PSN-L Email List Message

Subject: Re: Force Balence seismometer
From: Karl Cunningham karlc@.......
Date: Tue, 20 Jun 2000 09:35:13 -0700

Hi Tom --

Do you have the link to the Uni-Stuttgart page on capacitive transducers?

1)  I used an LVDT for my force balance seismometer.  It was small,
insensitive to electrostatic fields, and had low noise.  I am currently
experimenting with a capacitive transducer and I think it will eventually
be comparable in noise performance to the LVDT.  Others have used a VRDT
design with good results.

2) If you haven't already, check the archives at for discussions of magnetic
levitation.  Linearity, as you suggest, isn't crucial.  What is more of a
problem with magnetic levitation is its temperature drift and
susceptibility to changes in external magnetic fields (such as cars driving
by).  Even in seismometers that don't involve magnets, the problem of
external magnetic fields is serious enough that most designs try very hard
to use only materials that are not magnetic.

3) Rolling hinges tend to be zero-force devices.  Perhaps someone else can
suggest a source.  Flexures are used by a number of people with good
results.  They aren't zero-force, however.

The VBB seismometer design by Sean-Thomas Morrissey is a force-balance
design that can be built with readily available parts, and has impressive
performance.  See  There have
been several variations of this built by list members.

And you can see my design at

Karl Cunningham

At 10:47 AM 6/20/2000 -0400, you wrote:
>I am considering building a force balance seismometer. I have some
>(1) The recommended position sensor is a capacitive bridge, the details are
>on the Uni-Stuttgart geophysics page. Has anyone on the list done this? If
>so what problems, or more importantly, what solutions, ;) , evolved.
>(2) How important is linearity of the spring? My daughter asked me why we
>didn't float a beam using opposing magnets. That is a F = k(r^3) force to
>distance relationship and the perfect seismometer spring would have F =
>k(r). For vanishingly small displacements do the problems go away?
>(3) I seem to remember someone has a source of material for zero  force
>hinges.  Is it still available and who has it?
>Tom Schmitt
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Larry Cochrane <cochrane@..............>