[Avery Andrews 930925.1230]
Chris,
I think I better spend some time with this literature before venturing
any more remarks about it, but there is a little issue I'd be interested
in your opinion on (the lead-up is a bit on the long side, but the
actual quesiton reasonable short). In the motor control literature, which
I have spent some time on, there does seem to be some kind of campaign against
feedback going on, which leads to some assertions being made that
seem pretty odd to me (and tend to inflame Rick Marken). For
example, Abbs and Winstein, who have done a lot of work on
`distributed compensatory responses' (two effectors are involved
in producing an effect, such as a thumb and finger forming a grip,
the movement of one is inhibited, so the other goes further to
compensate), seem determined to deny that this is an example of
feedback.
They have a paper (1990: `Functional Contributions of Rapid and Automatic
Sensory-Based Adjustments to Motor Output', in Jeannerod, M (ed)
_Attention and Performance_ XIII, 627-652, which summarizes a lot
of information about the speed of various feedback loops (refuting
claims that were commonly made by motor control people in the late
seventies and early eighties that feedback was `too slow' to be
useful for various things).
But then they go on, in a section headed `Contributions outside the
closed-loop model' (p. 635):
"Technically, a _feedback_ sysem is one which an error signal directly
drives a corrective adjustment at the site where that error is
introduced. Recent experimental data indicate that many (or most)
important contributions of sensory informat to the on line control of
movement are not captured by this fundamental construct."
The intended effect of this is to put distributed compensatory responses
(their work, and other work by Carol Fowler) outside the `closed
loop model'. This seems seriously irrational to us, since it is
perfectly easy to design a feedback system to control an aperture
by sensing the difference between the desired and actual gap, and
sending appropriate signals to all the relevant effectors -- if one
is frozen, the other will just go further and faster to get the desired
result.
More seriously, and the real point of this longish story, is whether
this `technical' concept of what a feedback system is corresponds to
anything in your technical education. It isn't recognized by anybody
on CSGNet with a relevant background, and they cite no source for it.
Furthermore, I can't make any sense of it, and, the only interpretation
that sort of makes sense would exclude everybody's favorite example,
the home thermostat (disturbance introduced in the hall, when the front
door opens, corrected in the baseement, when the furnace comes on).
If you know where this `technical' concept came from, and/or know what
it is supposed to mean, I'd be very interested in hearing. Thanks.
Avery