[From Bill Powers (960701.1130 MDT)]
Hans Blom (960701) --
[Writing to Rick]
Control engineering was my major in University and it made up a
great deal of my professional activities after that. So don't tell
me I don't know the meaning of the word control. Has it ever
occurred to you that maybe YOU make the meaning so idiosyncratic
that discussions become almost impossible?
This is ONLY a matter of custom. What we are concerned with in PCT is a
system that can maintain its inputs at whatever momentary reference
value exists for the system. In the general case, which is the only one
worth pursuing in a general model, there will always be variations in
the reference signal at the same time that independent disturbing
variables are acting to alter the controlled input.
It's possible to consider special cases -- disturbance-free environment,
disturbances that occur only in one predictable pattern, constant
reference signals, square-wave variations, and so forth. But giving
different names to the system for each case hardly seems necessary, when
one basic model handles them all. It is possible that for special cases,
there are ways of optimizing the parameters of a system so it works best
for that case. But from all that I have seen, the differences in
parameters and in performance are minor and hardly worth making into an
issue. There is no difference in architecture between what you call a
"stabilizing" control system and a "regulating" control system. And in
human beings, EVERY control system must be usable for BOTH applications.
The only real issue here is whether we ought to use the word control
when talking about open-loop effects. In all such cases of which I know
where the word "control" is used, the loop is actually closed, but the
path by which it is closed is taken for granted and assumed to be
outside the range of the discussion. That is, one can make a case for
open-loop control only by ignoring the fact that someone or something,
somehow, must observe the output, compare it with a desired output, and
adjust the system so it continues to produce that output. True open-loop
control, in which the output is never observed or the observation is
never fed back to modify the system, is impossible. You couldn't even
set the system up to work properly.
Finally, Hans, don't forget that even though your major was control
engineering, you still learned what you know from other people, and they
learned what they know from still others. Wrong ideas are taught quite
as often as right ones, and errors can be propagated all too easily in
that way. There are, after all, departments of psychology in which all
sorts of ludicrous notions are passed solemnly on to succeeding
generations, all with the assurance that they have the stamp of
scientific respectability. It is possible that some of your mentors had
an ideosyncratic concept of what control means, and managed to persuade
you that it was universally accepted and objectively right. But that
doesn't mean they had a good definition of control -- only that they
insisted that you agree with it if you wanted to get a degree.
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Best,
Bill P.