PSN-L Email List Message

Subject: Re: stability of a Lehman
From: "Larry Cochrane" cochrane@..............
Date: Tue, 19 Nov 2002 16:18:09 -0800


Long period sensors like a Lehman are sensitive to ground tilt. The longer
the period the more sensitive it is to tilt changes. What happens is the
boom wonders around when the tilt of the ground changes. The changing of the
position of the boom effects the sensitivity since the distance between the
magnet and pickup coil changes over time.

My Lehman has even more of a problem because it is sitting on a wood floor.
Whenever the humidity changes I need to re adjust the sensor. At this point
I have given up on my Lehman do to this problem. I just don't have the time
to constantly monitor the sensor. To record teleseismic events I use my SG
sensor and for local events either a geophone or accelerometer.

Larry Cochrane
Redwood City, PSN

----- Original Message -----
From: "Charles R. Patton" 
Sent: Tuesday, November 19, 2002 9:00 AM
Subject: stability of a Lehman

> There have been numerous discussions about lengthening the period of a
> Lehman beyond the approximately 10 sec period and it becomes
> "unstable."
> The question is - unstable in what way?  The period changes, the
> centering fails, sensitivity goes haywire?  In particular, if the answer
> is, "The centering fails," does the center just move around, or does the
> beam "flop" to one side or the other?
> I'm doing some thought experiments during my long commute, and I'm
> thinking about the sources of error.  In particular it's been mentioned
> before that the center of rotation changes on the flexible hinge
> designs.  That's intuitive, but how much and in what direction - toward
> stability or instability? Has anybody figured out the actual path of the
> center of rotation for a typical Lehman?  Crossed-X flex hinges were
> mentioned, but in this application with side force, I don't think they
> would be any less susceptible to the same de-centering.
> Another question is the upright's rigidity.  Several pounds of weight at
> the end of a boom is a fair torque moment which is resisted by the
> spring constant of the 'pipe' (in the pipe constructed versions)
> vertical.  How much does this constant vary with temperature in standard
> steel?
> Anyway, if there is an answer I should have looked up in the past posts,
> just flame me, but please include an approximate date and subject header
> I can go looking for!
> Thanks,
> Charles R. Patton
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