of control in the Douglas lectures --  
at least in the two I've seen -- is that he clearly considers behavior  
like that of the ball at the bottom of a bowl to be an example of  
control; weak control perhaps, which is why he calls it "stability",  
but control nevertheless, since he considers control to be evidenced  
by a variable returning to its original state after a transient  
disturbance. It is this kind of mistake that has led psychologists to  
think that they can account for actual control behavior, such as  
control of limb position, using physical causal models. And this is  
one big reason for the rejection of PCT by conventional psychologists.  
Ergo, it upsets me when this kind of thing is presented as worthy of  
study by students of PCT. Perhaps it is worthy of such study, but as  
an object lesson in how people -- including control engineers!--have  
misunderstood what control is and why they have rejected purposeful  
(control) models of the behavior of living systems.

Best

Rick

> **···**
>
> --  
> Richard S. Marken PhD  
> www.mindreadings.com
> 
> The only thing that will redeem mankind is cooperation.  
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-- Bertrand Russell

---

<div class="post-metadata">

**Author:** ![MartinT](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/m/ee7513/32.png) [@MartinT](http://discourse.iapct.org/u/MartinT)\
**Post date:** [December 22, 2013, 4:28am UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/31 "2013-12-22T04:28:01Z")

</div>

[Martin Taylor 2013.12.21.23.13]

> [From Rick Marken (2013.12.21.1445)]
> 
> > Martin Taylor (2013.12.21.14.43)--
> > 
> > > RM: I believe that talking about the orbits -- over behavior of a possibly  
> > > controlled variable -- as a measure of "stability" leads to confusion  
> > > because it implies that the observable behavior of the variable (the  
> > > nature of the orbit) can be used to determine whether or not there is  
> > > control going on.
> > 
> > MT: Oh, I thought from the previous paragraph that you were getting it. Clearly  
> > you aren't. So, how would one be able to do what you suggest?
> 
> RM: Using the test for the controlled variable (TCV).

That is avoiding the question. You suggested that one might use the nature of the orbit to determine whether there is control. I wanted to know how one could do that. You can't sling in the TCV. That's like answering a question in Euclidean geometry by using algebra. You can certainly solve the problem that way, but not in the terms of the question.

Anyway, we were not discussing how one tells whether a particular dynamical system is a control system. I was trying to get you to understand that "stability" is a property of all dynamical systems, and is not excluded from being applicable to control systems. As with "causal" (below), it is a question of using language, where possible, the way it is understood by those whom you would like to appreciate and understand PCT.

> MT: Control is a real phenomenon. In my view, when combined with the internal  
> provision of the primary reference values, it is the defining characteristic  
> of life, but if you keep saying that it is different from causal phenomena,  
> you turn away from PCT all those who believe that even the biological world  
> is subject to physical laws.  
> RM: Sorry. I thought you understood that when I said "causal" I was  
> talking about the "open-loop" or lineal causal model of the physical  
> sciences (and, unfortunately, of the life sciences as well).

I understood very well, because from long acquaintance I know your language. The people who might not understand, and who prefer scientific theories not to depend on magic, are those to whom the word "causal" implies "physically realizable" -- most of the scientific world who have not been readers of CSGnet. PCT is physically realizable, and hence "causal".

As one who would very much like to see PCT have a larger influence in the world (as in your 50-year future imagined world), I find myself objecting strongly to things that I believe impede the possibility of that larger influence happening. Speaking a private language is in some respects unavoidable when dealing with an unconventional theory, but it is best to limit so far as possible the use of words that could seriously mislead. "Stability" is applicable to control systems, which are indeed "causal".

Martin

---

<div class="post-metadata">

**Author:** ![\_Boris\_Hartman](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/_/e8c25b/32.png) [@\_Boris\_Hartman](http://discourse.iapct.org/u/_Boris_Hartman)\
**Post date:** [December 22, 2013, 10:53am UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/32 "2013-12-22T10:53:38Z")

</div>

Hi Rick,

your text is not “full” clear to me, but anyway I’ll try to  
contribute some thoughts.

MT :

I still haven’t looked at the videos, but it strikes me that you are  
doing the old philosophers’ trick of taking a word that has a variety of  
meanings, and using a meaning appropriate in one context as though that were  
the meaning appropriate to a different context.

HB :

I agree with Martin about variety of meanings about terms you choose.  
It’s quite a slippy “terain”.

MT:

It so happens that the PCT definition of control is the maintenance of  
one particular value among the many different signal values in a negative  
feedback loop.

RM :

Actually, that’s not the PCT definition of control. The definition of  
control is “maintenance of a variable in a pre-selected state, protected  
from disturbance”. A negative feedback loop is a model of how control  
works. It’s not a tautology.

HB :

First of all I think you should try to find relevant  
“definitions” for the terms you are using. It seems to me, that  
central terms are control and stability. Sorry to say, but Martin’s definition  
seems to me much closer to PCT definition of control then yours.

There are quite some “foggy” conclusions in your definition  
Rick :

1. I think that PCT control begins with perceived states of “variables”  
not with variables themself. If you have to use variable in defitnion.

2. Another problem is term “protecting” you put in the  
definition. I think it would be better to use term canceling or compensating.  
It’s the definition we are talking about not “afternoon debate”.

3. Futher more I see problem in using “variable” in  
definition. Control by my oppinion doesn’t include only “outer controlled  
variables” but the whole organism with his close environment, with which  
it is interacting.

So with some necesary modifications I see your defintion of control in  
PCT sense as something going like this : " …control is maintenance  
of a pre-selected state in control system, by canceling disturbances, that  
affect it". My language could be a problem, but I hope I show the point.

TO PUT IT EXACTLY IN PCT SENSE, CONTROL IS : “Achievement and  
maintanance of a preselected perceptual state in the controlling system,  
through actions on the environment that also cancel the effects of  
disturbances” (B:CP, 2005).

Physiology is using terms control of “almost constant  
conditions” in organisms, what could mean that certain  
“variables” are kept in some genetically predefined limits.

So as I see it, control is initially not “protecting” act but  
**cancelling act** of output which  
compensate effects of disturbances so to maintain perceptual stability (some  
preselected perceptual state) in the controlling system. This could mean that  
“controlled variable” is already affected and new state perceived and  
controlled in comparator and act of canceling the effect or compensating the  
efect of disturbances realized.

To use term “protect” is something that means for me to act  
in advance, so to “prevent” some “controlled variable” or  
better predefined state of controlling system, from being disturbed, displaced,  
affected…etc.

Term “protecting” is probably kind of control but not in  
initial sense. By my oppinion is one of consequences of “pure” PCT  
control, which is by definition used with terms canceling, compensating,  
etc…

Something similar was Ashby’s “control” definition :  
“Every stable system has the property that if displaced from a state of  
equilibrium and released, the subsequent movement is so matched to the initial  
displacement that the system is brought back to the state of equilibrium”  
(Ashby, 1960).

I think that Ashby used “compensation” for description of  
“control”. And it seems to me that he used terms to describe actual  
“displacement” and actual “compensation” not something  
happening in advance.

He tried to give also definitions of dynamic system, variable and  
system, specifications of behaviour, “stability”,  
“equilibrium”, “steady-state”, and so on, as I think that  
Bill used some of this terms in Appendix to the book B:CP, 2005.

So the main point I see, is that whatever “controlled  
variable” is meant, is first “moved” (perceived displacement)  
from the predefined, initial state (reference state, equilibirum…) and  
than by canceling or compensating or opposing effects of action or whatever we  
call that (maybe behavior), again brought back to predefined, initial state  
(reference, equilibrium, whatever…).

If we say that “controlled variable” is protected than you  
probably assume that “controlled variable” was not disturbed yet, as  
disturbances has already been cancelled, compensated by control system in  
advance. But that can never happen if control system “has no  
experiences” with “moving controlled variable” from initial  
state with certain disturbances. How else could control system  
“choose” disturbances to act on, so to protect “controlled  
variable” from being affected if control system doesn’t know what kind of  
effect distrubances have on “controlled variable” and system itself.

I think that the selection of disturbances that could have effect on  
controlled variable, are those which in the past show tendency to  
“displace controlled variable” from initial state. So when control  
system has that “experience” than it can probably reorganize so to  
“protect” it from disturbances. But by my oppinion it has to be  
complex control system, build up with many control units, what could happen  
through evolution.

In such a complex organized control systems (more organized control  
units), certain control units serve the goal to really “prevent” or  
“protect” certain “controlled variable” from being  
disturbed, displaced or “moved” from initial state like in some  
physiological cases.

So I think it’s better to use initial terms when making defintion about  
control in PCT sense such as cancelling, compensating activity not  
“protecting” activity.

Your “exclusive” statement about Gordon…. :

RM : “Well, Mr. Douglas  
is now off my list of people to listen to about control theory. This lecture  
was awful”.

…has no sense to me.

By my oppinion Douglas tried methodically to show how control in  
different dynamic system works. And by my opinion his retorics and pictures  
about control is better then yours about “protecting controlled  
variables”. But both are insufficient (as probably mine is), but that  
doesn’t mean that we have to stop talking about your and his presentation of  
control, because they are “awfull”. I think that both are good as the  
bases to improve them, so they would show better how control theory works.

Rick, you are doing a good job, moderating on CSGnet. But nobody said  
that you couldn’t be better J

I can’t comment other discussions for the time being as I didn’t read  
them. Maybe I missed something important. I also didn’t entirely follow the  
discussion about B:CP (2005) so I’m interested if you make any comments about  
Appendix in the book. There are some interesting clarifications of terms  
control, stability…

Best,

Boris

![Attractors.jpg](http://discourse.iapct.org/uploads/default/original/1X/f95899be1fc710df351ee2dac6d13c29c52c1a91.jpeg)

> **···**
>
> * * *
> 
> **From:** Control Systems Group  
> Network (CSGnet) [[mailto:CSGNET@LISTSERV.ILLINOIS.EDU](mailto:CSGNET@LISTSERV.ILLINOIS.EDU)] **On Behalf Of** Richard Marken  
> **Sent:** Saturday, December 21, 2013  
> 2:38 AM  
> **To:** CSGNET@LISTSERV.ILLINOIS.EDU  
> **Subject:** Re: Classical Control  
> Systems Lectures
> 
> [From Rick Marken  
> (2013.12.20.1740)]
> 
> > Martin Taylor  
> > (2013.12.20.11.47)–
> 
> > > RM: Stability and control are two different _phenomena_, something that  
> > > it is now clear to me that Mr. Douglas is completely unaware of. These two  
> > > different phenomena are produced by two different kinds of systems; stability  
> > > is a kind of behavior exhibited by certain “open-loop” or what I call  
> > > “causal” systems.
> 
> > I still haven’t looked at the videos, but it strikes me that you are  
> > doing the old philosophers’ trick of taking a word that has a variety of  
> > meanings, and using a meaning appropriate in one context as though that were  
> > the meaning appropriate to a different context. Yes, “stability” and  
> > “control” are indeed different phenomena, in the same way  
> > “food” and “vegetable” are different concepts.
> 
> RM:  
> No,“stability” and “control”, as described by Douglas in  
> the very first control lecture, are different phenomena in the way  
> “food” and “poison” are different. A stable system  
> (according to the lecture) is one that returns to its original  
> (“equilibrium”) state after a transient disturbance; a control system  
> is is one that remains in a reference state during continuous disturbance.  
> Douglas should have talked about variables rather than systems but you get the  
> idea.
> 
> > MT: All feedback loops, in fact all dynamical systems, whether control  
> > systems or not, have stability criteria. Either they are stable or not. Some  
> > are more stable than others.
> 
> RM: Yes, they do. Indeed, I measure control in terms of stability  
> (observed/expected variance of the variable). In this case  
> “stability” is simply referring to a measure of the observed  
> variations in a variable and it can be used to measure the variability of a  
> controlled variable or an uncontrolled variable (like the variable position of  
> the ball in the bowl that Douglas refers to as a stable system).
> 
> > MT: Some are metastable,  
> > meaning they will maintain their current values until something momentarily  
> > disturbs one of their signal values. Some of those will continue to diverge  
> > from the original metastable value after the disturbance, some will just  
> > maintain the disturbed set of signal values without further change. Some are  
> > absolutely stable, meaning that after any kind of momentary disturbance they  
> > will return their values to their original levels. Most real systems don’t do  
> > that, and are stable only if the momentary disturbance doesn’t exceed some  
> > limit.
> > 
> > The key concept is the “orbit”. All systems that can be described by  
> > a vector of variables have a state. Their state is the vector of current  
> > variable values together with the rates of change of the current variables.  
> > That includes control systems, ball-in-a-bowl systems, the synapse strengths of  
> > networks of millions of neurons, etc. etc. If the system is at some location in  
> > the state space and is not further disturbed from outside, it will follow some  
> > track through the state space. That track is an orbit, and there is only one  
> > orbit through any point in the state space.
> > 
> > A stable system is one for which the orbit will converge to some track that is  
> > the same for all the initial locations in the state space. That track is called  
> > an “attractor”. The attractor may be a fixed point, a closed path  
> > (which represents a stable oscillator) or a “strange attractor”  
> > (which I won’t explain now). An unstable system is one for which the  
> > orbits diverge. Here are a couple of examples of attractors, or at least the  
> > projections of them into two dimensions, because even in 2-D, the orbit is  
> > actually in a space of four dimensions, two for location and two for velocity.  
> > I have omitted the velocity coordinates in these examples, and in the  
> > fixed-point example it is the velocity that distinguishes the orbits where two  
> > of them cross in the figure. In the 4-D state space, only one orbit passes  
> > through any particular point.
> > 
> > ![]()
> 
> RM: All of this simply describes the observed behavior of a variable.  
> Nothing about the shape of these orbits can tell you whether the variable is  
> controlled or not.
> 
> > MT: A control system is one for which the attractor converges in at  
> > least one dimension (the perception-value dimension), but that’s not the main  
> > criterion for differentiating the “ball-in-the-bowl” from a trivial  
> > control loop.
> 
> RM: That’s not only is not a “main” criterion; it’s not a  
> criterion at all. The only criterion for distinguishing the  
> “ball-in-the-bowl” from a controlled ball in the bowl is the criterion  
> John Kirkland  
> just mentioned: The criterion of The Test for the Controlled Variable, which is  
> whether there is less of an effect of a disturbance on the controlled variable  
> than expected. You simply cannot tell, by looking at just the observed behavior  
> of the “ball-in-the-bowl” (like the “fixed point” and  
> “stable oscillator” orbits pictured above) whether you are observing  
> the behavior of a controlled or uncontrolled variable. The orbits plotted above  
> could be the behavior of a controlled or uncontrolled ball. This is exactly  
> analogous to the situation in my mindreading demo ([http://www.mindreadings.com/ControlDemo/Mindread.html](http://www.mindreadings.com/ControlDemo/Mindread.html)). When  
> you move one avatar around the screen in a controlled manner, the other two  
> move as well; you can’t tell from the movements (orbits) of the avatars, which  
> is controlled and which are not. In order to determine control you have to  
> disturb the position of the avatars and see which avatar is affected _least_ by  
> the disturbance.
> 
> > MT: Bruce Abbott  
> > put his finger on it when he pointed out that the ball-in-the-bowl uses the  
> > energy supplied by the disturbance to return the ball to its fixed point,  
> > whereas the control loop uses an independent energy supply to oppose the effect  
> > of the disturbance on one (and only one) of the variables in the state space of  
> > the loop. The manner in which control is established is irrelevant.
> 
> RM: This is a description of models that produce the observed  
> behaviors: the open loop physics model for the “ball-in-the-bowl”;  
> closed-loop control for the controlled ball. The manner in which control  
> is established may be irrelevant (I have no idea what that means actually; the  
> only way I know of to establish that control is happening is by using the Test)  
> but one has to have established that control is going on in one case and that  
> it’s not going on in the other in order to apply the correct explanations  
> (models) to each case.
> 
> > MT: It so happens that the PCT definition of control is the maintenance  
> > of one particular value among the many different signal values in a negative  
> > feedback loop, so Rick’s comment “Control is produced _only_ by negative  
> > feedback control systems” is a tautology.
> 
> RM: Actually, that’s not the PCT definition of control. The  
> definition of control is “maintenance of a variable in a pre-selected  
> state, protected from disturbance”. A negative feedback loop is a model of  
> how control works. It’s not a tautology.
> 
> > > MT: Control is produced _only_ by negative feedback control systems. So  
> > > I couldn’t disagree with you more when you say “we need to be sure that  
> > > stability and control are not seen as belonging to different kinds of  
> > > systems”. In fact, we need to be VERY sure that we understand that  
> > > stability and control “belong” to two very different systems:  
> > > open-loop, causal systems for the former and closed loop negative  
> > > feedback systems for the latter. [MT: “you” here is Bruce Abbott.]
> 
> RM: So let me get this straight. Are you saying that the  
> “stability” of the behavior of the “ball-in-the-bowl” is  
> the same as the “stability” of the behavior of, say, the water level  
> in Ktesibios’ water clock?
> 
> Best
> 
> Rick
> 
> > To which I can only say  
> > that there are several applicable proverbs along the lines that one is better  
> > advised to listen and learn rather than to guess and pontificate. Bruce is  
> > quite right to say “we need to be sure that stability and control are not  
> > seen as belonging to different kinds of systems”. To contradict Bruce is  
> > to say something as nonsensical as “we need to be sure that leafiness and  
> > trees are not seen as belonging to different kinds of objects”.  
> > “Stability” applies to all kinds of dynamical systems, which control  
> > systems are.
> > 
> > Martin
> 
> –
> 
> Richard S. Marken  
> PhD  
> [www.mindreadings.com](http://www.mindreadings.com)
> 
> The only thing that will redeem mankind is cooperation.
> 
> ```
> -- Bertrand Russell
> 
> ```
> 
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> 
> Checked by AVG - [www.avg.com](http://www.avg.com)
> 
> Version: 2014.0.4259 / Virus Database: 3658/6937 - Release Date: 12/20/13
> 
> * * *
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> No virus found in this message.
> 
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**Author:** ![rsmarken](http://discourse.iapct.org/user_avatar/discourse.iapct.org/rsmarken/32/3553_2.png) [@rsmarken](http://discourse.iapct.org/u/rsmarken)\
**Post date:** [December 22, 2013, 6:26pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/33 "2013-12-22T18:26:08Z")

</div>

[From Rick Marken (2013.12.22.1025)]

> Martin Taylor (2013.12.21.23.13)--

> RM: I believe that talking about the orbits -- overt behavior of a  
> possibly controlled variable -- as a measure of "stability" leads to confusion  
> because it implies that the observable behavior of the variable (the  
> nature of the orbit) can be used to determine whether or not there is  
> control going on.

> MT: Oh, I thought from the previous paragraph that you were getting it.  
> Clearly you aren't. So, how would one be able to do what you suggest?

> RM: Using the test for the controlled variable (TCV).
> 
> MT: That is avoiding the question.

RM: I don't understand. What question did I avoid? You asked "So, how  
would one be able to do what you suggest?". I assumed that what you  
were referring to was my implicit suggestion that you have to do  
something other than simply observe the behavior of a variable in  
order to determine whether or not it is under control. So what I  
suggested is that what was needed to determine whether control was  
going on (or not) was to apply a disturbance and see if it had the  
expected effect, ie. use the TCV.

> MT: You suggested that one might use the nature  
> of the orbit to determine whether there is control.

RM: Actually, I think it's clear from the quote above that I suggested  
just the opposite: that it is impossible to determine whether or not  
control is going on by just observing (using) the behavior of the  
variable (the nature of the orbit in this case).

> MT: Anyway, we were not discussing how one tells whether a particular dynamical  
> system is a control system.

RM: Actually, that is exactly what I was discussing. You might not  
have been but I was.

> MT: I was trying to get you to understand that  
> "stability" is a property of all dynamical systems, and is not excluded from  
> being applicable to control systems.

RM: And I already said that I understood that. I even said that I use  
what are called "stability" measures to evaluate the quality of  
control. The only point I am making here is that the "stability"  
behavior of certain dynamical systems, like that of the ball in the  
bowl, is not control. It's pretty easy to see why. When a variable in  
a non-control dynamical system, like the ball in the bowl, is  
disturbed there is no resistance to the disturbance. The effect of a  
disturbance to the ball in the bowl, for example, is exactly what is  
expected based on physical law. If the disturbance is removed the ball  
returned to the initial state again as a result of physical law, not  
active disturbance resistance. A disturbance applied to a controlled  
variable is actively resisted; this active disturbance resistance is  
evidence that control is occurring. The return of a dynamical system  
to it's initial state after a transient disturbance, as when a ball  
returns to the bottom of the bowl after being pulled up the side and  
then let go, looks like control to the extent that a variable returns  
to what looks like a goal or reference state. It is this mistake --  
taking a non-control phenomenon for control -- that has caused  
enormous problems for acceptance of PCT -- or at least, for the  
acceptance of my papers in journals;-)

> RM: Sorry. I thought you understood that when I said "causal" I was  
> talking about the "open-loop" or lineal causal model of the physical  
> sciences (and, unfortunately, of the life sciences as well).
> 
> MT: I understood very well, because from long acquaintance I know your language.  
> The people who might not understand, and who prefer scientific theories not  
> to depend on magic, are those to whom the word "causal" implies "physically  
> realizable" -- most of the scientific world who have not been readers of  
> CSGnet. PCT is physically realizable, and hence "causal".

RM: OK. Point taken. I'll try to be more careful about that.

Best

Rick

> **···**
>
> --  
> Richard S. Marken PhD  
> www.mindreadings.com
> 
> The only thing that will redeem mankind is cooperation.  
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-- Bertrand Russell

---

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**Post date:** [December 22, 2013, 7:01pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/34 "2013-12-22T19:01:44Z")

</div>

[From Fred Nickols (2013.12.22.1355 EST)]

I'm doing my best to follow all this but I have a couple of questions. I  
don't want to derail the discussion but I would like to know the "correct"  
answers.

When we talk about a Disturbance affecting a Controlled Variable we are  
really talking about it affecting our perception of those effects; more  
specifically, changes in the perceived value of the CV. So, if there is a  
lag between the effects of a Disturbance on a CV and changes in our  
perception of that CV, it is theoretically possible for a Disturbance to  
alter a CV and we wouldn't know about it because our perception of it hasn't  
changed. Is that correct?

Error signals come about from the comparison of the Perceived Value of the  
CV with the reference signal for the CV. Errors can be created in two ways:  
(1) changes in the CV that are perceived in the form of changes in the  
perceived value of the CV and (2) changes in the reference signal itself.  
(I suppose a third way is through some combination of the two.) Is that  
correct?

Fred Nickols

> From: Richard Marken [mailto:rsmarken@GMAIL.COM]  
> Sent: Sunday, December 22, 2013 1:26 PM  
> To: CSGNET@LISTSERV.ILLINOIS.EDU  
> Subject: Re: Classical Control Systems Lectures
> 
> [From Rick Marken (2013.12.22.1025)]
> 
> \> Martin Taylor (2013.12.21.23.13)--
> 
> \> RM: I believe that talking about the orbits -- overt behavior of a  
> \> possibly controlled variable -- as a measure of "stability" leads to  
> \> confusion because it implies that the observable behavior of the  
> \> variable (the nature of the orbit) can be used to determine whether or  
> \> not there is control going on.
> 
> \> MT: Oh, I thought from the previous paragraph that you were getting it.  
> \> Clearly you aren't. So, how would one be able to do what you suggest?
> 
> \> RM: Using the test for the controlled variable (TCV).  
> \>  
> \> MT: That is avoiding the question.
> 
> RM: I don't understand. What question did I avoid? You asked "So, how  
> would one be able to do what you suggest?". I assumed that what you were  
> referring to was my implicit suggestion that you have to do something

other

> than simply observe the behavior of a variable in order to determine  
> whether or not it is under control. So what I suggested is that what was  
> needed to determine whether control was going on (or not) was to apply a  
> disturbance and see if it had the expected effect, ie. use the TCV.
> 
> \> MT: You suggested that one might use the nature of the orbit to  
> \> determine whether there is control.
> 
> RM: Actually, I think it's clear from the quote above that I suggested

just the

> opposite: that it is impossible to determine whether or not control is

going on

> by just observing (using) the behavior of the variable (the nature of the

orbit

> in this case).
> 
> \> MT: Anyway, we were not discussing how one tells whether a particular  
> \> dynamical system is a control system.
> 
> RM: Actually, that is exactly what I was discussing. You might not have

been

> but I was.
> 
> \> MT: I was trying to get you to understand that "stability" is a  
> \> property of all dynamical systems, and is not excluded from being  
> \> applicable to control systems.
> 
> RM: And I already said that I understood that. I even said that I use what

are

> called "stability" measures to evaluate the quality of control. The only

point I

> am making here is that the "stability"  
> behavior of certain dynamical systems, like that of the ball in the bowl,

is not

> control. It's pretty easy to see why. When a variable in a non-control  
> dynamical system, like the ball in the bowl, is disturbed there is no

resistance

> to the disturbance. The effect of a disturbance to the ball in the bowl,

for

> example, is exactly what is expected based on physical law. If the  
> disturbance is removed the ball returned to the initial state again as a

result

> of physical law, not active disturbance resistance. A disturbance applied

to a

> controlled variable is actively resisted; this active disturbance

resistance is

> evidence that control is occurring. The return of a dynamical system to

it's

> initial state after a transient disturbance, as when a ball returns to the  
> bottom of the bowl after being pulled up the side and then let go, looks

like

> control to the extent that a variable returns to what looks like a goal or  
> reference state. It is this mistake -- taking a non-control phenomenon for  
> control -- that has caused enormous problems for acceptance of PCT -- or

at

> **···**
>
> > -----Original Message-----  
> > least, for the acceptance of my papers in journals;-)
> > 
> > \> RM: Sorry. I thought you understood that when I said "causal" I was  
> > \> talking about the "open-loop" or lineal causal model of the physical  
> > \> sciences (and, unfortunately, of the life sciences as well).  
> > \>  
> > \> MT: I understood very well, because from long acquaintance I know your  
> > language.  
> > \> The people who might not understand, and who prefer scientific  
> > \> theories not to depend on magic, are those to whom the word "causal"  
> > \> implies "physically realizable" -- most of the scientific world who  
> > \> have not been readers of CSGnet. PCT is physically realizable, and hence  
> > "causal".
> > 
> > RM: OK. Point taken. I'll try to be more careful about that.
> > 
> > Best
> > 
> > Rick  
> > --  
> > Richard S. Marken PhD  
> > www.mindreadings.com
> > 
> > The only thing that will redeem mankind is cooperation.  
> > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-- Bertrand Russell

---

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**Author:** ![rsmarken](http://discourse.iapct.org/user_avatar/discourse.iapct.org/rsmarken/32/3553_2.png) [@rsmarken](http://discourse.iapct.org/u/rsmarken)\
**Post date:** [December 22, 2013, 7:12pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/35 "2013-12-22T19:12:05Z")

</div>

[From Rick Marken (2013.12.22.1110)]

![Attractors.jpg](http://discourse.iapct.org/uploads/default/original/1X/f95899be1fc710df351ee2dac6d13c29c52c1a91.jpeg)

> **···**
>
> On Sun, Dec 22, 2013 at 2:53 AM, Boris Hartman [boris.hartman@masicom.net](mailto:boris.hartman@masicom.net) wrote:
> 
> > BH: TO PUT IT EXACTLY IN PCT SENSE, CONTROL IS : “Achievement and  
> > maintanance of a preselected perceptual state in the controlling system,  
> > through actions on the environment that also cancel the effects of  
> > disturbances” (B:CP, 2005).
> 
> RM: Yes, that’s a good one too;-) It’s tough to give a definition of control that is completely theory free and I don’t think Bill was trying to do that when he came up with this definition. Indeed, part of Bill’s goal in giving this definition of control was to include the important new assumptions of what has come to be called PCT. I don’t think it was until many years after the publication of B:CP that he actively promoted the idea that control was a phenomenon in and of itsef – an objective _fact_ (as in the subtitle to LCS III) – that is explained by the theory of control: control theory.
> 
> I think the definition above can be “objectified:” a bit by changing “perceptual state in a controlling system” to “variable”. The idea that it is a perceptual variable that is controlled is really part of the theory – a very important part of the theory but part of the theory nevertheless.
> 
> But one can observe control without making any assumptions about _how_ control works. We can do this by observing a variable, such as the distance between cursor and target in a tracking task, and noting that disturbances have little or none of their expected effect on this variable and that this is because the effects of these disturbances are being cancelled by observed actions (mouse movements). So control is happening because we are seeing the:
> 
> “Achievement and  
> maintenance of a variable in a particular state  
> through actions on the environment that also cancel the effects of  
> disturbances”.
> 
> I also got rid of the term “pre-selected” because this also makes theoretical assumptions about what is going on inside the system doing the controlling. Of course the “variable” referred to in the above definition is a perception but then everything is a perception so it’s really unnecessary to mention it.
> 
> > 
> 
> > BH: So as I see it, control is initially not “protecting” act but  
> > **cancelling act** of output which  
> > compensate effects of disturbances so to maintain perceptual stability (some  
> > preselected perceptual state) in the controlling system.
> 
> RM: That’s fine. The verbal description matters less to me than the functional model that actually makes control work. I think that control can be correctly described as involving “cancelling” the effect of disturbances or “protecting” a controlled variable from the effects of disturbance. But if “protecting” doesn’t work for you then feel free not to use it.
> 
> Best
> 
> Rick
> 
> > This could mean that  
> > “controlled variable” is already affected and new state perceived and  
> > controlled in comparator and act of canceling the effect or compensating the  
> > efect of disturbances realized.
> 
> > To use term “protect” is something that means for me to act  
> > in advance, so to “prevent” some “controlled variable” or  
> > better predefined state of controlling system, from being disturbed, displaced,  
> > affected…etc.
> 
> > Term “protecting” is probably kind of control but not in  
> > initial sense. By my oppinion is one of consequences of “pure” PCT  
> > control, which is by definition used with terms canceling, compensating,  
> > etc…
> 
> > 
> 
> > Something similar was Ashby’s “control” definition :  
> > “Every stable system has the property that if displaced from a state of  
> > equilibrium and released, the subsequent movement is so matched to the initial  
> > displacement that the system is brought back to the state of equilibrium”  
> > (Ashby, 1960).
> 
> > 
> 
> > I think that Ashby used “compensation” for description of  
> > “control”. And it seems to me that he used terms to describe actual  
> > “displacement” and actual “compensation” not something  
> > happening in advance.
> 
> > He tried to give also definitions of dynamic system, variable and  
> > system, specifications of behaviour, “stability”,  
> > “equilibrium”, “steady-state”, and so on, as I think that  
> > Bill used some of this terms in Appendix to the book B:CP, 2005.
> 
> > 
> 
> > So the main point I see, is that whatever “controlled  
> > variable” is meant, is first “moved” (perceived displacement)  
> > from the predefined, initial state (reference state, equilibirum…) and  
> > than by canceling or compensating or opposing effects of action or whatever we  
> > call that (maybe behavior), again brought back to predefined, initial state  
> > (reference, equilibrium, whatever…).
> 
> > 
> 
> > If we say that “controlled variable” is protected than you  
> > probably assume that “controlled variable” was not disturbed yet, as  
> > disturbances has already been cancelled, compensated by control system in  
> > advance. But that can never happen if control system “has no  
> > experiences” with “moving controlled variable” from initial  
> > state with certain disturbances. How else could control system  
> > “choose” disturbances to act on, so to protect “controlled  
> > variable” from being affected if control system doesn’t know what kind of  
> > effect distrubances have on “controlled variable” and system itself.
> 
> > 
> 
> > I think that the selection of disturbances that could have effect on  
> > controlled variable, are those which in the past show tendency to  
> > “displace controlled variable” from initial state. So when control  
> > system has that “experience” than it can probably reorganize so to  
> > “protect” it from disturbances. But by my oppinion it has to be  
> > complex control system, build up with many control units, what could happen  
> > through evolution.
> 
> > 
> 
> > In such a complex organized control systems (more organized control  
> > units), certain control units serve the goal to really “prevent” or  
> > “protect” certain “controlled variable” from being  
> > disturbed, displaced or “moved” from initial state like in some  
> > physiological cases.
> 
> > 
> 
> > So I think it’s better to use initial terms when making defintion about  
> > control in PCT sense such as cancelling, compensating activity not  
> > “protecting” activity.
> 
> > 
> 
> > Your “exclusive” statement about Gordon…. :
> 
> > RM : “Well, Mr. Douglas  
> > is now off my list of people to listen to about control theory. This lecture  
> > was awful”.
> 
> > 
> 
> > …has no sense to me.
> 
> > 
> 
> > By my oppinion Douglas tried methodically to show how control in  
> > different dynamic system works. And by my opinion his retorics and pictures  
> > about control is better then yours about “protecting controlled  
> > variables”. But both are insufficient (as probably mine is), but that  
> > doesn’t mean that we have to stop talking about your and his presentation of  
> > control, because they are “awfull”. I think that both are good as the  
> > bases to improve them, so they would show better how control theory works.
> 
> > 
> 
> > Rick, you are doing a good job, moderating on CSGnet. But nobody said  
> > that you couldn’t be better J
> 
> > 
> 
> > I can’t comment other discussions for the time being as I didn’t read  
> > them. Maybe I missed something important. I also didn’t entirely follow the  
> > discussion about B:CP (2005) so I’m interested if you make any comments about  
> > Appendix in the book. There are some interesting clarifications of terms  
> > control, stability…
> 
> > 
> 
> > Best,
> 
> > 
> 
> > Boris
> 
> > 
> 
> > 
> 
> > * * *
> 
> > **From:** Control Systems Group  
> > Network (CSGnet) [[mailto:CSGNET@LISTSERV.ILLINOIS.EDU](mailto:CSGNET@LISTSERV.ILLINOIS.EDU)] **On Behalf Of** Richard Marken  
> > **Sent:** Saturday, December 21, 2013  
> > 2:38 AM  
> > **To:** CSGNET@LISTSERV.ILLINOIS.EDU  
> > **Subject:** Re: Classical Control  
> > Systems Lectures
> 
> > 
> 
> > [From Rick Marken  
> > (2013.12.20.1740)]
> 
> > 
> 
> > > Martin Taylor  
> > > (2013.12.20.11.47)–
> 
> > > > RM: Stability and control are two different _phenomena_, something that  
> > > > it is now clear to me that Mr. Douglas is completely unaware of. These two  
> > > > different phenomena are produced by two different kinds of systems; stability  
> > > > is a kind of behavior exhibited by certain “open-loop” or what I call  
> > > > “causal” systems.
> 
> > >
> 
> > > I still haven’t looked at the videos, but it strikes me that you are  
> > > doing the old philosophers’ trick of taking a word that has a variety of  
> > > meanings, and using a meaning appropriate in one context as though that were  
> > > the meaning appropriate to a different context. Yes, “stability” and  
> > > “control” are indeed different phenomena, in the same way  
> > > “food” and “vegetable” are different concepts.
> 
> > 
> 
> > RM:  
> > No,“stability” and “control”, as described by Douglas in  
> > the very first control lecture, are different phenomena in the way  
> > “food” and “poison” are different. A stable system  
> > (according to the lecture) is one that returns to its original  
> > (“equilibrium”) state after a transient disturbance; a control system  
> > is is one that remains in a reference state during continuous disturbance.  
> > Douglas should have talked about variables rather than systems but you get the  
> > idea.
> 
> > >
> 
> > > MT: All feedback loops, in fact all dynamical systems, whether control  
> > > systems or not, have stability criteria. Either they are stable or not. Some  
> > > are more stable than others.
> 
> > 
> 
> > RM: Yes, they do. Indeed, I measure control in terms of stability  
> > (observed/expected variance of the variable). In this case  
> > “stability” is simply referring to a measure of the observed  
> > variations in a variable and it can be used to measure the variability of a  
> > controlled variable or an uncontrolled variable (like the variable position of  
> > the ball in the bowl that Douglas refers to as a stable system).
> 
> > 
> 
> > > MT: Some are metastable,  
> > > meaning they will maintain their current values until something momentarily  
> > > disturbs one of their signal values. Some of those will continue to diverge  
> > > from the original metastable value after the disturbance, some will just  
> > > maintain the disturbed set of signal values without further change. Some are  
> > > absolutely stable, meaning that after any kind of momentary disturbance they  
> > > will return their values to their original levels. Most real systems don’t do  
> > > that, and are stable only if the momentary disturbance doesn’t exceed some  
> > > limit.
> > > 
> > > The key concept is the “orbit”. All systems that can be described by  
> > > a vector of variables have a state. Their state is the vector of current  
> > > variable values together with the rates of change of the current variables.  
> > > That includes control systems, ball-in-a-bowl systems, the synapse strengths of  
> > > networks of millions of neurons, etc. etc. If the system is at some location in  
> > > the state space and is not further disturbed from outside, it will follow some  
> > > track through the state space. That track is an orbit, and there is only one  
> > > orbit through any point in the state space.
> > > 
> > > A stable system is one for which the orbit will converge to some track that is  
> > > the same for all the initial locations in the state space. That track is called  
> > > an “attractor”. The attractor may be a fixed point, a closed path  
> > > (which represents a stable oscillator) or a “strange attractor”  
> > > (which I won’t explain now). An unstable system is one for which the  
> > > orbits diverge. Here are a couple of examples of attractors, or at least the  
> > > projections of them into two dimensions, because even in 2-D, the orbit is  
> > > actually in a space of four dimensions, two for location and two for velocity.  
> > > I have omitted the velocity coordinates in these examples, and in the  
> > > fixed-point example it is the velocity that distinguishes the orbits where two  
> > > of them cross in the figure. In the 4-D state space, only one orbit passes  
> > > through any particular point.
> 
> > 
> 
> > RM: All of this simply describes the observed behavior of a variable.  
> > Nothing about the shape of these orbits can tell you whether the variable is  
> > controlled or not.
> 
> > > MT: A control system is one for which the attractor converges in at  
> > > least one dimension (the perception-value dimension), but that’s not the main  
> > > criterion for differentiating the “ball-in-the-bowl” from a trivial  
> > > control loop.
> 
> > 
> 
> > RM: That’s not only is not a “main” criterion; it’s not a  
> > criterion at all. The only criterion for distinguishing the  
> > “ball-in-the-bowl” from a controlled ball in the bowl is the criterion  
> > John Kirkland  
> > just mentioned: The criterion of The Test for the Controlled Variable, which is  
> > whether there is less of an effect of a disturbance on the controlled variable  
> > than expected. You simply cannot tell, by looking at just the observed behavior  
> > of the “ball-in-the-bowl” (like the “fixed point” and  
> > “stable oscillator” orbits pictured above) whether you are observing  
> > the behavior of a controlled or uncontrolled variable. The orbits plotted above  
> > could be the behavior of a controlled or uncontrolled ball. This is exactly  
> > analogous to the situation in my mindreading demo ([http://www.mindreadings.com/ControlDemo/Mindread.html](http://www.mindreadings.com/ControlDemo/Mindread.html)). When  
> > you move one avatar around the screen in a controlled manner, the other two  
> > move as well; you can’t tell from the movements (orbits) of the avatars, which  
> > is controlled and which are not. In order to determine control you have to  
> > disturb the position of the avatars and see which avatar is affected _least_ by  
> > the disturbance.
> 
> > 
> 
> > > MT: Bruce Abbott  
> > > put his finger on it when he pointed out that the ball-in-the-bowl uses the  
> > > energy supplied by the disturbance to return the ball to its fixed point,  
> > > whereas the control loop uses an independent energy supply to oppose the effect  
> > > of the disturbance on one (and only one) of the variables in the state space of  
> > > the loop. The manner in which control is established is irrelevant.
> 
> > 
> 
> > RM: This is a description of models that produce the observed  
> > behaviors: the open loop physics model for the “ball-in-the-bowl”;  
> > closed-loop control for the controlled ball. The manner in which control  
> > is established may be irrelevant (I have no idea what that means actually; the  
> > only way I know of to establish that control is happening is by using the Test)  
> > but one has to have established that control is going on in one case and that  
> > it’s not going on in the other in order to apply the correct explanations  
> > (models) to each case.
> 
> > 
> 
> > > MT: It so happens that the PCT definition of control is the maintenance  
> > > of one particular value among the many different signal values in a negative  
> > > feedback loop, so Rick’s comment “Control is produced _only_ by negative  
> > > feedback control systems” is a tautology.
> 
> > 
> 
> > RM: Actually, that’s not the PCT definition of control. The  
> > definition of control is “maintenance of a variable in a pre-selected  
> > state, protected from disturbance”. A negative feedback loop is a model of  
> > how control works. It’s not a tautology.
> 
> > >
> 
> > > > MT: Control is produced _only_ by negative feedback control systems. So  
> > > > I couldn’t disagree with you more when you say “we need to be sure that  
> > > > stability and control are not seen as belonging to different kinds of  
> > > > systems”. In fact, we need to be VERY sure that we understand that  
> > > > stability and control “belong” to two very different systems:  
> > > > open-loop, causal systems for the former and closed loop negative  
> > > > feedback systems for the latter. [MT: “you” here is Bruce Abbott.]
> 
> > RM: So let me get this straight. Are you saying that the  
> > “stability” of the behavior of the “ball-in-the-bowl” is  
> > the same as the “stability” of the behavior of, say, the water level  
> > in Ktesibios’ water clock?
> 
> > 
> 
> > Best
> 
> > Rick
> 
> > 
> 
> > > >
> 
> > > >
> 
> > > To which I can only say  
> > > that there are several applicable proverbs along the lines that one is better  
> > > advised to listen and learn rather than to guess and pontificate. Bruce is  
> > > quite right to say “we need to be sure that stability and control are not  
> > > seen as belonging to different kinds of systems”. To contradict Bruce is  
> > > to say something as nonsensical as “we need to be sure that leafiness and  
> > > trees are not seen as belonging to different kinds of objects”.  
> > > “Stability” applies to all kinds of dynamical systems, which control  
> > > systems are.
> > > 
> > > Martin
> 
> > –
> 
> > Richard S. Marken  
> > PhD  
> > [www.mindreadings.com](http://www.mindreadings.com)
> 
> > 
> 
> > The only thing that will redeem mankind is cooperation.
> > 
> > ```
> > -- Bertrand Russell
> > 
> > ```
> 
> > No virus found in this message.
> 
> > Checked by AVG - [www.avg.com](http://www.avg.com)
> 
> > Version: 2014.0.4259 / Virus Database: 3658/6937 - Release Date: 12/20/13
> 
> > * * *
> 
> > No virus found in this message.
> > 
> > Checked by AVG - [www.avg.com](http://www.avg.com)
> 
> > Version: 2014.0.4259 / Virus Database: 3658/6937 - Release Date: 12/20/13
> 
> –  
> Richard S. Marken PhD  
> [www.mindreadings.com](http://www.mindreadings.com)  
> The only thing that will redeem mankind is cooperation.
> 
> ```
> -- Bertrand Russell
> 
> ```

---

<div class="post-metadata">

**Author:** ![\_Boris\_Hartman](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/_/e8c25b/32.png) [@\_Boris\_Hartman](http://discourse.iapct.org/u/_Boris_Hartman)\
**Post date:** [December 25, 2013, 11:55am UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/36 "2013-12-25T11:55:41Z")

</div>

Sory Martin to put my text into your discussion, but it seems to me  
that something odd is going on here.

> **···**
>
> -----Original Message-----  
> From: Control Systems Group Network (CSGnet)  
> [mailto:CSGNET@LISTSERV.ILLINOIS.EDU] On Behalf Of Richard Marken  
> Sent: Saturday, December 21, 2013 11:45 PM  
> To: CSGNET@LISTSERV.ILLINOIS.EDU  
> Subject: Re: Classical Control Systems Lectures
> 
> [From Rick Marken (2013.12.21.1445)]
> 
> > Martin Taylor (2013.12.21.14.43)--
> 
> > > RM: How the behavior comes about is a theoretical question. The prior  
> > > question must be "what kind of behavior is it --control or  
> > > non-control",
> 
> HB :  
> What do you mean by this Rick ? Any behavior is controlled. Organisms  
> (Living Control Systems) are controlling all the time. The prior question is  
> probably "which perception do people control" ? So it's very important "how  
> behavior comes about" to understand that behaviors can't be controlled or  
> uncontrolled.
> 
> Bill's definition of ANY BEHAVIOR : The purpose of any given behavior is to  
> prevent controlled perception from changing away from the reference  
> condition. Purpose implies goal : The goal of ANY BEHAVIOR is defined as the  
> reference condition of the controlled perception" (B:CP, p. 50, 2005)
> 
> If you meant that some behaviors in relation to you seems to be controlled  
> or uncontrolled, I suppose you mean that they can have some special meaning  
> for your control.  
> So from your point of view you are probably classifying other people or  
> living beings Behavior as controlled or non-controlled.
> 
> Any categorization of behaviors on controlled and uncontrolled  
> has some subjective criteria, not objective. If somebody is following you,  
> that means that you are judging the "followers" behavior as controlled in  
> relation to you. But that doesn't mean that behaviors of other people in  
> shared environment are uncontrolled, just because they do not have  
> any effect on your control.
> 
> RM :  
> ..a question that can only be answered using a version of
> 
> > > the Test for the Controlled Variable.
> 
> HB :  
> The problem I see here is not the use of the "test for the controlled  
> variable", but the "test for which perception is controlled".  
> People usually try to "read" the purpose of other people in everyday  
> relationship or in war situation, or in sport (for example boxing, karate,  
> basketball, football.). You really think that they are using the TCV in your  
> scientific sense to determine the purpose of other people ?
> 
> RM :  
> What you can't tell by looking
> 
> > > at the orbits (the visible behavior of a variable alone) is whether  
> > > the variable doing the orbit is a controlled or uncontrolled variable.
> > 
> > MT: Yes, you are getting the pointt.
> > 
> > > RM: I believe that talking about the orbits -- over behavior of a
> 
> possibly
> 
> > > controlled variable -- as a measure of "stability" leads to confusion  
> > > because it implies that the observable behavior of the variable (the  
> > > nature of the orbit) can be used to determine whether or not there is  
> > > control going on.
> 
> > MT: Oh, I thought from the previous paragraph that you were getting it.
> 
> Clearly
> 
> > you aren't. So, how would one be able to do what you suggest?
> 
> RM: Using the test for the controlled variable (TCV). The orbits are  
> one part of the TCV: the observed variations in the state of the  
> hypothetical controlled variable. The other part of the TCV is the  
> concomitant variations in a disturbance. If there is little  
> relationship between temporal variations in the disturbance and the  
> temporal variations in the hypotetical controlled variable (your  
> orbits) then the variable is likely to be under control. If the  
> variations in the hypothetical controlled variable are what are  
> expected (on physical grounds) then the variable is \_not\_ under  
> control.
> 
> > MT: Control is a real phenomenon. In my view, when combined with the
> 
> internal
> 
> > provision of the primary reference values, it is the defining
> 
> characteristic
> 
> > of life, but if you keep saying that it is different from causal
> 
> phenomena,
> 
> > you turn away from PCT all those who believe that even the biological
> 
> world
> 
> > is subject to physical laws.
> 
> RM: Sorry. I thought you understood that when I said "causal" I was  
> talking about the "open-loop" or lineal causal model of the physical  
> sciences (and, unfortunately, of the life sciences as well).
> 
> > MT: To understand PCT, it is NOT necessary to believe in acausality (i.e.
> 
> magic, divine
> 
> > intervention, or something like that).
> 
> RM: Right. You have to understand circular causality in order to understand  
> PCT.
> 
> > MT: PCT is a product of causality, and cannot be understood if you invoke  
> > some distinction between control and causality, where analytically,  
> > experimentally, and philosophically there is none. Without causality there  
> > can be no control.
> 
> RM: I don't believe I've ever said that there was a distinction  
> between control and causality. But perhaps I was not clear. The  
> distinction I make (and that Bill made, of course) is between control  
> and non-control behavior. Non-control behavior is exemplified by the  
> behavior of the ball in the bowl; no control is involved because there  
> is no resistance to disturbance. Such behavior can be accounted for by  
> causal models, by which I mean the lineal causal models of physics.  
> Control behavior is exemplified by the behavior of the distance  
> between target and cursor in a tracking task; control is involved  
> because there is nearly perfect resistance to disturbance. Such  
> behavior can be accounted for only by circular causal models --  
> negative feedback control models.
> 
> The problem with the treatment of control in the Douglas lectures --  
> at least in the two I've seen -- is that he clearly considers behavior  
> like that of the ball at the bottom of a bowl to be an example of  
> control; weak control perhaps, which is why he calls it "stability",  
> but control nevertheless, since he considers control to be evidenced  
> by a variable returning to its original state after a transient  
> disturbance. It is this kind of mistake that has led psychologists to  
> think that they can account for actual control behavior, such as  
> control of limb position, using physical causal models. And this is  
> one big reason for the rejection of PCT by conventional psychologists.  
> Ergo, it upsets me when this kind of thing is presented as worthy of  
> study by students of PCT. Perhaps it is worthy of such study, but as  
> an object lesson in how people -- including control engineers!--have  
> misunderstood what control is and why they have rejected purposeful  
> (control) models of the behavior of living systems.
> 
> Best
> 
> Rick
> 
> --  
> Richard S. Marken PhD  
> www.mindreadings.com
> 
> The only thing that will redeem mankind is cooperation.  
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-- Bertrand Russell
> 
> -----  
> No virus found in this message.  
> Checked by AVG - www.avg.com  
> Version: 2014.0.4259 / Virus Database: 3658/6939 - Release Date: 12/21/13

---

<div class="post-metadata">

**Author:** ![\_Boris\_Hartman](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/_/e8c25b/32.png) [@\_Boris\_Hartman](http://discourse.iapct.org/u/_Boris_Hartman)\
**Post date:** [December 25, 2013, 12:42pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/37 "2013-12-25T12:42:22Z")

</div>

Well Rick, I must admitt I am worried…where this “PCT  
boat” is goimg…

[From Rick Marken (2013.12.22.1110)]

![Attractors.jpg](http://discourse.iapct.org/uploads/default/original/1X/f95899be1fc710df351ee2dac6d13c29c52c1a91.jpeg)

> **···**
>
> On Sun, Dec 22, 2013 at 2:53 AM, Boris Hartman [boris.hartman@masicom.net](mailto:boris.hartman@masicom.net) wrote:
> 
> HB:
> 
> TO PUT IT EXACTLY IN PCT SENSE, CONTROL IS : “Achievement  
> and maintanance of a preselected perceptual state in the controlling system,  
> through actions on the environment that also cancel the effects of  
> disturbances” (B:CP, 2005).
> 
> RM:
> 
> Yes, that’s a good one too;-) It’s tough to give a definition of  
> control that is completely theory free and I don’t think Bill was trying to do  
> that when he came up with this definition. Indeed, part of Bill’s goal in  
> giving this definition of control was to include the important new assumptions of  
> what has come to be called PCT. I don’t think it was until many years after the  
> publication of B:CP that he actively promoted the idea that control was a  
> phenomenon in and of itsef – an objective _fact_ (as in the subtitle to LCS  
> III) – that is explained by the theory of control: control theory.
> 
> HB :
> 
> I’m wondering if you become an “officiall interpreter” of  
> Bill’s knowledge ? J. I thought you are just moderating. And from one  
> subtitle you concluded that “control” is “objective” fact.
> 
> The “fact” in subtitle could also mean that there are so many  
> evidence and models that we can conclude the generality of control in living  
> beings. If you think that Bill showed for “objectivity” in external  
> environment, you’ll with no doubt find some evidence in the book LCS III and  
> show me that you are right.
> 
> It seems to me, that you are trying to reduce PCT to some special case  
> of PCT. But in this way PCT will fall apart like “castle from cards”.  
> Beleive me. Bill was smart guy, knowing what he was doing, and why he wrote  
> definitions as they are. He kept generality of PCT model. And you are not.  
> Control units can be used in many ways, not just for “protecting” the  
> “controlled” variable from disturbances. Think of it.
> 
> Control “being a fact” is by my oppinion also contradiciting  
> your previous statement that “control” is phenomenon, so it’s by my  
> research in vocabulary, entirely dependent from perception. And perception can  
> never be “objective”. Maybe you had in mind some other meaning of  
> “phenomenon”.
> 
> There is no “objective fact” on itself, because you have to  
> prove that you somehow directly access to “reality” (outer  
> environment) and know it in every detail. But still humans are knowing about  
> “reality” only through their perception (some transformations) of  
> “reality”. And perception is not only limited, but it’s also just  
> partly presenting the “reality” or outside environment (as much input  
> functions you have) and input gain. Mostly enough for good control, but  
> sometimes is not enough. Accidents happens.
> 
> And more perceptions of the same “reality” more “objective  
> fact”. “Reality” is never mirrored into your consciousnes to be  
> “objective”. Whatever you are perceiving is just a “model”,  
> “perceptual construction”, “abstract system”, whatever you  
> call that what you perceive.
> 
> RM :
> 
> I think the definition above can be “objectified:” a bit by changing  
> “perceptual state in a controlling system” to “variable”.  
> The idea that it is a perceptual variable that is controlled is really part of  
> the theory – a very important part of the theory but part of the theory  
> nevertheless.
> 
> HB :
> 
> “Objectifying” Bill’s definition with “variable” is  
> not good idea. As I said before. I’m pretty sure that Bill knew what he was  
> doing. But I’m not sure that you do. I think that’s why his definitions and generic  
> diagram survived so much time, and I beleive it will survive much more time in  
> future, if you will not change or modify it. Putting the controlled “variable”  
> into Bill’s definition and consequently into “functional” diagram  
> could by my oppinion destroy it’s generality. It could be meant as just one  
> special case of PCT.
> 
> RM :
> 
> But one can observe control without making any assumptions about _how_ control  
> works. We can do this by observing a variable, such as the distance between  
> cursor and target in a tracking task, and noting that disturbances have little  
> or none of their expected effect on this variable and that this is because the  
> effects of these disturbances are being cancelled by observed actions (mouse  
> movements).
> 
> HB :
> 
> I thought that you are “protecting” the “controlled”  
> variable form distrubances not canceling the effects of disturbances.
> 
> Why did you use term cancel instead of term “protecting”, if  
> they have the same meaning by your oppinion ? And sorry I didn’t get it, which  
> is “controlled” variable in this case that you are “protecting”  
> it from disturbances ?
> 
> RM :
> 
> So control is happening because we are seeing the:
> 
> “Achievement and maintenance of a variable in a particular state  
> through actions on the environment that also cancel the effects of disturbances”.
> 
> HB :
> 
> So you are also “officialy” promoted to change Bill’s definitions.  
> To me it’s obviously that you shouldn’t do that. Now as you changed definition,  
> you’ll have to change also his generic diagram and put the “controlled  
> variable” into outer environment. Well I’m wondering, are you going to  
> change whole his theory? Into what, RCT ?
> 
> RM :
> 
> I also got rid of the term “pre-selected” because  
> this also makes theoretical assumptions about what is going on inside the  
> system doing the controlling.
> 
> HB :
> 
> You got rid of Bill’s term in his definition (???). I’m really  
> wondering who authorised you to do that ?
> 
> RM :
> 
> Of course the “variable” referred to in the above  
> definition is a perception but then everything is a perception so it’s really  
> unnecessary to mention it.
> 
> HB :
> 
> Better. Nothing is “objective”. All is just perception. Variables  
> are just perceptual characteristics. But in your case I see it necesary to  
> mention it, as you are mixing “objective facts” with perceived states  
> of variables.
> 
> And I’m asking you once again that you use sysmbols HB not BH. As I  
> will thought that you are reffering to Bob Hintz.
> 
> BH: So as I see it, control is initially not “protecting” act  
> but **cancelling act** of output which  
> compensate effects of disturbances so to maintain perceptual stability (some  
> preselected perceptual state) in the controlling system.
> 
> RM: That’s fine. The  
> verbal description matters less to me than the functional model that actually  
> makes control work.
> 
> HB :
> 
> Well, I’m glad that you are satisfied with my verbal descriptions. But  
> I still think that verbal description are mostly closely related to “functional  
> models” in our heads. And I’m pretty interested if you can show me your  
> "functional« model (not adapted Bill’s), that will show how control  
> unit is “protecting” controlled variable in outer environment. I’d  
> just like to see how much your verbal description is not important to you and  
> how “actualy makes control work”.
> 
> RM :
> 
> I think that control can be correctly described as involving  
> “cancelling” the effect of disturbances or “protecting” a  
> controlled variable from the effects of disturbance. But if  
> “protecting” doesn’t work for you then feel free not to use it.
> 
> HB:
> 
> This one is very »foggy« and »slippy«  
> conclusion. I think that you shouldn’t use this “equatation”,  
> specially not in Bill’s defintions. But I think you could use it in some of your  
> theories.
> 
> But to test rightness of your “equality”, I’d be glad if you  
> show us how examples about PCT that Bill used with his favourite terms work. So  
> please show us how you would verbalize his examples with term “protection”.  
> Maybe something like this :
> 
> 1. 
> 
> ```
> driving
> 
> ```
> 
> control (“protection” of speed and position on the road),
> 
> 1. 
> 
> ```
> tracking
> 
> ```
> 
> experiment (“protection” of position of cursor)….
> 
> 1. 
> 
> ```
> maybe
> 
> ```
> 
> you’ll remember some more.
> 
> As always maybe I misunderstood something…sorry…
> 
> Best
> 
> Boris
> 
> * * *
> 
> **From:** Control Systems Group  
> Network (CSGnet) [[mailto:CSGNET@LISTSERV.ILLINOIS.EDU](mailto:CSGNET@LISTSERV.ILLINOIS.EDU)] **On Behalf Of** Richard Marken  
> **Sent:** Sunday, December 22, 2013  
> 8:12 PM  
> **To:** CSGNET@LISTSERV.ILLINOIS.EDU  
> **Subject:** Re: Classical Control  
> Systems Lectures
> 
> [From Rick Marken (2013.12.22.1110)]
> 
> On Sun, Dec 22, 2013 at 2:53 AM, Boris Hartman [boris.hartman@masicom.net](mailto:boris.hartman@masicom.net) wrote:
> 
> > BH: TO PUT IT  
> > EXACTLY IN PCT SENSE, CONTROL IS : “Achievement and maintanance of a  
> > preselected perceptual state in the controlling system, through actions on the  
> > environment that also cancel the effects of disturbances” (B:CP, 2005).
> 
> RM: Yes, that’s a good  
> one too;-) It’s tough to give a definition of control that is completely theory  
> free and I don’t think Bill was trying to do that when he came up with this  
> definition. Indeed, part of Bill’s goal in giving this definition of control  
> was to include the important new assumptions of what has come to be called PCT.  
> I don’t think it was until many years after the publication of B:CP that he  
> actively promoted the idea that control was a phenomenon in and of itsef – an  
> objective _fact_ (as in the subtitle to LCS III) – that is explained by the  
> theory of control: control theory.
> 
> I think the definition above can be “objectified:” a bit by changing  
> “perceptual state in a controlling system” to “variable”.  
> The idea that it is a perceptual variable that is controlled is really part of  
> the theory – a very important part of the theory but part of the theory  
> nevertheless.
> 
> But one can observe control without making any assumptions about _how_ control  
> works. We can do this by observing a variable, such as the distance between  
> cursor and target in a tracking task, and noting that disturbances have little  
> or none of their expected effect on this variable and that this is because the  
> effects of these disturbances are being cancelled by observed actions (mouse  
> movements). So control is happening because we are seeing the:
> 
> “Achievement and maintenance of a variable in a particular state  
> through actions on the environment that also cancel the effects of  
> disturbances”.
> 
> I also got rid of the term “pre-selected” because  
> this also makes theoretical assumptions about what is going on inside the  
> system doing the controlling. Of course the “variable” referred to in  
> the above definition is a perception but then everything is a perception so  
> it’s really unnecessary to mention it.
> 
> > BH: So as I see it, control is initially not “protecting” act  
> > but **cancelling act** of output which  
> > compensate effects of disturbances so to maintain perceptual stability (some  
> > preselected perceptual state) in the controlling system.
> 
> RM: That’s fine. The  
> verbal description matters less to me than the functional model that actually  
> makes control work. I think that control can be correctly described as  
> involving “cancelling” the effect of disturbances or  
> “protecting” a controlled variable from the effects of disturbance.  
> But if “protecting” doesn’t work for you then feel free not to use  
> it.
> 
> Best
> 
> Rick
> 
> > This could mean that “controlled variable” is already  
> > affected and new state perceived and controlled in comparator and act of  
> > canceling the effect or compensating the efect of disturbances realized.
> 
> > To use term “protect”  
> > is something that means for me to act in advance, so to “prevent”  
> > some “controlled variable” or better predefined state of controlling  
> > system, from being disturbed, displaced, affected…etc.
> 
> > Term  
> > “protecting” is probably kind of control but not in initial sense. By  
> > my oppinion is one of consequences of “pure” PCT control, which is by  
> > definition used with terms canceling, compensating, etc…
> 
> > Something similar  
> > was Ashby’s “control” definition : “Every stable system has the  
> > property that if displaced from a state of equilibrium and released, the  
> > subsequent movement is so matched to the initial displacement that the system  
> > is brought back to the state of equilibrium” (Ashby, 1960).
> 
> > I think that Ashby  
> > used “compensation” for description of “control”. And it  
> > seems to me that he used terms to describe actual “displacement” and  
> > actual “compensation” not something happening in advance.
> 
> > He tried to give  
> > also definitions of dynamic system, variable and system, specifications of  
> > behaviour, “stability”, “equilibrium”,  
> > “steady-state”, and so on, as I think that Bill used some of this  
> > terms in Appendix to the book B:CP, 2005.
> 
> > So the main point  
> > I see, is that whatever “controlled variable” is meant, is first  
> > “moved” (perceived displacement) from the predefined, initial state  
> > (reference state, equilibirum…) and than by canceling or compensating or  
> > opposing effects of action or whatever we call that (maybe behavior), again  
> > brought back to predefined, initial state (reference, equilibrium, whatever…).
> 
> > If we say that  
> > “controlled variable” is protected than you probably assume that  
> > “controlled variable” was not disturbed yet, as disturbances has  
> > already been cancelled, compensated by control system in advance. But that can  
> > never happen if control system “has no experiences” with “moving  
> > controlled variable” from initial state with certain disturbances. How  
> > else could control system “choose” disturbances to act on, so to  
> > protect “controlled variable” from being affected if control system  
> > doesn’t know what kind of effect distrubances have on “controlled  
> > variable” and system itself.
> 
> > I think that the  
> > selection of disturbances that could have effect on controlled variable, are  
> > those which in the past show tendency to “displace controlled  
> > variable” from initial state. So when control system has that  
> > “experience” than it can probably reorganize so to  
> > “protect” it from disturbances. But by my oppinion it has to be  
> > complex control system, build up with many control units, what could happen  
> > through evolution.
> 
> > In such a complex  
> > organized control systems (more organized control units), certain control units  
> > serve the goal to really “prevent” or “protect” certain  
> > “controlled variable” from being disturbed, displaced or  
> > “moved” from initial state like in some physiological cases.
> 
> > So I think it’s  
> > better to use initial terms when making defintion about control in PCT sense  
> > such as cancelling, compensating activity not “protecting” activity.
> 
> > Your  
> > “exclusive” statement about Gordon…. :
> 
> > RM : “Well,  
> > Mr. Douglas is now off my list of people to  
> > listen to about control theory. This lecture was awful”.
> 
> > …has no  
> > sense to me.
> 
> > By my oppinion  
> > Douglas tried methodically to show how control in different dynamic system  
> > works. And by my opinion his retorics and pictures about control is better then  
> > yours about “protecting controlled variables”. But both are  
> > insufficient (as probably mine is), but that doesn’t mean that we have to stop  
> > talking about your and his presentation of control, because they are  
> > “awfull”. I think that both are good as the bases to improve them, so  
> > they would show better how control theory works.
> 
> > Rick, you are  
> > doing a good job, moderating on CSGnet. But nobody said that you couldn’t be  
> > better J
> 
> > I can’t comment  
> > other discussions for the time being as I didn’t read them. Maybe I missed  
> > something important. I also didn’t entirely follow the discussion about B:CP  
> > (2005) so I’m interested if you make any comments about Appendix in the book.  
> > There are some interesting clarifications of terms control, stability…
> 
> > Best,
> 
> > Boris
> 
> > 
> 
> > 
> 
> > * * *
> 
> > **From:** Control Systems Group  
> > Network (CSGnet) [[mailto:CSGNET@LISTSERV.ILLINOIS.EDU](mailto:CSGNET@LISTSERV.ILLINOIS.EDU)]  
> > **On Behalf Of** Richard Marken  
> > **Sent:** Saturday, December 21, 2013  
> > 2:38 AM  
> > **To:** CSGNET@LISTSERV.ILLINOIS.EDU  
> > **Subject:** Re: Classical Control  
> > Systems Lectures
> 
> > [From Rick Marken (2013.12.20.1740)]
> 
> > > Martin Taylor  
> > > (2013.12.20.11.47)–
> 
> > > > RM: Stability and  
> > > > control are two different _phenomena_, something that it is now clear to me  
> > > > that Mr. Douglas is completely unaware of. These two different phenomena are  
> > > > produced by two different kinds of systems; stability is a kind of behavior  
> > > > exhibited by certain “open-loop” or what I call “causal”  
> > > > systems.
> 
> > > I still haven’t  
> > > looked at the videos, but it strikes me that you are doing the old  
> > > philosophers’ trick of taking a word that has a variety of meanings, and using  
> > > a meaning appropriate in one context as though that were the meaning  
> > > appropriate to a different context. Yes, “stability” and  
> > > “control” are indeed different phenomena, in the same way  
> > > “food” and “vegetable” are different concepts.
> 
> > RM:  
> > No,“stability” and “control”, as described by Douglas in  
> > the very first control lecture, are different phenomena in the way  
> > “food” and “poison” are different. A stable system  
> > (according to the lecture) is one that returns to its original  
> > (“equilibrium”) state after a transient disturbance; a control system  
> > is is one that remains in a reference state during continuous disturbance.  
> > Douglas should have talked about variables rather than systems but you get the  
> > idea.
> 
> > > MT: All feedback  
> > > loops, in fact all dynamical systems, whether control systems or not, have  
> > > stability criteria. Either they are stable or not. Some are more stable than  
> > > others.
> 
> > RM: Yes, they do.  
> > Indeed, I measure control in terms of stability (observed/expected variance of  
> > the variable). In this case “stability” is simply referring to a  
> > measure of the observed variations in a variable and it can be used to measure  
> > the variability of a controlled variable or an uncontrolled variable (like the  
> > variable position of the ball in the bowl that Douglas refers to as a stable  
> > system).
> 
> > > MT: Some are  
> > > metastable, meaning they will maintain their current values until something  
> > > momentarily disturbs one of their signal values. Some of those will continue to  
> > > diverge from the original metastable value after the disturbance, some will  
> > > just maintain the disturbed set of signal values without further change. Some  
> > > are absolutely stable, meaning that after any kind of momentary disturbance  
> > > they will return their values to their original levels. Most real systems don’t  
> > > do that, and are stable only if the momentary disturbance doesn’t exceed some  
> > > limit.
> > > 
> > > The key concept is the “orbit”. All systems that can be described by  
> > > a vector of variables have a state. Their state is the vector of current  
> > > variable values together with the rates of change of the current variables.  
> > > That includes control systems, ball-in-a-bowl systems, the synapse strengths of  
> > > networks of millions of neurons, etc. etc. If the system is at some location in  
> > > the state space and is not further disturbed from outside, it will follow some  
> > > track through the state space. That track is an orbit, and there is only one  
> > > orbit through any point in the state space.
> > > 
> > > A stable system is one for which the orbit will converge to some track that is  
> > > the same for all the initial locations in the state space. That track is called  
> > > an “attractor”. The attractor may be a fixed point, a closed path  
> > > (which represents a stable oscillator) or a “strange attractor”  
> > > (which I won’t explain now). An unstable system is one for which the  
> > > orbits diverge. Here are a couple of examples of attractors, or at least the  
> > > projections of them into two dimensions, because even in 2-D, the orbit is  
> > > actually in a space of four dimensions, two for location and two for velocity.  
> > > I have omitted the velocity coordinates in these examples, and in the  
> > > fixed-point example it is the velocity that distinguishes the orbits where two  
> > > of them cross in the figure. In the 4-D state space, only one orbit passes  
> > > through any particular point.
> > > 
> > > ![]()
> 
> > RM: All of this  
> > simply describes the observed behavior of a variable. Nothing about the shape  
> > of these orbits can tell you whether the variable is controlled or not.
> 
> > > MT: A control  
> > > system is one for which the attractor converges in at least one dimension (the  
> > > perception-value dimension), but that’s not the main criterion for  
> > > differentiating the “ball-in-the-bowl” from a trivial control loop.
> 
> > RM: That’s not  
> > only is not a “main” criterion; it’s not a criterion at all. The only  
> > criterion for distinguishing the “ball-in-the-bowl” from a controlled  
> > ball in the bowl is the criterion John Kirkland just mentioned: The criterion of The  
> > Test for the Controlled Variable, which is whether there is less of an effect  
> > of a disturbance on the controlled variable than expected. You simply cannot  
> > tell, by looking at just the observed behavior of the  
> > “ball-in-the-bowl” (like the “fixed point” and “stable  
> > oscillator” orbits pictured above) whether you are observing the behavior  
> > of a controlled or uncontrolled variable. The orbits plotted above could be the  
> > behavior of a controlled or uncontrolled ball. This is exactly analogous to the  
> > situation in my mindreading  
> > demo ([http://www.mindreadings.com/ControlDemo/Mindread.html](http://www.mindreadings.com/ControlDemo/Mindread.html)). When you move one avatar around the  
> > screen in a controlled manner, the other two move as well; you can’t tell from  
> > the movements (orbits) of the avatars, which is controlled and which are not.  
> > In order to determine control you have to disturb the position of the avatars  
> > and see which avatar is affected _least_ by the disturbance.
> 
> > > MT: Bruce Abbott put his finger  
> > > on it when he pointed out that the ball-in-the-bowl uses the energy supplied by  
> > > the disturbance to return the ball to its fixed point, whereas the control loop  
> > > uses an independent energy supply to oppose the effect of the disturbance on  
> > > one (and only one) of the variables in the state space of the loop. The manner  
> > > in which control is established is irrelevant.
> 
> > RM: This is a  
> > description of models that produce the observed behaviors: the open loop  
> > physics model for the “ball-in-the-bowl”; closed-loop control for the  
> > controlled ball. The manner in which control is established may be  
> > irrelevant (I have no idea what that means actually; the only way I know of to  
> > establish that control is happening is by using the Test) but one has to have  
> > established that control is going on in one case and that it’s not going on in  
> > the other in order to apply the correct explanations (models) to each case.
> 
> > > MT: It so happens  
> > > that the PCT definition of control is the maintenance of one particular value  
> > > among the many different signal values in a negative feedback loop, so Rick’s  
> > > comment “Control is produced _only_ by negative feedback control  
> > > systems” is a tautology.
> 
> > RM:  
> > Actually, that’s not the PCT definition of control. The definition of control  
> > is “maintenance of a variable in a pre-selected state, protected from  
> > disturbance”. A negative feedback loop is a model of how control works. It’s  
> > not a tautology.
> 
> > > > MT: Control is  
> > > > produced _only_ by negative feedback control systems. So I couldn’t disagree  
> > > > with you more when you say “we need to be sure that stability and control  
> > > > are not seen as belonging to different kinds of systems”. In fact, we need  
> > > > to be VERY sure that we understand that stability and control  
> > > > “belong” to two very different systems: open-loop, causal  
> > > > systems for the former and closed loop negative feedback systems for the  
> > > > latter. [MT: “you” here is Bruce Abbott.]
> 
> > RM: So let me get  
> > this straight. Are you saying that the “stability” of the  
> > behavior of the “ball-in-the-bowl” is the same as the  
> > “stability” of the behavior of, say, the water level in Ktesibios’  
> > water clock?
> 
> > Best
> 
> > Rick
> 
> > > To which I can  
> > > only say that there are several applicable proverbs along the lines that one is  
> > > better advised to listen and learn rather than to guess and pontificate. Bruce  
> > > is quite right to say “we need to be sure that stability and control are  
> > > not seen as belonging to different kinds of systems”. To contradict Bruce  
> > > is to say something as nonsensical as “we need to be sure that leafiness  
> > > and trees are not seen as belonging to different kinds of objects”.  
> > > “Stability” applies to all kinds of dynamical systems, which control  
> > > systems are.
> > > 
> > > Martin
> 
> > –
> 
> > Richard S. Marken PhD  
> > [www.mindreadings.com](http://www.mindreadings.com)
> 
> > The only thing  
> > that will redeem mankind is cooperation.
> > 
> > ```
> > -- Bertrand Russell
> > 
> > ```
> 
> > No virus found in this message.
> 
> > Checked by AVG - [www.avg.com](http://www.avg.com)
> 
> > Version: 2014.0.4259 / Virus Database: 3658/6937 - Release Date:  
> > 12/20/13
> 
> > * * *
> 
> > No virus found in this message.
> > 
> > Checked by AVG - [www.avg.com](http://www.avg.com)
> 
> > Version: 2014.0.4259 / Virus Database: 3658/6937 - Release Date:  
> > 12/20/13
> 
> –
> 
> Richard S. Marken  
> PhD  
> [www.mindreadings.com](http://www.mindreadings.com)
> 
> The only thing that will redeem mankind is cooperation.
> 
> ```
> -- Bertrand Russell
> 
> ```
> 
> No virus found in this message.
> 
> Checked by AVG - [www.avg.com](http://www.avg.com)
> 
> Version: 2014.0.4259 / Virus Database: 3658/6941 - Release Date: 12/22/13

---

<div class="post-metadata">

**Author:** ![FredNickols](http://discourse.iapct.org/user_avatar/discourse.iapct.org/frednickols/32/2645_2.png) [@FredNickols](http://discourse.iapct.org/u/FredNickols)\
**Post date:** [December 25, 2013, 1:32pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/38 "2013-12-25T13:32:45Z")

</div>

[From Fred Nickols (2013.12.24.0828 EST)]

I wouldn’t worry too much about it, Boris. Rick is probably as close to being the crown prince of PCT as anyone but he’s far from infallible. The CSG archives are full of instances wherein Bill corrected Rick’s thinking. Sadly, without Bill, there’s no one to do that anymore. So, Rick will doubtless put forth PCT as he sees it. But if he errs he won’t go unchallenged, just uncorrected. And that’s okay; his grasp of PCT is probably as good as any others out there, just slightly different in some ways, as attested to by the occasional disputes and discussions between he and other equally savvy PCTers.

So relax and have a happy holiday season. The new year should keep us all busy.

Fred Nickols

![Attractors.jpg](http://discourse.iapct.org/uploads/default/original/1X/f95899be1fc710df351ee2dac6d13c29c52c1a91.jpeg)

> **···**
>
> **From:** Boris Hartman [[mailto:boris.hartman@MASICOM.NET](mailto:boris.hartman@MASICOM.NET)]  
> **Sent:** Wednesday, December 25, 2013 7:42 AM  
> **To:** CSGNET@LISTSERV.ILLINOIS.EDU  
> **Subject:** Re: Classical Control Systems Lectures
> 
> Well Rick, I must admitt I am worried…where this “PCT boat” is goimg…
> 
> [From Rick Marken (2013.12.22.1110)]
> 
> On Sun, Dec 22, 2013 at 2:53 AM, Boris Hartman [boris.hartman@masicom.net](mailto:boris.hartman@masicom.net) wrote:
> 
> HB:
> 
> TO PUT IT EXACTLY IN PCT SENSE, CONTROL IS : “Achievement and maintanance of a preselected perceptual state in the controlling system, through actions on the environment that also cancel the effects of disturbances” (B:CP, 2005).
> 
> RM:
> 
> Yes, that’s a good one too;-) It’s tough to give a definition of control that is completely theory free and I don’t think Bill was trying to do that when he came up with this definition. Indeed, part of Bill’s goal in giving this definition of control was to include the important new assumptions of what has come to be called PCT. I don’t think it was until many years after the publication of B:CP that he actively promoted the idea that control was a phenomenon in and of itsef – an objective _fact_ (as in the subtitle to LCS III) – that is explained by the theory of control: control theory.
> 
> HB :
> 
> I’m wondering if you become an “officiall interpreter” of Bill’s knowledge ? J. I thought you are just moderating. And from one subtitle you concluded that “control” is “objective” fact.
> 
> The “fact” in subtitle could also mean that there are so many evidence and models that we can conclude the generality of control in living beings. If you think that Bill showed for “objectivity” in external environment, you’ll with no doubt find some evidence in the book LCS III and show me that you are right.
> 
> It seems to me, that you are trying to reduce PCT to some special case of PCT. But in this way PCT will fall apart like “castle from cards”. Beleive me. Bill was smart guy, knowing what he was doing, and why he wrote definitions as they are. He kept generality of PCT model. And you are not. Control units can be used in many ways, not just for “protecting” the “controlled” variable from disturbances. Think of it.
> 
> Control “being a fact” is by my oppinion also contradiciting your previous statement that “control” is phenomenon, so it’s by my research in vocabulary, entirely dependent from perception. And perception can never be “objective”. Maybe you had in mind some other meaning of “phenomenon”.
> 
> There is no “objective fact” on itself, because you have to prove that you somehow directly access to “reality” (outer environment) and know it in every detail. But still humans are knowing about “reality” only through their perception (some transformations) of “reality”. And perception is not only limited, but it’s also just partly presenting the “reality” or outside environment (as much input functions you have) and input gain. Mostly enough for good control, but sometimes is not enough. Accidents happens.
> 
> And more perceptions of the same “reality” more “objective fact”. “Reality” is never mirrored into your consciousnes to be “objective”. Whatever you are perceiving is just a “model”, “perceptual construction”, “abstract system”, whatever you call that what you perceive.
> 
> RM :  
> I think the definition above can be “objectified:” a bit by changing “perceptual state in a controlling system” to “variable”. The idea that it is a perceptual variable that is controlled is really part of the theory – a very important part of the theory but part of the theory nevertheless.
> 
> HB :
> 
> “Objectifying” Bill’s definition with “variable” is not good idea. As I said before. I’m pretty sure that Bill knew what he was doing. But I’m not sure that you do. I think that’s why his definitions and generic diagram survived so much time, and I beleive it will survive much more time in future, if you will not change or modify it. Putting the controlled “variable” into Bill’s definition and consequently into “functional” diagram could by my oppinion destroy it’s generality. It could be meant as just one special case of PCT.
> 
> RM :  
> But one can observe control without making any assumptions about _how_ control works. We can do this by observing a variable, such as the distance between cursor and target in a tracking task, and noting that disturbances have little or none of their expected effect on this variable and that this is because the effects of these disturbances are being cancelled by observed actions (mouse movements).
> 
> HB :
> 
> I thought that you are “protecting” the “controlled” variable form distrubances not canceling the effects of disturbances.
> 
> Why did you use term cancel instead of term “protecting”, if they have the same meaning by your oppinion ? And sorry I didn’t get it, which is “controlled” variable in this case that you are “protecting” it from disturbances ?
> 
> RM :
> 
> So control is happening because we are seeing the:  
> “Achievement and maintenance of a variable in a particular state through actions on the environment that also cancel the effects of disturbances”.
> 
> HB :
> 
> So you are also “officialy” promoted to change Bill’s definitions. To me it’s obviously that you shouldn’t do that. Now as you changed definition, you’ll have to change also his generic diagram and put the “controlled variable” into outer environment. Well I’m wondering, are you going to change whole his theory? Into what, RCT ?
> 
> RM :
> 
> I also got rid of the term “pre-selected” because this also makes theoretical assumptions about what is going on inside the system doing the controlling.
> 
> HB :
> 
> You got rid of Bill’s term in his definition (???). I’m really wondering who authorised you to do that ?
> 
> RM :
> 
> Of course the “variable” referred to in the above definition is a perception but then everything is a perception so it’s really unnecessary to mention it.
> 
> HB :
> 
> Better. Nothing is “objective”. All is just perception. Variables are just perceptual characteristics. But in your case I see it necesary to mention it, as you are mixing “objective facts” with perceived states of variables.
> 
> And I’m asking you once again that you use sysmbols HB not BH. As I will thought that you are reffering to Bob Hintz.
> 
> BH: So as I see it, control is initially not “protecting” act but **cancelling act** of output which compensate effects of disturbances so to maintain perceptual stability (some preselected perceptual state) in the controlling system.
> 
> RM: That’s fine. The verbal description matters less to me than the functional model that actually makes control work.
> 
> HB :
> 
> Well, I’m glad that you are satisfied with my verbal descriptions. But I still think that verbal description are mostly closely related to “functional models” in our heads. And I’m pretty interested if you can show me your "functional« model (not adapted Bill’s), that will show how control unit is “protecting” controlled variable in outer environment. I’d just like to see how much your verbal description is not important to you and how “actualy makes control work”.
> 
> RM :
> 
> I think that control can be correctly described as involving “cancelling” the effect of disturbances or “protecting” a controlled variable from the effects of disturbance. But if “protecting” doesn’t work for you then feel free not to use it.
> 
> HB:
> 
> This one is very »foggy« and »slippy« conclusion. I think that you shouldn’t use this “equatation”, specially not in Bill’s defintions. But I think you could use it in some of your theories.
> 
> But to test rightness of your “equality”, I’d be glad if you show us how examples about PCT that Bill used with his favourite terms work. So please show us how you would verbalize his examples with term “protection”. Maybe something like this :
> 
> 1. driving control (“protection” of speed and position on the road),
> 
> 2. tracking experiment (“protection” of position of cursor)….
> 
> 3. maybe you’ll remember some more.
> 
> As always maybe I misunderstood something…sorry…
> 
> Best
> 
> Boris
> 
> * * *
> 
> **From:** Control Systems Group Network (CSGnet) [[mailto:CSGNET@LISTSERV.ILLINOIS.EDU](mailto:CSGNET@LISTSERV.ILLINOIS.EDU)] **On Behalf Of** Richard Marken  
> **Sent:** Sunday, December 22, 2013 8:12 PM  
> **To:** CSGNET@LISTSERV.ILLINOIS.EDU  
> **Subject:** Re: Classical Control Systems Lectures
> 
> [From Rick Marken (2013.12.22.1110)]
> 
> On Sun, Dec 22, 2013 at 2:53 AM, Boris Hartman [boris.hartman@masicom.net](mailto:boris.hartman@masicom.net) wrote:
> 
> > BH: TO PUT IT EXACTLY IN PCT SENSE, CONTROL IS : “Achievement and maintanance of a preselected perceptual state in the controlling system, through actions on the environment that also cancel the effects of disturbances” (B:CP, 2005).
> 
> RM: Yes, that’s a good one too;-) It’s tough to give a definition of control that is completely theory free and I don’t think Bill was trying to do that when he came up with this definition. Indeed, part of Bill’s goal in giving this definition of control was to include the important new assumptions of what has come to be called PCT. I don’t think it was until many years after the publication of B:CP that he actively promoted the idea that control was a phenomenon in and of itsef – an objective _fact_ (as in the subtitle to LCS III) – that is explained by the theory of control: control theory.
> 
> I think the definition above can be “objectified:” a bit by changing “perceptual state in a controlling system” to “variable”. The idea that it is a perceptual variable that is controlled is really part of the theory – a very important part of the theory but part of the theory nevertheless.
> 
> But one can observe control without making any assumptions about _how_ control works. We can do this by observing a variable, such as the distance between cursor and target in a tracking task, and noting that disturbances have little or none of their expected effect on this variable and that this is because the effects of these disturbances are being cancelled by observed actions (mouse movements). So control is happening because we are seeing the:
> 
> “Achievement and maintenance of a variable in a particular state through actions on the environment that also cancel the effects of disturbances”.
> 
> I also got rid of the term “pre-selected” because this also makes theoretical assumptions about what is going on inside the system doing the controlling. Of course the “variable” referred to in the above definition is a perception but then everything is a perception so it’s really unnecessary to mention it.
> 
> > BH: So as I see it, control is initially not “protecting” act but **cancelling act** of output which compensate effects of disturbances so to maintain perceptual stability (some preselected perceptual state) in the controlling system.
> 
> RM: That’s fine. The verbal description matters less to me than the functional model that actually makes control work. I think that control can be correctly described as involving “cancelling” the effect of disturbances or “protecting” a controlled variable from the effects of disturbance. But if “protecting” doesn’t work for you then feel free not to use it.
> 
> Best
> 
> Rick
> 
> > This could mean that “controlled variable” is already affected and new state perceived and controlled in comparator and act of canceling the effect or compensating the efect of disturbances realized.
> 
> > To use term “protect” is something that means for me to act in advance, so to “prevent” some “controlled variable” or better predefined state of controlling system, from being disturbed, displaced, affected…etc.
> 
> > Term “protecting” is probably kind of control but not in initial sense. By my oppinion is one of consequences of “pure” PCT control, which is by definition used with terms canceling, compensating, etc…
> 
> > Something similar was Ashby’s “control” definition : “Every stable system has the property that if displaced from a state of equilibrium and released, the subsequent movement is so matched to the initial displacement that the system is brought back to the state of equilibrium” (Ashby, 1960).
> 
> > I think that Ashby used “compensation” for description of “control”. And it seems to me that he used terms to describe actual “displacement” and actual “compensation” not something happening in advance.
> 
> > He tried to give also definitions of dynamic system, variable and system, specifications of behaviour, “stability”, “equilibrium”, “steady-state”, and so on, as I think that Bill used some of this terms in Appendix to the book B:CP, 2005.
> 
> > So the main point I see, is that whatever “controlled variable” is meant, is first “moved” (perceived displacement) from the predefined, initial state (reference state, equilibirum…) and than by canceling or compensating or opposing effects of action or whatever we call that (maybe behavior), again brought back to predefined, initial state (reference, equilibrium, whatever…).
> 
> > If we say that “controlled variable” is protected than you probably assume that “controlled variable” was not disturbed yet, as disturbances has already been cancelled, compensated by control system in advance. But that can never happen if control system “has no experiences” with “moving controlled variable” from initial state with certain disturbances. How else could control system “choose” disturbances to act on, so to protect “controlled variable” from being affected if control system doesn’t know what kind of effect distrubances have on “controlled variable” and system itself.
> 
> > I think that the selection of disturbances that could have effect on controlled variable, are those which in the past show tendency to “displace controlled variable” from initial state. So when control system has that “experience” than it can probably reorganize so to “protect” it from disturbances. But by my oppinion it has to be complex control system, build up with many control units, what could happen through evolution.
> 
> > In such a complex organized control systems (more organized control units), certain control units serve the goal to really “prevent” or “protect” certain “controlled variable” from being disturbed, displaced or “moved” from initial state like in some physiological cases.
> 
> > So I think it’s better to use initial terms when making defintion about control in PCT sense such as cancelling, compensating activity not “protecting” activity.
> 
> > Your “exclusive” statement about Gordon…. :
> 
> > RM : “Well, Mr. Douglas is now off my list of people to listen to about control theory. This lecture was awful”.
> 
> > …has no sense to me.
> 
> > By my oppinion Douglas tried methodically to show how control in different dynamic system works. And by my opinion his retorics and pictures about control is better then yours about “protecting controlled variables”. But both are insufficient (as probably mine is), but that doesn’t mean that we have to stop talking about your and his presentation of control, because they are “awfull”. I think that both are good as the bases to improve them, so they would show better how control theory works.
> 
> > Rick, you are doing a good job, moderating on CSGnet. But nobody said that you couldn’t be better J
> 
> > I can’t comment other discussions for the time being as I didn’t read them. Maybe I missed something important. I also didn’t entirely follow the discussion about B:CP (2005) so I’m interested if you make any comments about Appendix in the book. There are some interesting clarifications of terms control, stability…
> 
> > Best,
> 
> > Boris
> 
> > 
> 
> > 
> 
> > * * *
> 
> > **From:** Control Systems Group Network (CSGnet) [[mailto:CSGNET@LISTSERV.ILLINOIS.EDU](mailto:CSGNET@LISTSERV.ILLINOIS.EDU)] **On Behalf Of** Richard Marken  
> > **Sent:** Saturday, December 21, 2013 2:38 AM  
> > **To:** CSGNET@LISTSERV.ILLINOIS.EDU  
> > **Subject:** Re: Classical Control Systems Lectures
> 
> > [From Rick Marken (2013.12.20.1740)]
> 
> > > Martin Taylor (2013.12.20.11.47)–
> 
> > > > RM: Stability and control are two different _phenomena_, something that it is now clear to me that Mr. Douglas is completely unaware of. These two different phenomena are produced by two different kinds of systems; stability is a kind of behavior exhibited by certain “open-loop” or what I call “causal” systems.
> 
> > > I still haven’t looked at the videos, but it strikes me that you are doing the old philosophers’ trick of taking a word that has a variety of meanings, and using a meaning appropriate in one context as though that were the meaning appropriate to a different context. Yes, “stability” and “control” are indeed different phenomena, in the same way “food” and “vegetable” are different concepts.
> 
> > RM: No,“stability” and “control”, as described by Douglas in the very first control lecture, are different phenomena in the way “food” and “poison” are different. A stable system (according to the lecture) is one that returns to its original (“equilibrium”) state after a transient disturbance; a control system is is one that remains in a reference state during continuous disturbance. Douglas should have talked about variables rather than systems but you get the idea.
> 
> > > MT: All feedback loops, in fact all dynamical systems, whether control systems or not, have stability criteria. Either they are stable or not. Some are more stable than others.
> 
> > RM: Yes, they do. Indeed, I measure control in terms of stability (observed/expected variance of the variable). In this case “stability” is simply referring to a measure of the observed variations in a variable and it can be used to measure the variability of a controlled variable or an uncontrolled variable (like the variable position of the ball in the bowl that Douglas refers to as a stable system).
> 
> > > MT: Some are metastable, meaning they will maintain their current values until something momentarily disturbs one of their signal values. Some of those will continue to diverge from the original metastable value after the disturbance, some will just maintain the disturbed set of signal values without further change. Some are absolutely stable, meaning that after any kind of momentary disturbance they will return their values to their original levels. Most real systems don’t do that, and are stable only if the momentary disturbance doesn’t exceed some limit.
> > > 
> > > The key concept is the “orbit”. All systems that can be described by a vector of variables have a state. Their state is the vector of current variable values together with the rates of change of the current variables. That includes control systems, ball-in-a-bowl systems, the synapse strengths of networks of millions of neurons, etc. etc. If the system is at some location in the state space and is not further disturbed from outside, it will follow some track through the state space. That track is an orbit, and there is only one orbit through any point in the state space.
> > > 
> > > A stable system is one for which the orbit will converge to some track that is the same for all the initial locations in the state space. That track is called an “attractor”. The attractor may be a fixed point, a closed path (which represents a stable oscillator) or a “strange attractor” (which I won’t explain now). An unstable system is one for which the orbits diverge. Here are a couple of examples of attractors, or at least the projections of them into two dimensions, because even in 2-D, the orbit is actually in a space of four dimensions, two for location and two for velocity. I have omitted the velocity coordinates in these examples, and in the fixed-point example it is the velocity that distinguishes the orbits where two of them cross in the figure. In the 4-D state space, only one orbit passes through any particular point.
> > > 
> > > ![]()
> 
> > RM: All of this simply describes the observed behavior of a variable. Nothing about the shape of these orbits can tell you whether the variable is controlled or not.
> 
> > > MT: A control system is one for which the attractor converges in at least one dimension (the perception-value dimension), but that’s not the main criterion for differentiating the “ball-in-the-bowl” from a trivial control loop.
> 
> > RM: That’s not only is not a “main” criterion; it’s not a criterion at all. The only criterion for distinguishing the “ball-in-the-bowl” from a controlled ball in the bowl is the criterion John Kirkland just mentioned: The criterion of The Test for the Controlled Variable, which is whether there is less of an effect of a disturbance on the controlled variable than expected. You simply cannot tell, by looking at just the observed behavior of the “ball-in-the-bowl” (like the “fixed point” and “stable oscillator” orbits pictured above) whether you are observing the behavior of a controlled or uncontrolled variable. The orbits plotted above could be the behavior of a controlled or uncontrolled ball. This is exactly analogous to the situation in my mindreading demo ([http://www.mindreadings.com/ControlDemo/Mindread.html](http://www.mindreadings.com/ControlDemo/Mindread.html)). When you move one avatar around the screen in a controlled manner, the other two move as well; you can’t tell from the movements (orbits) of the avatars, which is controlled and which are not. In order to determine control you have to disturb the position of the avatars and see which avatar is affected _least_ by the disturbance.
> 
> > > MT: Bruce Abbott put his finger on it when he pointed out that the ball-in-the-bowl uses the energy supplied by the disturbance to return the ball to its fixed point, whereas the control loop uses an independent energy supply to oppose the effect of the disturbance on one (and only one) of the variables in the state space of the loop. The manner in which control is established is irrelevant.
> 
> > RM: This is a description of models that produce the observed behaviors: the open loop physics model for the “ball-in-the-bowl”; closed-loop control for the controlled ball. The manner in which control is established may be irrelevant (I have no idea what that means actually; the only way I know of to establish that control is happening is by using the Test) but one has to have established that control is going on in one case and that it’s not going on in the other in order to apply the correct explanations (models) to each case.
> 
> > > MT: It so happens that the PCT definition of control is the maintenance of one particular value among the many different signal values in a negative feedback loop, so Rick’s comment “Control is produced _only_ by negative feedback control systems” is a tautology.
> 
> > RM: Actually, that’s not the PCT definition of control. The definition of control is “maintenance of a variable in a pre-selected state, protected from disturbance”. A negative feedback loop is a model of how control works. It’s not a tautology.
> 
> > > > MT: Control is produced _only_ by negative feedback control systems. So I couldn’t disagree with you more when you say “we need to be sure that stability and control are not seen as belonging to different kinds of systems”. In fact, we need to be VERY sure that we understand that stability and control “belong” to two very different systems: open-loop, causal systems for the former and closed loop negative feedback systems for the latter. [MT: “you” here is Bruce Abbott.]
> 
> > RM: So let me get this straight. Are you saying that the “stability” of the behavior of the “ball-in-the-bowl” is the same as the “stability” of the behavior of, say, the water level in Ktesibios’ water clock?
> 
> > Best
> 
> > Rick
> 
> > > To which I can only say that there are several applicable proverbs along the lines that one is better advised to listen and learn rather than to guess and pontificate. Bruce is quite right to say “we need to be sure that stability and control are not seen as belonging to different kinds of systems”. To contradict Bruce is to say something as nonsensical as “we need to be sure that leafiness and trees are not seen as belonging to different kinds of objects”. “Stability” applies to all kinds of dynamical systems, which control systems are.
> > > 
> > > Martin
> 
> > –
> 
> > Richard S. Marken PhD  
> > [www.mindreadings.com](http://www.mindreadings.com)
> 
> > The only thing that will redeem mankind is cooperation.  
> > – Bertrand Russell
> 
> > No virus found in this message.
> 
> > Checked by AVG - [www.avg.com](http://www.avg.com)
> 
> > Version: 2014.0.4259 / Virus Database: 3658/6937 - Release Date: 12/20/13
> 
> > * * *
> 
> > No virus found in this message.  
> > Checked by AVG - [www.avg.com](http://www.avg.com)
> 
> > Version: 2014.0.4259 / Virus Database: 3658/6937 - Release Date: 12/20/13
> 
> –
> 
> Richard S. Marken PhD  
> [www.mindreadings.com](http://www.mindreadings.com)
> 
> The only thing that will redeem mankind is cooperation.  
> – Bertrand Russell
> 
> No virus found in this message.  
> Checked by AVG - [www.avg.com](http://www.avg.com)  
> Version: 2014.0.4259 / Virus Database: 3658/6941 - Release Date: 12/22/13

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**Author:** ![rsmarken](http://discourse.iapct.org/user_avatar/discourse.iapct.org/rsmarken/32/3553_2.png) [@rsmarken](http://discourse.iapct.org/u/rsmarken)\
**Post date:** [December 25, 2013, 7:40pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/39 "2013-12-25T19:40:21Z")

</div>

[From Rick Marken (2013.12.25.1140)]

> **···**
>
> On Wed, Dec 25, 2013 at 3:55 AM, Boris Hartman [boris.hartman@masicom.net](mailto:boris.hartman@masicom.net) wrote:
> 
> > > > RM: How the behavior comes about is a theoretical question. The prior
> > 
> > > > question must be "what kind of behavior is it --control or
> > 
> > > > non-control",
> 
> > BH :What do you mean by this Rick ?
> 
> RM: What I mean is that the behavior called “control” is a fact; an observable phenomenon. Control is happening when a variable (the possible “disturbance”) that should have an effect on another variable (the possible “controlled variable”) is observed to have far less of an effect than expected based on a physical analysis of the situation. The behavior of most physical systems – inanimate systems such as pendulums and balls in bowls – does not involve control; the behavior of living systems seems to always involve control. Once one has determined that control is occurring then the next step is to try to explain it. The explanation of control is a theory and so far the best theoretical explanation of control is control theory applied properly to living systems in the form of PCT.
> 
> > BH: Bill’s definition of ANY BEHAVIOR : The purpose of any given behavior is to
> > 
> > prevent controlled perception from changing away from the reference
> > 
> > condition.
> 
> RM: I would say that is a claim (or hypothesis) about the behavior of living systems (not non-living system); it is not a definition. Whether or not any behavior (of a living or non-living system) involves control is an empirical question which can be answered by test (the TCV), not by definition or theory.
> 
> > BH: Any categorization of behaviors on controlled and uncontrolled
> > 
> > has some subjective criteria, not objective.
> 
> RM: I would say that the TCV is an objective basis for categorizing behavior as controlled or uncontrolled. The fact that the TCV can be done automatically, by a computer program – as is the case in my Mind Reading demo ([http://www.mindreadings.com/ControlDemo/Mindread.html](http://www.mindreadings.com/ControlDemo/Mindread.html)) – is my basis for calling the TCV an objective means of discriminating control from non-control behavior.
> 
> > BH: The problem I see here is not the use of the "test for the controlled
> > 
> > variable", but the “test for which perception is controlled”.
> > 
> > People usually try to “read” the purpose of other people in everyday
> > 
> > relationship or in war situation, or in sport (for example boxing, karate,
> > 
> > basketball, football.). You really think that they are using the TCV in your
> > 
> > scientific sense to determine the purpose of other people ?
> 
> RM: I’m sure they are not. The TCV is a formal tool that can be used (at least in principle) to make quantitatively precise identifications of the perceptual variables that are being controlled when we observe any particular example of controlling (such as intercepting objects or moving or keeping a cursor on target).
> 
> Best
> 
> Rick
> 
> –  
> Richard S. Marken PhD  
> [www.mindreadings.com](http://www.mindreadings.com)  
> The only thing that will redeem mankind is cooperation.  
> – Bertrand Russell

---

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**Author:** ![rsmarken](http://discourse.iapct.org/user_avatar/discourse.iapct.org/rsmarken/32/3553_2.png) [@rsmarken](http://discourse.iapct.org/u/rsmarken)\
**Post date:** [December 25, 2013, 8:36pm UTC](http://discourse.iapct.org/t/classical-control-systems-lectures/2570/40 "2013-12-25T20:36:34Z")

</div>

[From Rick Marken (2013.12.25.1240)]

![Attractors.jpg](http://discourse.iapct.org/uploads/default/original/1X/f95899be1fc710df351ee2dac6d13c29c52c1a91.jpeg)

> **···**
>
> On Wed, Dec 25, 2013 at 4:42 AM, Boris Hartman [boris.hartman@masicom.net](mailto:boris.hartman@masicom.net) wrote:
> 
> > HB: Well Rick, I must admitt I am worried…where this “PCT  
> > boat” is goimg…
> 
> RM: You really never know with science. But wherever we are going we’re doing it with the best navigation tools available (models) and with our eyes wide open (experimental test). Well, some of us are, anyway.
> 
> > HB: I’m wondering if you become an “officiall interpreter” of  
> > Bill’s knowledge ?
> 
> RM: Nope. Completely unofficial.
> 
> > HB. I thought you are just moderating.
> 
> RM: No, just contributing. This is an un-moderated list.
> 
> > HB: And from one  
> > subtitle you concluded that “control” is “objective” fact.
> 
> RM: My conclusion that control is a fact is based on my own thinking. I wrote a paper on the topic some time ago; it’s the first paper in my “Mind Readings” book; it’s called “The Nature of Behavior: Control as Fact and Theory”. I think Bill liked the idea (of distinguishing control as fact from control as theory) and that’s why he eventually incorporated the “fact of control” idea into the subtitle of his book. Seeing control as a fact (rather than as just a theory) is really one of the most important contributions Bill Powers made to our understanding of living systems: living systems _control_; that’s the fact, Jack.
> 
> > HB: Control “being a fact” is by my oppinion also contradiciting  
> > your previous statement that “control” is phenomenon, so it’s by my  
> > research in vocabulary, entirely dependent from perception. And perception can  
> > never be “objective”. Maybe you had in mind some other meaning of  
> > “phenomenon”.
> 
> RM: All facts are perceptions. So in that sense all facts are subjective. When I say that control is an objective phenomenon what I mean is that there are procedures (called the TCV) that can be used by anyone to demonstrate the fact (perception or phenomenon) of control to themselves. Control is an objective fact in the same way linear acceleration is an objective fact. The phenomenon of linear acceleration is a perception – a rather complex perception but still a perception – that anyone can demonstrate to themselves by carrying out the procedures Galileo used (rolling a ball down an inclined plane and measuring the time it takes to get from one equally spaced point to another as it descends) to demonstrate it to himself and others. Similarly anyone can demonstrate the perception of control to themselves using the TCV.
> 
> > HB: There is no “objective fact” on itself, because you have to  
> > prove that you somehow directly access to “reality” (outer  
> > environment) and know it in every detail.
> 
> RM: Obviously I did not mean “objective” in that sense. I am perfectly aware that “it’s all perception” and we don’t have (and never will have) access to external “reality”.
> 
> > HB : I thought that you are “protecting” the “controlled”  
> > variable form distrubances not canceling the effects of disturbances.
> 
> RM: I think of these as just two different ways of saying the same thing.
> 
> > HB: Why did you use term cancel instead of term “protecting”, if  
> > they have the same meaning by your oppinion ?
> 
> RM: I like “protecting” better than “canceling” in some cases because there are usually many different sources of disturbance acting on a CV so actions are not canceling the effect of a disturbance (as in CV = d - o, output cancels disturbance) but are protecting the controlled variable from the net effect of many disturbing influences on the CV.
> 
> > HB: And sorry I didn’t get it, which  
> > is “controlled” variable in this case that you are “protecting”  
> > it from disturbances ?
> 
> RM: The controlled variable is the variable around which behavior is organized. So if the behavior is catching a fly ball, one controlled variable is vertical optical velocity and to get under the ball the fielder must keep that variable at 0. So the fielder moves backward or forward as necessary to keep this variable at 0, protected from the effects of disturbance, the main disturbance being the trajectory of the ball.
> 
> > RM : So control is happening because we are seeing the:
> 
> > “Achievement and maintenance of a variable in a particular state  
> > through actions on the environment that also cancel the effects of disturbances”.
> 
> > 
> 
> > HB: So you are also “officialy” promoted to change Bill’s definitions.
> 
> RM: Anyone can propose changes to anything about PCT and try to justify them. And anyone else is also free to reject the changes and justify the rejection. This is not a cult.
> 
> > HB: To me it’s obviously that you shouldn’t do that. Now as you changed definition,  
> > you’ll have to change also his generic diagram and put the “controlled  
> > variable” into outer environment.
> 
> RM: The controlled variable already is in the outer environment in the “generic” diagram. It’s called the “controlled quantity” when it’s out there.
> 
> Best
> 
> Rick
> 
> > 
> 
> > 
> 
> > 
> 
> > * * *
> 
> > **From:** Control Systems Group  
> > Network (CSGnet) [[mailto:CSGNET@LISTSERV.ILLINOIS.EDU](mailto:CSGNET@LISTSERV.ILLINOIS.EDU)] **On Behalf Of** Richard Marken  
> > **Sent:** Sunday, December 22, 2013  
> > 8:12 PM
> 
> > **To:** CSGNET@LISTSERV.ILLINOIS.EDU  
> > **Subject:** Re: Classical Control  
> > Systems Lectures
> 
> > 
> 
> > [From Rick Marken (2013.12.22.1110)]
> 
> > 
> 
> > On Sun, Dec 22, 2013 at 2:53 AM, Boris  
> > Hartman [boris.hartman@masicom.net](mailto:boris.hartman@masicom.net) wrote:
> 
> > 
> 
> > > BH: TO PUT IT  
> > > EXACTLY IN PCT SENSE, CONTROL IS : “Achievement and maintanance of a  
> > > preselected perceptual state in the controlling system, through actions on the  
> > > environment that also cancel the effects of disturbances” (B:CP, 2005).
> 
> > 
> 
> > RM: Yes, that’s a good  
> > one too;-) It’s tough to give a definition of control that is completely theory  
> > free and I don’t think Bill was trying to do that when he came up with this  
> > definition. Indeed, part of Bill’s goal in giving this definition of control  
> > was to include the important new assumptions of what has come to be called PCT.  
> > I don’t think it was until many years after the publication of B:CP that he  
> > actively promoted the idea that control was a phenomenon in and of itsef – an  
> > objective _fact_ (as in the subtitle to LCS III) – that is explained by the  
> > theory of control: control theory.
> > 
> > I think the definition above can be “objectified:” a bit by changing  
> > “perceptual state in a controlling system” to “variable”.  
> > The idea that it is a perceptual variable that is controlled is really part of  
> > the theory – a very important part of the theory but part of the theory  
> > nevertheless.
> > 
> > But one can observe control without making any assumptions about _how_ control  
> > works. We can do this by observing a variable, such as the distance between  
> > cursor and target in a tracking task, and noting that disturbances have little  
> > or none of their expected effect on this variable and that this is because the  
> > effects of these disturbances are being cancelled by observed actions (mouse  
> > movements). So control is happening because we are seeing the:
> > 
> > “Achievement and maintenance of a variable in a particular state  
> > through actions on the environment that also cancel the effects of  
> > disturbances”.
> 
> > I also got rid of the term “pre-selected” because  
> > this also makes theoretical assumptions about what is going on inside the  
> > system doing the controlling. Of course the “variable” referred to in  
> > the above definition is a perception but then everything is a perception so  
> > it’s really unnecessary to mention it.
> 
> > >
> 
> > > BH: So as I see it, control is initially not “protecting” act  
> > > but **cancelling act** of output which  
> > > compensate effects of disturbances so to maintain perceptual stability (some  
> > > preselected perceptual state) in the controlling system.
> 
> > 
> 
> > RM: That’s fine. The  
> > verbal description matters less to me than the functional model that actually  
> > makes control work. I think that control can be correctly described as  
> > involving “cancelling” the effect of disturbances or  
> > “protecting” a controlled variable from the effects of disturbance.  
> > But if “protecting” doesn’t work for you then feel free not to use  
> > it.
> 
> > Best
> > 
> > Rick
> 
> > 
> 
> > > This could mean that “controlled variable” is already  
> > > affected and new state perceived and controlled in comparator and act of  
> > > canceling the effect or compensating the efect of disturbances realized.
> 
> > >
> 
> > > To use term “protect”  
> > > is something that means for me to act in advance, so to “prevent”  
> > > some “controlled variable” or better predefined state of controlling  
> > > system, from being disturbed, displaced, affected…etc.
> 
> > > Term  
> > > “protecting” is probably kind of control but not in initial sense. By  
> > > my oppinion is one of consequences of “pure” PCT control, which is by  
> > > definition used with terms canceling, compensating, etc…
> 
> > >
> 
> > > Something similar  
> > > was Ashby’s “control” definition : “Every stable system has the  
> > > property that if displaced from a state of equilibrium and released, the  
> > > subsequent movement is so matched to the initial displacement that the system  
> > > is brought back to the state of equilibrium” (Ashby, 1960).
> 
> > >
> 
> > > I think that Ashby  
> > > used “compensation” for description of “control”. And it  
> > > seems to me that he used terms to describe actual “displacement” and  
> > > actual “compensation” not something happening in advance.
> 
> > > He tried to give  
> > > also definitions of dynamic system, variable and system, specifications of  
> > > behaviour, “stability”, “equilibrium”,  
> > > “steady-state”, and so on, as I think that Bill used some of this  
> > > terms in Appendix to the book B:CP, 2005.
> 
> > >
> 
> > > So the main point  
> > > I see, is that whatever “controlled variable” is meant, is first  
> > > “moved” (perceived displacement) from the predefined, initial state  
> > > (reference state, equilibirum…) and than by canceling or compensating or  
> > > opposing effects of action or whatever we call that (maybe behavior), again  
> > > brought back to predefined, initial state (reference, equilibrium, whatever…).
> 
> > >
> 
> > > If we say that  
> > > “controlled variable” is protected than you probably assume that  
> > > “controlled variable” was not disturbed yet, as disturbances has  
> > > already been cancelled, compensated by control system in advance. But that can  
> > > never happen if control system “has no experiences” with “moving  
> > > controlled variable” from initial state with certain disturbances. How  
> > > else could control system “choose” disturbances to act on, so to  
> > > protect “controlled variable” from being affected if control system  
> > > doesn’t know what kind of effect distrubances have on “controlled  
> > > variable” and system itself.
> 
> > >
> 
> > > I think that the  
> > > selection of disturbances that could have effect on controlled variable, are  
> > > those which in the past show tendency to “displace controlled  
> > > variable” from initial state. So when control system has that  
> > > “experience” than it can probably reorganize so to  
> > > “protect” it from disturbances. But by my oppinion it has to be  
> > > complex control system, build up with many control units, what could happen  
> > > through evolution.
> 
> > >
> 
> > > In such a complex  
> > > organized control systems (more organized control units), certain control units  
> > > serve the goal to really “prevent” or “protect” certain  
> > > “controlled variable” from being disturbed, displaced or  
> > > “moved” from initial state like in some physiological cases.
> 
> > >
> 
> > > So I think it’s  
> > > better to use initial terms when making defintion about control in PCT sense  
> > > such as cancelling, compensating activity not “protecting” activity.
> 
> > >
> 
> > > Your  
> > > “exclusive” statement about Gordon…. :
> 
> > > RM : “Well,  
> > > Mr. Douglas is now off my list of people to  
> > > listen to about control theory. This lecture was awful”.
> 
> > >
> 
> > > …has no  
> > > sense to me.
> 
> > >
> 
> > > By my oppinion  
> > > Douglas tried methodically to show how control in different dynamic system  
> > > works. And by my opinion his retorics and pictures about control is better then  
> > > yours about “protecting controlled variables”. But both are  
> > > insufficient (as probably mine is), but that doesn’t mean that we have to stop  
> > > talking about your and his presentation of control, because they are  
> > > “awfull”. I think that both are good as the bases to improve them, so  
> > > they would show better how control theory works.
> 
> > >
> 
> > > Rick, you are  
> > > doing a good job, moderating on CSGnet. But nobody said that you couldn’t be  
> > > better J
> 
> > >
> 
> > > I can’t comment  
> > > other discussions for the time being as I didn’t read them. Maybe I missed  
> > > something important. I also didn’t entirely follow the discussion about B:CP  
> > > (2005) so I’m interested if you make any comments about Appendix in the book.  
> > > There are some interesting clarifications of terms control, stability…
> 
> > >
> 
> > > Best,
> 
> > >
> 
> > > Boris
> 
> > >
> 
> > >
> 
> > > * * *
> 
> > > **From:** Control Systems Group  
> > > Network (CSGnet) [[mailto:CSGNET@LISTSERV.ILLINOIS.EDU](mailto:CSGNET@LISTSERV.ILLINOIS.EDU)]  
> > > **On Behalf Of** Richard Marken  
> > > **Sent:** Saturday, December 21, 2013  
> > > 2:38 AM  
> > > **To:** CSGNET@LISTSERV.ILLINOIS.EDU  
> > > **Subject:** Re: Classical Control  
> > > Systems Lectures
> 
> > >
> 
> > > [From Rick Marken (2013.12.20.1740)]
> 
> > >
> 
> > > > Martin Taylor  
> > > > (2013.12.20.11.47)–
> 
> > > > > RM: Stability and  
> > > > > control are two different _phenomena_, something that it is now clear to me  
> > > > > that Mr. Douglas is completely unaware of. These two different phenomena are  
> > > > > produced by two different kinds of systems; stability is a kind of behavior  
> > > > > exhibited by certain “open-loop” or what I call “causal”  
> > > > > systems.
> 
> > > >
> 
> > > > I still haven’t  
> > > > looked at the videos, but it strikes me that you are doing the old  
> > > > philosophers’ trick of taking a word that has a variety of meanings, and using  
> > > > a meaning appropriate in one context as though that were the meaning  
> > > > appropriate to a different context. Yes, “stability” and  
> > > > “control” are indeed different phenomena, in the same way  
> > > > “food” and “vegetable” are different concepts.
> 
> > >
> 
> > > RM:  
> > > No,“stability” and “control”, as described by Douglas in  
> > > the very first control lecture, are different phenomena in the way  
> > > “food” and “poison” are different. A stable system  
> > > (according to the lecture) is one that returns to its original  
> > > (“equilibrium”) state after a transient disturbance; a control system  
> > > is is one that remains in a reference state during continuous disturbance.  
> > > Douglas should have talked about variables rather than systems but you get the  
> > > idea.
> 
> > > >
> 
> > > > MT: All feedback  
> > > > loops, in fact all dynamical systems, whether control systems or not, have  
> > > > stability criteria. Either they are stable or not. Some are more stable than  
> > > > others.
> 
> > >
> 
> > > RM: Yes, they do.  
> > > Indeed, I measure control in terms of stability (observed/expected variance of  
> > > the variable). In this case “stability” is simply referring to a  
> > > measure of the observed variations in a variable and it can be used to measure  
> > > the variability of a controlled variable or an uncontrolled variable (like the  
> > > variable position of the ball in the bowl that Douglas refers to as a stable  
> > > system).
> 
> > >
> 
> > > > MT: Some are  
> > > > metastable, meaning they will maintain their current values until something  
> > > > momentarily disturbs one of their signal values. Some of those will continue to  
> > > > diverge from the original metastable value after the disturbance, some will  
> > > > just maintain the disturbed set of signal values without further change. Some  
> > > > are absolutely stable, meaning that after any kind of momentary disturbance  
> > > > they will return their values to their original levels. Most real systems don’t  
> > > > do that, and are stable only if the momentary disturbance doesn’t exceed some  
> > > > limit.
> > > > 
> > > > The key concept is the “orbit”. All systems that can be described by  
> > > > a vector of variables have a state. Their state is the vector of current  
> > > > variable values together with the rates of change of the current variables.  
> > > > That includes control systems, ball-in-a-bowl systems, the synapse strengths of  
> > > > networks of millions of neurons, etc. etc. If the system is at some location in  
> > > > the state space and is not further disturbed from outside, it will follow some  
> > > > track through the state space. That track is an orbit, and there is only one  
> > > > orbit through any point in the state space.
> > > > 
> > > > A stable system is one for which the orbit will converge to some track that is  
> > > > the same for all the initial locations in the state space. That track is called  
> > > > an “attractor”. The attractor may be a fixed point, a closed path  
> > > > (which represents a stable oscillator) or a “strange attractor”  
> > > > (which I won’t explain now). An unstable system is one for which the  
> > > > orbits diverge. Here are a couple of examples of attractors, or at least the  
> > > > projections of them into two dimensions, because even in 2-D, the orbit is  
> > > > actually in a space of four dimensions, two for location and two for velocity.  
> > > > I have omitted the velocity coordinates in these examples, and in the  
> > > > fixed-point example it is the velocity that distinguishes the orbits where two  
> > > > of them cross in the figure. In the 4-D state space, only one orbit passes  
> > > > through any particular point.
> 
> > >
> 
> > > RM: All of this  
> > > simply describes the observed behavior of a variable. Nothing about the shape  
> > > of these orbits can tell you whether the variable is controlled or not.
> 
> > > > MT: A control  
> > > > system is one for which the attractor converges in at least one dimension (the  
> > > > perception-value dimension), but that’s not the main criterion for  
> > > > differentiating the “ball-in-the-bowl” from a trivial control loop.
> 
> > >
> 
> > > RM: That’s not  
> > > only is not a “main” criterion; it’s not a criterion at all. The only  
> > > criterion for distinguishing the “ball-in-the-bowl” from a controlled  
> > > ball in the bowl is the criterion John Kirkland just mentioned: The criterion of The  
> > > Test for the Controlled Variable, which is whether there is less of an effect  
> > > of a disturbance on the controlled variable than expected. You simply cannot  
> > > tell, by looking at just the observed behavior of the  
> > > “ball-in-the-bowl” (like the “fixed point” and “stable  
> > > oscillator” orbits pictured above) whether you are observing the behavior  
> > > of a controlled or uncontrolled variable. The orbits plotted above could be the  
> > > behavior of a controlled or uncontrolled ball. This is exactly analogous to the  
> > > situation in my mindreading  
> > > demo ([http://www.mindreadings.com/ControlDemo/Mindread.html](http://www.mindreadings.com/ControlDemo/Mindread.html)). When you move one avatar around the  
> > > screen in a controlled manner, the other two move as well; you can’t tell from  
> > > the movements (orbits) of the avatars, which is controlled and which are not.  
> > > In order to determine control you have to disturb the position of the avatars  
> > > and see which avatar is affected _least_ by the disturbance.
> 
> > >
> 
> > > > MT: Bruce Abbott put his finger  
> > > > on it when he pointed out that the ball-in-the-bowl uses the energy supplied by  
> > > > the disturbance to return the ball to its fixed point, whereas the control loop  
> > > > uses an independent energy supply to oppose the effect of the disturbance on  
> > > > one (and only one) of the variables in the state space of the loop. The manner  
> > > > in which control is established is irrelevant.
> 
> > >
> 
> > > RM: This is a  
> > > description of models that produce the observed behaviors: the open loop  
> > > physics model for the “ball-in-the-bowl”; closed-loop control for the  
> > > controlled ball. The manner in which control is established may be  
> > > irrelevant (I have no idea what that means actually; the only way I know of to  
> > > establish that control is happening is by using the Test) but one has to have  
> > > established that control is going on in one case and that it’s not going on in  
> > > the other in order to apply the correct explanations (models) to each case.
> 
> > >
> 
> > > > MT: It so happens  
> > > > that the PCT definition of control is the maintenance of one particular value  
> > > > among the many different signal values in a negative feedback loop, so Rick’s  
> > > > comment “Control is produced _only_ by negative feedback control  
> > > > systems” is a tautology.
> 
> > >
> 
> > > RM:  
> > > Actually, that’s not the PCT definition of control. The definition of control  
> > > is “maintenance of a variable in a pre-selected state, protected from  
> > > disturbance”. A negative feedback loop is a model of how control works. It’s  
> > > not a tautology.
> 
> > > >
> 
> > > > > MT: Control is  
> > > > > produced _only_ by negative feedback control systems. So I couldn’t disagree  
> > > > > with you more when you say “we need to be sure that stability and control  
> > > > > are not seen as belonging to different kinds of systems”. In fact, we need  
> > > > > to be VERY sure that we understand that stability and control  
> > > > > “belong” to two very different systems: open-loop, causal  
> > > > > systems for the former and closed loop negative feedback systems for the  
> > > > > latter. [MT: “you” here is Bruce Abbott.]
> 
> > > RM: So let me get  
> > > this straight. Are you saying that the “stability” of the  
> > > behavior of the “ball-in-the-bowl” is the same as the  
> > > “stability” of the behavior of, say, the water level in Ktesibios’  
> > > water clock?
> 
> > >
> 
> > > Best
> 
> > > Rick
> 
> > >
> 
> > > > >
> 
> > > > >
> 
> > > > To which I can  
> > > > only say that there are several applicable proverbs along the lines that one is  
> > > > better advised to listen and learn rather than to guess and pontificate. Bruce  
> > > > is quite right to say “we need to be sure that stability and control are  
> > > > not seen as belonging to different kinds of systems”. To contradict Bruce  
> > > > is to say something as nonsensical as “we need to be sure that leafiness  
> > > > and trees are not seen as belonging to different kinds of objects”.  
> > > > “Stability” applies to all kinds of dynamical systems, which control  
> > > > systems are.
> > > > 
> > > > Martin
> 
> > > –
> 
> > > Richard S. Marken PhD  
> > > [www.mindreadings.com](http://www.mindreadings.com)
> 
> > >
> 
> > > The only thing  
> > > that will redeem mankind is cooperation.
> > > 
> > > ```
> > > -- Bertrand Russell
> > > 
> > > ```
> 
> > > No virus found in this message.
> 
> > > Checked by AVG - [www.avg.com](http://www.avg.com)
> 
> > > Version: 2014.0.4259 / Virus Database: 3658/6937 - Release Date:  
> > > 12/20/13
> 
> > > * * *
> 
> > > No virus found in this message.
> > > 
> > > Checked by AVG - [www.avg.com](http://www.avg.com)
> 
> > > Version: 2014.0.4259 / Virus Database: 3658/6937 - Release Date:  
> > > 12/20/13
> 
> > –
> 
> > Richard S. Marken  
> > PhD  
> > [www.mindreadings.com](http://www.mindreadings.com)
> 
> > 
> 
> > The only thing that will redeem mankind is cooperation.
> > 
> > ```
> > -- Bertrand Russell
> > 
> > ```
> 
> > No virus found in this message.
> > 
> > Checked by AVG - [www.avg.com](http://www.avg.com)
> 
> > Version: 2014.0.4259 / Virus Database: 3658/6941 - Release Date: 12/22/13
> 
> –  
> Richard S. Marken PhD  
> [www.mindreadings.com](http://www.mindreadings.com)  
> The only thing that will redeem mankind is cooperation.
> 
> ```
> -- Bertrand Russell
> 
> ```

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