Tim Wescott <[email protected]> wrote in
news:[email protected]:

> I can either say "that's very clever of you to have caught that", or I
> can fess up to leaving an "almost" out of there somewhere.
>
> Certainly the performance of many control loops out there are limited by
> delay, and the performance of almost all "high performance" control
> loops ends up getting limited by the phase shift in the plant, sensors
> or controller.
>
> --
>


Most likely the latter. I can remember spending considerable time in class
figuring out just how must delay a feedback system can tolerate before
going unstable, but "almost" all of them could tolerate "some", if I recall
correctly, based upon just when the phase would kick a nice negative
feedback into the much less pleasant positive variety. Of course, delay
generally hurts performance, and often simple stability is much less than
what one needs.

While this example is extremely nonlinear, one of my favorite lab exercises
in my biomedical engineering course was simply using a pot to zero a chart
recorder (yeah, messy ink, green paper, clogged pens.... that's the stuff!)
line, but the pot was wired through a delay line. We changed the delay
until we started to oscillate. Because our brains tend to "sample" in
strange ways, and we produce semi-ballistic responses, people did
considerably better than a run of the mill feedback system would do in the
same situation. Wonderfully laid out lab, though.


--
Scott
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