<[Bill Leach 950520.17:23 U.S. Eastern Time Zone]
Message: 10190 on Sat, 20 May 1995 14:45:50 +0330
Author : Mohammad Ghavam-Zadeh <GHAVAMZD%IREARN.BITNET@uga.cc.uga.edu>
Welcome to the net. There is a monthly posting by Dag Forssell that will
be of interest to you. This posting provides a brief summary of PCT,
CSG-L and sources for additional information. In addition it provides
information concerning some "net practices".
You stated that you are working on "positive feedback control systems"
and have generated a paper in this area...
I believe that there might be a "bit of a rough time" coming to terms
with the discussions that take place here. The term "feedback" and the
term "positive feedback" are used in their strict Engineered Control
Systems Theory meaning.
Thus from the standpoint of PCT, there is no such animal as a "Positive
Feedback Control System" that actually controls. That is, no real
control system that actually maintains an input signal at a constant
reference value against random and unpredictable disturbance to the
"controlled" element in the environment can have a net positive feedback
and still function.
That does not deny the potential usefulness of some limited amount of
"positive feedback" in certain control situations as long as the overall
feedback signal is still negative.
Note that specifically, "negative feedback" as used here on CSG-L is
referring to an input signal that is 180 out of phase with the output
signal in the mathematical sense.
Control in this sense is possibly best typlified by the "unity gain Op
Amp" circuit. The "reference" (or goal) is applied at the
"non-inverting" input to the amp. The output is (ultimately) the
"behaviour" and the line wired from the output back to the "inverting"
input is both the feedback (which is negative since signal feeds the
inverting input) and the "perception". Note that of course in other
"real world" controllers, there are components or functions present in
the line.
Of course there are other, rather imprecise, meaning for the term
feedback -- such as when I tell an associate that "he has done an
outstanding job". Usually this would be referred to as "my providing
positive feedback" to the associate. There are pretty compelling reasons
why a PCTer would assert that this is an improper use for the term.
The common psychological use of the term feedback to mean "good" or "bad"
is a serious disservice and leads to great difficulty when attempting to
communicate a simple but vital concept for understanding control systems.
Additionally what I or an observer might lable as "good feedback" or
"positive feedback", meaning of course that what has been so labled is
"good" for the subject could very well be "bad" from the subject's point
of view (which is the only one that matters as far as understanding
behaviour is concerned).
You might expand a bit upon what you are referring to when you say
"positive feedback control systems".
-bill