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Subject: feedback limitations due to damping anharmonicity
From: Randall Peters PETERS_RD@..........
Date: Sat, 09 Feb 2008 14:35:10 -0500

    I don't see an obvioius 'showstopper' with what you've mentioned, but then I
certainly don't understand all the nuances.  Let me give you one of the thoughts that I
posed on this matter the better part of  a decade ago.  The following is copied from one
of my webpages:

3.3  Gedanken to illustrate that limitations exist
Lest one believe that force-balance feedback is infinitely superior to conventional
seismometry, consider the following logic. Why even bother with the leaf-spring that is
commonly used to support the test mass in these instruments? Why not just add a feedback
network to a solid state mass balance instrument that works with resistive strain gauges?
Place a big test mass on the pan of the modified mass balance (mmb), add a magnetic
transducer of some type to provide a significant lifting force on the mass, and
``voila''-with proper feedback adjustment we suddenly can see earthquakes with the
simplest of instruments. Hopefully everyone will quickly recognize the folly of this
reasoning and know that such a modified mass measuring instrument is not capable of
functioning as a bonafide seismometer. But why? The answer to this question lies in the
following observation. System adaptability is no better than the integrity of the
``spring'' used in generation of the error signal. As noted earlier, any error signal
requires the measurement of strain. In the case of the hypothetical modified mmb, the
``spring'', in the absence of feedback, has an exceedingly large k. In the case of the
W/S leaf-spring seismometer, the leaf has a considerably larger k than that of the
conventional seismometer. Can electronics soften even the hardest springs? The answer is
obviously no! What are the limitations to softening? I submit to the reader that there
are a host of unanswered questions in the matter. It is easy to see that electronics
limitations (addressed earlier) pose an ultimate upper limit on the size of k. But
anelasticity of the support is probably even more important than the electronics-and the
problems borne of it are mostly unstudied. This is true in spite of the fact that
practictioners understand that an instrument must be allowed to settle for some time
after initial loading, before it becomes dependable. This settling is necessary to
minimize the effects of anelasticity, through a type of work-hardening.

There is a factor in all this that is unavoidable and of much greater influence than I
ever expected until some experiments that I did in the last year--concerned with creep.
The results will be reported in the  Chapter 1 that I wrote titled "Building on old
foundations with new technologies", of Nova's "Science Education in the 21st
Century"--due out this quarter.
There is apparently no level below which creep isn't significant and it's influence
depends on which way the temperature was moving at the time the system is observed (total
temperature swings of only about 5 C over 24 h.)  I found significant, peculiar
disruptions due to creep at energy levels of the pendulum at only 10^(-12) J.  Bottom
line--engineering the feedback network to compensate for the multiplicity of anomalous
possibilities appears to me to be a staggering proposition.
    My own opinion is that it is best to let the seismometer 'find its own best
equilbirium', rather than forcing it into the 'one preferred by the chosen point of the
feedback circuitry'.  Why mess with mother nature's preference?

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