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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: Wed, 13 Nov 2002 16:19:58 GMT
NNTP-Posting-Date: Wed, 13 Nov 2002 08:19:58 PST
> "Chuck Simmons" wrote in message
> > Ban wrote:
> > > digital circuits work with saturated transistors, no feedback there. A
> > > filter does not use feedback.
> > I beg your pardon, in an FIR filter, what is the output term(s) doing in
> > the equation if not feedback?
> Depends on the filter. In the filter Ban's thinking of, the output is a
> function of the inputs, but not the ouput - it's strictly feedforward. In
> the more general digital filter structure, of course, there are feedforward
> and feedback terms.
I really should have been more awake. IIR is what is used for control
systems. This is because the output from an IRR digital filter at any
iteration requires at most one multiplication and one addition. This is
because the filter can be computed but for the last sample in the
interval from output to sample. FIR approximations of a typical IIR are
unusable in control because the order is much higher for the
approximation (theoretically infinite) and the calculation time gets to
be impossible. It is easy to show that linear systems of order two or
greater cannot be controlled with FIR filters (compensators) in general.
However, appropriate IIR filters are quite simple and easy to design for
typical compensators of this kind.
BTW, delay in digital control systems generally means that performance
is poorer than analog controllers. For example, computational delay
produces increased overshoot and reduction of stability margins. I know
of no case in digital control of dynamical systems where digital control
gives better performance than analog control except in nonlinear
There are some papers from the 80s by Tomizuka et al. at Berkeley CA
that examine high order FIR filters to deal with repeatable error in
control systems. This is a feedback technique in realization. It is
pretty difficult in that the filter may be order 100 or greater (I was
looking at order 120 the last time I worked on this) and with feedback,
stability was very hard to determine. I did get it to work well at order
32 years ago. The main problem is computing the filter itself because
the order of that computation is several thousand and numerical
stability is absolutely out to lunch. Simulating such systems in Matlab
totally fails. All of my work was experimental given the failure of
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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