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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Historical question: negative feedback and the op amp
Date: Fri, 15 Nov 2002 03:32:52 GMT
NNTP-Posting-Date: Thu, 14 Nov 2002 19:32:52 PST
John Woodgate wrote:
> I read in sci.electronics.design that Chuck Simmons
> wrote (in <3DD3BBC3.6430E797@webaccess.net>)
> about 'Historical question: negative feedback and the op amp', on Thu,
> 14 Nov 2002:
> >The problem comes about because you
> >cannot control pole locations and sero locations at the same time. Since
> >pole locations determine stability, it is necessary to let the zeros
> >fall where they may.
> Do you always have too few degrees of freedom to fix everything
This depends on what you mean by optimally but yes. The problem comes
from the fact that when you look at the rational function that pops out
of the eigenvalue computation for the system, you usually find that the
degrees of the numerator and denominator are either the same or differ
by one (the latter typically comes from the plant). This simply means
that you can fix poles or zeros but not both. You can do a similar
analysis with the matrix representation of the system but it is not
quite so slick.
> >There is one other case somewhat interesting to me which is changing
> >from one control law to another on the fly. Generally, this can be done
> >smoothly with a full state space implementation so that the old states
> >can be plugged into the new control law.
> This is sort of like drawing a complex curve with French curves.
It is but it is non-intuitive but true that slowly changing the control
law from one to another where every intermediate controller is itself
stable can be unstable. The stability of an abrupt change in control law
is actually a simpler problem.
My young engineer got his master's degree and my group got useful
information for one of our immediate projects so we were all happy
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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