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:
a message dated 09/03/03, dyouden@......... writes:
As for the Australian references,
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
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.
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
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
The operating frequency is 50 Khz, and yes, it is a phase sensitive
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 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
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
use of aquarium cement comes from Roger Baker's work on his
Gravimeter/Seismometer. Here's a quote from his web site, which is
.. 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.
Have a good one,
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