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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: Thu, 14 Nov 2002 15:04:53 GMT
NNTP-Posting-Date: Thu, 14 Nov 2002 07:04:53 PST
> see how your question finally has developed to a high-class discussion,
> where everyone gets out these dusty DSP books and carefully checks his
> arguments before sending them.
> There are at least 3 main themes,
> 1. history
> 2. feedback
> 3. digital(with and without fb) vs. analog
> I personally enjoy mainly point3. which is where it is happening now.
> Certainly digital signal processing will revolutionize our technical
> evolution as much as the invention of the transistor or operational
> More and more new ideas like adaptive, convolution and lattice filters are
> being refined to accomplish formerly impossible tasks.
> On the other hand Chuck is right when promoting IIR-filters.
> A minimum phase filter has the fastest possible response time and that is
> what analog filters are (hopefully).
> Only an IIR-filter can be equal, and then also the phase response will be
> similar to the analog counterpart. (which means high frequencies will be
> delayed less)
There is a practical problem that comes up in digital control of
dynamical systems. Optimizing sampling, conversion and calculation,
there is still a small delay between the sampling instant and the output
of the command. I generally like to keep this at 10% of the sample
period or less although I have allowed this to go as high as 20%. The
problem is that the delay produces a non-minimum phase zero which is
slightly embarrassing for settling. 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.
However, aside from doing an analog implementation, the IIR does indeed
allow me to meet goals in control systems. I must accept the small
settling difficulty as the price of a versatile controller with the
ability to handle complex exceptional conditions.
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 computationally clumsy
and strategies for changing IIR coefficients on the fly with minimum
glitch are interesting. Back in the 80s I had one of my engineers going
for a masters and he took on this question for a thesis. In particular
he looked at conditions to switch from a stable controller to an
unstable controller (a PID controller is unstable due to the integral
action). The results, though unsurprising, were useful.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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