from Dick Robertson,2009.12.23.1240CDT- I think]
[From Bill Powers (2009.12.23.0812 MDT)]
Bruce Abbott (2009.12.13.0820 EST) –
BA: Pure feedforward systems can work well if the effects of
the disturbance on the variable to be controlled are predictable
and there are no other significant disturbances that go undetected and thus
uncorrected. That’s a tall order in most cases, however; consequently a practical
system is likely to include both feedforward and feedback.BP: Well, yes, it is a tall order – you might as well say “…
can
work well if everything in the environment is arranged so it can
work
well.” I wouldn’t dispute that. Unfortunately, that usually
takes a
lot of arranging as you note: a tall order.The temperature-compensated pendulum clock was a very nice
application of feedforward. A column of mercury (as in a
thermometer)
mounted on a clock pendulum would expand upward, moving mass
toward
the pivot to compensate for the increase in length of the other
material in the pendulum as temperature increased. That was in
1721
(good old Google). It produced clocks accurate to one second a
week
instead of one second per day.
That’s neat. I didn’t know that.
By freezing, and not liking it. There is feedback; it’s just
delayed
a bit and averaged over a number of trials. A feedforward system
has
no feedback; its input is not affected by its output.BA: But once outside, I may discover that it’s actually warmer
than
I thought it would be and, as I begin to heat up, I take the
coat
off. I would venture to guess that most examples of feedforward
are
like this. That is, feedforward is piggybacked onto a negative
feedback control system.
BA: Another common example involving feedforward is classical
conditioning. For example, in eyeblink conditioning, a tone
immediately precedes a puff of air directed onto the cornea of
the >eye. The puff of air stimulates sensors in the cornea, and
ordinary negative feedback control acts to close the eyelid to counteract the
effect of this disturbance to the corneal sensations. But after
a sufficient number of pairings between the signal predicting the
puff of air (the tone) and the air puff, the tone becomes effective
at eliciting the same closure of the eyelid, in advance of the air
puff. The tone has become a feedforward signal and the result
is to further limit the effect of the air puff on those corneal sensations.BP: I would say that the perceptual input function has been reorganized to add the tone to its definition of the controlled variable. It perceives and controls “puff + tone → food” instead of just “puff → food”. This is not an open-loop system: the
reaction to “puff + tone” definitely feeds back to affect “puff + tone.”
A disturbance of either individual variable will result in action that opposes the change in the composite variable. And of course the initial reaction to “puff” is probably a reaction to the drying effect of the puff, with the blink restoring the wetness. Resistance to cooling also might play a part.I expect Dick Robertson to chime in here with his observations
Well, OK, here is what we reported in IMP I: "Some early students of control- theory psychology became interested in repeating a classical laboratory experiment to determine whether they could identify the controlled variable in a simple control system. The classic “eyeblink reflex” was chosen… There is a legend among psychologists that it provides a nice illustration of the conditioning of a reflex. The apparatus is simple [chin rest, pipette next to S’s side of face; tube with rubber ball to send puff of air to S’s eyeball] + making an audible click before the puff of air.
My students set up this experiment with one added feature. The asked the S to work arithmetic problems which were on the blackboard in the foom, telling him/her that they were studying wht\ether the process is affected by the experimental conditions, which they did not clarify. This complication was imposed in hopes of keeping the subject from attending to the airblast as something to be concerned about.
The first S performed as expected. After a time, he blinked even when the airblast did not come. However, with the second S something different happened. He began to blink more frequently, but randomly as related to the time of the airblast.
The explanation for these differences draw upon higher levels of control, in which the eyeblink is only the final output. The S who showed conditioning turned out upon follow-up to have accepted the task as it was construed for him by the experimenters He tried to go ahead working the problems, paying as little attention to…distractions as possible. The other S said he quickly noticed the distraction, figured that it played a part in the experiment, and assumed that he was to get the distraction “under control.” He did this by resetting his reference signal for more frequent blinking.
The perception of the eyeblink as the main subject of interest was from the experimenters’ point of view, and it did n’t take into account the fact that each of these subjects constructed a different control system for the (different) variable defined by his [S’s] purpose.
[and we renamed the the experimental variable under control in terms of a “corneal-lubricating control system.]”
For what it’s worth, I consulted one of my colleagures who usually taught exper-psych, about the eyeblink reflex setup. He said, as I recall, “Yes, it’s a difficult experiment to get consistent results from.”
Best,
Dick R.