PSN-L Email List Message

Subject: Re: Force Balence seismometer
From: "Tom Schmitt" tschmitt@..............
Date: Tue, 20 Jun 2000 13:41:10 -0400


Thanks for the very fine reply.

The Stuttgsrt links are:

or if I got some things wrong just go to

and work through from their research/forshung pages to seismometry.

There is a discussion of capacitive sensors and a bibliography that incudes
articles on design, which I have not read yet.

I hope to get down to GaTech next week to photcopy them.

Tom Schmitt

----- Original Message -----
From: Karl Cunningham 
Sent: Tuesday, June 20, 2000 12:35 PM
Subject: Re: Force Balence seismometer

> 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
> 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:
> >Question
> >
> >I am considering building a force balance seismometer. I have some
> >questions>
> >
> >(1) The recommended position sensor is a capacitive bridge, the details
> >on the Uni-Stuttgart geophysics page. Has anyone on the list done this?
> >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?
> >
> >Thanks
> >Tom Schmitt
> >tschmitt@..............
> >
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Larry Cochrane <cochrane@..............>