PSN-L Email List Message

Subject: feedback semantics
From: Randall Peters PETERS_RD@..........
Date: Mon, 11 Feb 2008 18:18:31 -0500

    I am not very familiar with the terminology used in the world of engineering controls.
My term 'soft' for the feedback scheme that I have used seemed reasonable to me for the
following reasons:  (i) it is as you noted, happerning at a much lower frequency than most of
the signals of interest (say teleseisms at 20 s period,  and (ii) it is nowhere near as
powerful as the forces required to do force-balance; i.e., keep the mass from ostensibly
    You mention the matter of using feedback with a pendulum.  I can't imagine a reasonably
simple pendulum for seismic purposes where feedback would ever be necessary.  The primary
source of motion at very low frequencies is the change in shape of the earth.  Every mass
part of our planet contributes to the local field, and so a plumb bob provides information
concerning eigenmodes and tides (as the VolksMeter has demonstrated).
    Your question about my use of the word 'noise' relative to force balance
systems--anything that works with the derivative of the position of the inertial mass (flat
to velocity sensor) will fail to see earth motions at really long periods (starting around a
few thousand seconds or even less).  Even though the period of the VolksMeter's simple
pendulum is only about 1 s, it is well suited to the study of earth changes happening over
days, months, and even years.


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