[Martin Taylor 2006.04.28.10.05]
[From Bjorn Simonsen (2006.04.28,11:55 EUST)]
Martin Taylor 2006.04.27.17.03
>>I agree that the pendulum idea and the mass spring
idea can't serve as models of purposeful behavior,
but it looks to me that the pendulum system is
influenced by negative feedback after the pendulum
bob is released
>The difference between the restoration of the pendulum
(or a spring, and the like) is that for them the energy
required for the restoration is provided by whatever
moved the item away from its equilibrium position.
It's just a conversion of the form of the energy to
jn the case of the pendulum) gravitational potential
energy provided by whatever pushed the pendulum,
and, on restoration, the dissipation of that energy into heat.
May I go into some hair-splitting comments?
Many systems have their distinctive stamp. Isn't it possible to think upon a pendulum as a simple, a very simple negative feedback system.
Write the equations, and that will tell. If you have an equation in which thre is something of the form x(t) = f(x(t-delta), y, z, ....), then you have shown it is a feedback system. Whether the feedback is positive or negative depends on the actual equation.
In our learned hierarchy, which mostly is a negative feedback system, the reference signal most of the time represent potential energy.
Where did THAT come from?
The reference signal always represents a desired state of a perceptual signal. The perceptual signal may represent, say, whether my wife's face looks happy, or whether the hole I'm digging is 1 metre deep. I can't reconcile either of those, or most of my other (consciously available) reference signals with a desire to see something have a particular value of potential energy.
The same is to say about the perceptual signal.
Actually, my comment above applies primarily to the perceptual signal, because that is what is affected by the state of the outer world. The reference signal doesn't "represent" anything other than the desired value of the perceptual signal, so if it "represents" something in the outer world, that something is whatever the perceptual signal represents.
In a pendulum system the reference is zero potential energy,
If there's a control system involved (which is more than just a negative feedback system, there must be some variable that is set. The equation must be of the form x(t) = f(x(t-delta1), r(t-delta2), y, z, ...), where r(t) is the reference signal, and the form of the equation is such that
x(t) - r(t-delta2) < x(t-delta1) - r(t-delta2)
Now, I grant you that r(t) might be a constant, in which case you might be tempted to elide it from the representation. But it's always there. A simple negative feedback system doesn't have that r(t) term.
You are a control system. When you stand on a ships deck in heavy weather, you have a reference for standing upright. Your output counteracts the disturbance.
But if I were like a pendulum, I would stay tilted as long as the deck stayed tilted. So there's a difference between me and the pendulum. I make myself more upright when the deck tilts -- or try to.
>In a control system, whether it be physical or chemical
or whatever, the energy required for restoration comes
from a source independent of the energy suplied by the
Is it energy that controls a thermostat to experience a temperature of 20 degrees centigrade. Do a thermostat controlling for 25 degrees possess more energy than a thermostat controlling for 20 degrees?
As I pointed out previously, the perceptual sensor takes as little energy from the perceived system as it possibly can. It's the furnace, not the thermostat, that provides the energy to maintain the temperature at 20, and the furnace does use more energy to maintain the temperature at 25 (assuming it's winter).
Is it the energy or the organization that result in negative feedback? If you cut through the feedback channel in a negative feedback system, you change the organization. And you loose the purpose concept.
Perfectly true. A control system is a special form of negative feedback system, in which there exists a reference value, and in which the output gain exceeds the input gain and the energy for counteracting a disturbance is obtained from a separate source.
I just ask.
>The control system's "perception" extracts
negligible energy from the disturbed system, but the
output can supply as much energy as is available from
its supply (conceivably up to the energy released by an H-bomb!).
Is an exploding H-bomb an effect of the output signal in a negative feedback system?
Depends how it is used. If it is used as the output in a control system, as was the case before the Test ban treaty came into force, then yes.
>Equilibrium systems do tend toward restoration of
their central values, but they do so without outside
>energy sources (other than that supplied by whatever
disturbed them away from the equilibrium in the first place).
Yes, but they also do so with outside energy sources. If hydrogen iodide is supplied with thermal energy, you get iodine and hydrogen when the temperature is more than 180 degrees centigrade.
2.5 kcal + 2HJ �> H2 + J2 (exhalation)
I think the mass effect constant (dependent on temperature) tells us the reference for the concentration of iodide. If we take the heat away, the equilibrium is moved to the left.
And if you put a magnet near a pendulum with an irom bob, its equilibrium is moved away from vertical. In neither case do I see a separate energy source being used to couteract the effect of an outside disturbance that is supplying energy to change the state of a system.
However, if a control system used the magnet to counter the effect of the wind on the pendulum, that would be using the separate energy source (magnet) to counter the energy of the disturbance (wind). If the magnet is moved (or its power level altered) by the output process of the control system, it's an energy source separate from the disturbance. If it is placed by an ouside agency, it's part of the disturbance.
In your HJ example, if the 2.5 kcal is supplied from a source varied according to whether a control system wants to see H + J or HJ, that's one thing. If the HJ sample is blown near a heat source by the wind, the heat energy is just part of the disturbance.
Equilibrium and control aren't the same, even if sometimes the simple dynamics are indistinguishable.