PSN-L Email List Message

Subject: Re: Fw: Folded Pendulum Seismometer
From: "David H. Youden" dyouden@.........
Date: Tue, 11 Mar 2003 06:08:30 -0500

At 02:05 AM 3/10/03 -0500, you wrote:
In a message dated 09/03/03, dyouden@......... writes:

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.

Hi Dave,

        Thanks very much for the references. When I call up the Woo reference and go to 'Seismographic Instrumentation', I just get empty squares instead of diagrams or photos. Do you get the same problems? Also, I can't print the text directly. I have to copy it and then print it. There must be something odd about the text, since I have not had this problem before.

        I have experienced the same problem. I think that the server is just very slow at downloading the diagrams. As for the text, I had to do just what you did - cut and paste the paper together. A pain, but worth the effort, I thought.

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.

        I would certainly be interested, please. There are few modern circuits about and an up to date capacitative transducer would be a valuable addition. John Lahr has a very useful website and is most helpful.

Both Larry, and John Lahr have agreed to post the electronics, so as soon as I get them in shape you'll have the documentation.

        What is the operating frequency of the transducer? Is it a phase sensitive detector?

The operating frequency is 50 Khz, and yes, it is a phase sensitive detector.

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 characterising 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.

        You do not use a triple feedback loop then? What seismic mass do you use?
        I am not familiar with the term 'centre of percussion'?

By triple feedback I presume you mean PID. I messed with a PID loop, but could see no real advantage, so I have used a proportional-differential system.

The total mass of the carriage and the adjustable weight is about 550 grams, not much but enough for the purpose.

For a discussion of "center of percussion" see
It's not a difficult subject and has real implications for pendulum design.

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.

        Roger had plenty of enthusiasm, but his work seemed to be over hyped by Carlson at Scientific American. He was right about a lot of things, but also wrong about several.

        You can get a polyurethane mastic called Sikaflex, used on yachts, which sets by moisture, is extremely strong and sticks most things. You can also stick glass with epoxy and get extremely strong joints. Epoxy does not stick to water at all well and cold glass has several layers of the stuff. The trick is to bake the glass to 150 C in an electric oven, take it out with cotton gloves and immediately stick it with a slow setting epoxy. Al also benefits from this treatment.

Thanks for this tip. I'll try it. I'm sure that it will work.

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

      It helps considerably, thank you. That would be great.


      Chris Chapman

Have a good one,

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