PSN-L Email List Message

Subject: Re: Fw: Folded Pendulum Seismometer
From: "David H. Youden" dyouden@.........
Date: Sun, 09 Mar 2003 07:34:56 -0500


I'll try to answer your questions one at a time.

As for the Australian references, try        Lai Mun Woo has done interesting research. She is a student of D.G. Blair. Her page entitled references lists critical papers, notably "Performance of an ultra low-frequency folded pendulum" by D. G Blair. This paper is not available on the web as far as I know, but any good library (books, remember them?) can get it for you.

Also look at      This is a LIGO website which has the necessary math.

For a different approach, go to  A UWA site about the Roberts linkage.

For the circuit, If Larry gives his OK I'll send him a document that details the circuitry that I used. It'll take a couple of days to prepare it for publication, but I will gladly do it if there is interest. Basically, it is the same stuff that is used for an S-G instrument, except for the capacitance gauge stuff.

My information on setup and performance is limited. Basically setup involves levelling the instrument and then adjusting the counterweight to achieve the maximum sustainable period of oscillation (With the feedback turned off). Then you connect the feedback and adjust its level for critical damping. I have not spent a lot of time characterizing the performance of the device. It is more sensitive than the big Lehman that sits next to it, but beyond that, there is little that I can tell you.

My use of aquarium cement comes from Roger Baker's work on his Gravimeter/Seismometer. Here's a quote from his web site, which is currently at .. I built a copy of his rig, but was unhappy with its performance.

Note: One unusual feature of the construction of practically my entire instrument is the use of silicone rubber as an adhesive, frequently in combination with glass. Many kinds of instrumental prototypes can be constructed with nothing more than glass carefully cut with a carbide wheel cutter, and perhaps ground with a diamond wheel to give a precision fit, and finally bonded with silicone rubber to to give a permanent and slightly flexible bond. Most of the metal parts can be cut with a jeweler's saw and soldered as needed.
Whereas some be tempted to think that prototypes made in this way would be floppy instead of rigid, this is not so. The thing to keep in mind is that glass is cheap, very easy to cut, easy to grind to fit, and very stiff and permanent, and that thixotropic silicone rubber makes setup easy while giving a nearly perfect bond. If there are rules to be kept in mind when doing such work, they are probably the need to use enough pressure always keep the rubber bonds very thin, and to make use of right angle pieces of glass to brace the various components of the instrument. If this is all kept in mind it will be found possible to rapidly build prototypes that are just as rigid and more permanent, in many cases, than similar structures made of metal, and also rapidly and with very few tools being required. It is possible to force a razor blade into the bond and disassemble and then reassemble the various components in a slightly different position if necessary."

I hope this all helps, Stay in touch, and, with Larry's approval,  I'll do something about the electronics.


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