# engineering control theory

**URL:** <http://discourse.iapct.org/t/engineering-control-theory/13695>\
**Category:** CSG1999\
**Created:** [April 22, 1999, 6:40pm UTC](http://discourse.iapct.org/t/engineering-control-theory/13695 "1999-04-22T18:40:49Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Bill\_Powers1](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/b/8e7dd6/32.png) [@Bill\_Powers1](http://discourse.iapct.org/u/Bill_Powers1)\
**Post date:** [April 22, 1999, 6:40pm UTC](http://discourse.iapct.org/t/engineering-control-theory/13695/1 "1999-04-22T18:40:49Z")

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[from Mary Powers 9904.22]

Some comments on the control theory thread beginning 9904.20 or so:

About control engineers knowing what they are doing - they are designing  
devices that \_do\_ things in the "real world", from thermostats on up. That  
the real world (including what their control systems are doing) is a  
perceptual phenomenon in their minds is irrelevant (they think). That the  
devices themselves also deal with a perceptual phenomenon that is a function  
of the transducers they are given is perhaps an unusual point of view but  
one which can certainly be helpful. Events like Three Mile Island would  
never have happened if the engineers had considered what the control system  
they designed could perceive - which was not the condition of the valve, or  
the flow of water which depended on the position of the valve, but simply  
the information that a signal to the valve had indeed been sent. This is  
all the system could sense, and it was blind to the fact that the signal had  
failed to activate the valve. So it proceeded on the basis that the valve  
was activated, with the unfortunate results we all know. Obviously the  
engineers involved did not imagine correctly what needed to be sensed.

In WTP's career of designing control systems, he found that taking the  
system's "point of view" was a strategy that solved many problems so easily  
it felt like cheating. But this maneuver, of imagining what the system he  
was designing could perceive with the sensors he was giving it, occurred to  
him mainly because he was learning engineering control theory and developing  
PCT essentially simultaneously - a fortuitous circumstance unlikely to  
happen again. This is what makes his view of control systems unusual, but  
not wrong. It may, indeed, be better than what most engineers emerge from  
school with.

It is rather insulting to claim that all PCT is based on chapter 1 of  
control system texts. A great deal of what comes in later chapters has do  
do with achieving the results one wants from the system one is building -  
and involves stability issues, the inevitable crankiness and variations in  
the supposedly standardized parts one is using, various unexpected  
consequences of joining one system to another, etc., etc. Something people  
tend to forget, I think, that underlying Bill's discussion of control lies a  
technical knowledge (from later chapters in those books) that rarely  
surfaces because it isn't called for in this forum. And one major reason it  
isn't required is the fact that on the net we are dealing with \_living\_  
control systems, which have evolved over millenia and eons to \_be\_ stable  
and to work. The work Richard Kennaway is doing with Bill off the net on a  
\_simulated\_ bug definitely requires those later chapters.

As for why choose PCT over traditional engineering: the thing that is being  
sneered at here, PCT's "simplicity", is its virtue. Can you imagine a  
bacterium packing the equipment necessary for model-based control? To be  
able to describe the performance of bacteria, and increasing complex ones  
such as humans, by the proliferation of the same simple element into huge  
hierarchies, is parsimonious and elegant, and takes evolution into account  
as well as things-as-they-are.

Mary P.

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**Author:** ![Bruce\_Gregory](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/b/4bbf92/32.png) [@Bruce\_Gregory](http://discourse.iapct.org/u/Bruce_Gregory)\
**Post date:** [April 22, 1999, 7:33pm UTC](http://discourse.iapct.org/t/engineering-control-theory/13695/2 "1999-04-22T19:33:54Z")

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[From Bruce Gregory (990422.1533 EDT)]

Mary Powers 9904.22

> It is rather insulting to claim that all PCT is based on chapter 1 of  
> control system texts.

Since I made that statement, I want to be clear that I was speaking from  
a possible control engineer's point of view. The basics of PCT can be  
demonstrated in a very simple control loop. This makes PCT look almost  
trivial \_if\_ you think you understand how a thermostat works. As you  
point out in reference to Three Mile Island, not all engineers  
understand as much as they think they do. Taking the system's point of  
view is obviously a \_major\_ change in viewpoint that many people are  
unable to make. I suspect the reason is that they do not realize just  
how big a step it is.

> As for why choose PCT over traditional engineering: the  
> thing that is being  
> sneered at here, PCT's "simplicity", is its virtue.

I don't know who was sneering, but it wasn't me. It's simplicity is  
without question its strength.

> Can you imagine a  
> bacterium packing the equipment necessary for model-based  
> control? To be  
> able to describe the performance of bacteria, and increasing  
> complex ones  
> such as humans, by the proliferation of the same simple  
> element into huge  
> hierarchies, is parsimonious and elegant, and takes evolution  
> into account  
> as well as things-as-they-are.

No question about it. It's elegance is more than impressive, it is  
compelling.

Bruce Gregory

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**Author:** ![Wolfgang\_Zocher1](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/w/919ad9/32.png) [@Wolfgang\_Zocher1](http://discourse.iapct.org/u/Wolfgang_Zocher1)\
**Post date:** [April 23, 1999, 7:06am UTC](http://discourse.iapct.org/t/engineering-control-theory/13695/3 "1999-04-23T07:06:14Z")

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> [from Wolfgang Zocher 9904.23]

WONDERFUL post, Mary! Thank you!

> **···**
>
> --  
> Wolfgang Zocher
> 
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**Author:** ![Philip\_Runkel](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/p/8dc957/32.png) [@Philip\_Runkel](http://discourse.iapct.org/u/Philip_Runkel)\
**Post date:** [April 23, 1999, 5:41pm UTC](http://discourse.iapct.org/t/engineering-control-theory/13695/4 "1999-04-23T17:41:39Z")

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from Phil Runkel on 23 April 99:

Ah, Mary, dear Mary,

when I have been forging my way through the brambles of a series of  
vituperative and presumptuous messages, how welcome is the balm and  
sanity in a message such as yours of 22 April! When I see your name at  
the top of a posting, I always know that I am going to feel once again on  
firm footing and in good balance. Thanks very much for your careful and  
timely comment.