PSN-L Email List Message
Subject: Re: seismometer performance
From: Brett Nordgren Brett3mr@.............
Date: Fri, 15 Feb 2008 22:40:57 -0500
At 09:13 PM 2/15/2008 -0500, you wrote:
> Brett, you did a beautiful job with your paper on force feedback. I can
> almost understand it! One issue discussed but unresolved is how to handle
> drift. Have you ever looked at the efforts of one of our PSN members,
> Allan Coleman. He has built a number of force feedback sensors of both
> horizontal and vertical types. His designs feature the use of motors to
> recenter the pendulum.
Thanks for the kind words. Yes, I had studied what Allan was doing, though
I should go back and see what's new. He's had some very interesting
ideas. I completely agree that some form of mechanical rebalancing process
will be essential, extremely so in a vertical. The real value of using
feedback to aid in centering is to reduce how often you need to
mechanically rebalance, and minimize zero point drift between adjustments.
If you think about it, any systematic mechanical rebalancing of the system
is also feedback, though it sometimes involves a human in the feedback
loop, and it will be a little hard to describe well with numbers.
If carried down to low enough frequency, integral feedback can go a long
way toward resisting instrument drift. This is 'just' a matter of making
the instrument force/acceleration response approach zero at very low
Unfortunately, there is dirty little secret about using R-C integral
feedback to resist drift error forces, and that is evident when the
integrator is 'straining' to cancel a fairly strong unbalance force, in
which event there will be a substantial voltage across the integrating
capacitor. The cap. has a temperature coefficient of C which is of the
same order of magnitude as the Temp. Coeff. of a steel spring i.e. pretty
large. Since in a feedback integrator the charge, Q in the cap. changes
relatively slowly and can be considered to be constant as a first
approximation, if its capacitance goes up with temperature, its voltage
goes down in proportion because of Q=CV thus introducing its own rather
large drift effect. It will only work as expected if the system is already
reasonably well balanced mechanically making the voltage across the cap.
not too large. Allan's got the right approach.
Thanks for Allan's links. I'll go take a look.
> His website is:
> You can also access it from a link in:
> Look for "Allan Coleman's seismometer designs".
> Force rebalance is a necessity for network sensors. All need a flat
> response that is known and stable. As for me, there is too much involved
> circuitry to cope with. I choose to use conventional open loop sensors of
> known period and damping, and then to make their response flat and
> broadband using my inverse filter program, "WQFilter.exe". This utility
> is available for download from
>PSN Station REM
>Locust Valley, NY
>The year's hottest artists on the red carpet at the Grammy Awards. Go to
My e-mail address above should be working, but if not
you can always use my mail form at: http://bnordgren.org/contactB.html
using your Web browser.
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