I sent this quite a bit earlier. It hasn’t shown up but other messages have since then so I’m resending it.
[From Fred Nickols (2015.04.05.0644 EDT)]
I think we can and do take steps to protect against disturbances but doing so requires forethought, conscious thinking and deliberate action. I don’t think the crosswinds example is a good fit with “protect.” It seems a much better fit with “counter.” To protect is to guard against what would happen and that is to imagine or envision some future state that we don’t want and we act so as to head it off. That is what it means to “protect” – to guard against.
When I’m driving my car and crosswinds are buffeting it I generally keep the wheel turned a little farther against the wind than I would if the wind weren’t blowing it. If the wind stops I have to move the wheel the opposite direction or else I will veer from my lane. So if the controlled variable is my car’s position in the lane and the wind disturbs my reference for that variable (e.g., centered in the lane) I compensate by turning the wheel. I don’t turn the wheel against imagined wind. The wind is a disturbance and when it gusts my car moves and my sensing that movement is the occasion to correct for its effects.
I think the proof of this pudding is that in gusty crosswinds the actual position of my car varies with respect to its reference state more than it does when the wind is not blowing.
I’ll also admit that this is a somewhat complicated situation because compensating or correcting for or countering the effects of the wind does in fact protect against my car getting too far off center (unless of course the wind overwhelms my attempts to keep it in its lane).
Fred Nickols
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From: Richard Marken (rsmarken@gmail.com via csgnet Mailing List) [mailto:csgnet@lists.illinois.edu]
Sent: Saturday, April 04, 2015 2:05 PM
To: csgnet@lists.illinois.edu
Subject: Re: Re: Draft revision of entry on control theory
[From Rick Marken (2015.04.04.1100)]
On Sat, Apr 4, 2015 at 1:36 AM, “Boris Hartman” csgnet@lists.illinois.edu wrote:
HB: You must be a magician. O.K. show me please how can I protect course (path) of my car on the road from disturbances – cross wind ?
RM: By varying the steering wheel appropriately to counter the effects of the cross-wind on the car’s path.
HB: Did I understand right that you are saying that i don’t COUNTER-ACT or CANCEL THE EFFECTS of disturbances (cross-wind)
RM: No, you didn’t understand me right. You protect a controlled variable from disturbances by acting so as to counter the net effect that disturbances would have had on the controlled variable. Saying that a control system “protects” a controlled variable is just another way of saying that a control system acts to counter the effects these disturbances would have had. I like describing it as “protection” from disturbance because it has less of an S-R flavor to it than describing it as “countering”. Describing disturbance resistance as “countering” sounds a bit like saying that the countering actions are a reaction to the disturbance, which they are not.
Regards,
Fred Nickols, CPT
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