Karl Thanks for the very fine reply. The Stuttgsrt links are: http://www.geophys.uni-stuttgart.de/seismometry/man_html/index.html and http://www.geophys.uni-stuttgart.de/seismometry/hbk_html/index.html or if I got some things wrong just go to http://www.geophys.uni-stuttgart.de 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 tschmitt@.............. ----- Original Message ----- From: Karl CunninghamTo: 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 > http://psn.quake.net/maillist.html#archives 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 http://www.eas.slu.edu/People/STMorrissey/ There have > been several variations of this built by list members. > > And you can see my design at http://www.jps.net/karlc > > 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 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? > > > >Thanks > >Tom Schmitt > >tschmitt@.............. > > > >__________________________________________________________ > > > >Public Seismic Network Mailing List (PSN-L) > > > >To leave this list email PSN-L-REQUEST@.............. with > >the body of the message (first line only): unsubscribe > >See http://www.seismicnet.com/maillist.html for more information. > > > > > __________________________________________________________ > > Public Seismic Network Mailing List (PSN-L) > > To leave this list email PSN-L-REQUEST@.............. with > the body of the message (first line only): unsubscribe > See http://www.seismicnet.com/maillist.html for more information. __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>