[Martin Taylor 970421 07:30]
Bruce Abbott (970420.2230 EST)
If r is fixed, d = 0, and output varies, then o is itself a
disturbance to p. Output varies because the residual effects
of earlier variations in d are still reverberating in the loop.
So what I said is _not_ false: if p wiggles, then p is being
disturbed. By definition.Actually, this is NOT the definition of disturbance. See Bill Powers
(970419.1830 MST).Disturbance it is; THE disturbance it isn't. When _I_ use a word, it means
what I want it to mean; that's why I pay it so well. (;->
I know you turn this into a joke, but it's not really a good idea. We've
had four or five uears of confusion because of a misunderstanding about
what is meant by the word "disturbance," a misunderstanding that should
have been cleared up in ten seconds. You risk another.
Let's not quibble over words; the reverberations of previous disturbance
appear in the form of output wiggles, which shake p from its tranquility.
If an action itself results in the production of further error, it is a
disturbance in my book. It makes more sense to me to call this a form of
disturbance when, for example, it leads to oscillations, damped or undamped.
After the first impulse, the system keeps disturbing _itself_.
Sure it does, in the everyday usage of the word. Not in the technical PCT
usage, though. In the technical PCT sense, "disturbance" and its related
phrases refer to one of the two external inputs to the loop.
The issue does raise a critical point about the behaviour of feedback loops,
though. A stable loop quickly damps out these self-induced fluctuations. If
they persist for long, it means that the loop's outputs depend more on the
loop parameters than on the input values (reference or disturbing influence).
A loop whose outputs depend mainly on its parameters is not a good control
loop.
Martin (with disturbed sleep)