[Bulk] Re: A chemical equation of equilibrium and
negative
[Martin Taylor 2006.04.27.17.03]
[From Bjorn Simonsen (2006.04.27,21:35
EUST)]
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. I think upon the tension in the pendulum
suspension as the “perceptual signal” and a tension in the
pendulum suspension like the gravity force as the reference (this
happens when the pendulum hang still.
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.
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 disturbnace. 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!).
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).
Martin
[Martin Taylor 2006.04.29.13.11]
[From Rick Marken (2006.04.29.0840)]
Bjorn Simonsen (2006.04.29,08:45 EUST)
I absolutely agree that _if_ the pendulum is a control system it is not a _good_ control system. There are many not good control systems. Is a thermostat a good control system? If you open the window in your living room a very cold winter day, it takes about the same time to create an equal and opposite force before the room temperature comes back to the set point temperature.
I think this misses the fact that pendulum barely controls at all while the thermostat, though not perfect, controls quite well. ...
So you can say that both the pendulum and thermostat are poor controllers but that the pendulum is worse. ...
I don't think you can say the pendulum is a controller of any kind, good or bad. It doesn't have any of the three main characteristics of a controller:
(1) the existence of a signal that represents the state of something outside itself,
(2) use of a source of energy independent of whatever might disturb the state represented by the signal, and
(3) an asymmetry between the force applied to the outer state to produce the signal and the force available to oppose changes in the outer state.
Whether a control system is "good" or "poor", it has all of those features. The pendulum has none.
I suppose you could say that Newton's law that a force is opposed by an equal an opposite reaction (e.g. a pendulum held at an angle) means that all mechanical effects involve negative feedback, but I'd call that stretching a point beyond the horizon! But even if you have an over-the-horizon radar, not all negative feedback systems are control systems.
Martin
PS. In a few minutes, I'm gone for a week or ten days, by which time this thread will probably be dead, so unless you disagree with the above in some arguable way, I don't expect to contribute to it further.