PSN-L Email List Message
Subject: Re: A Free Mass seismometer
From: "David H. Youden" dyouden@.........
Date: Sun, 06 Apr 2003 06:18:00 -0400
At 09:45 AM 4/5/03 -0800, you wrote:
Is there any force keeping the free mass centered? Air flow
changes? If so then you are back to having a natural
There is no centering force other than that
generated by whatever driver is installed for force feedback. Sometimes
the air bearings can produce a slight decentering force, but this looks
like tilt and can be compensated.
I'm not a fluids guy, but I
wonder if small scale turbulence in air flow would cause an unpredictable
wandering and the fall to the floor you mention. Is there an active
way to keep the thing centered, maybe radiation pressure from
lasers? Any active correction could be measured and removed from
The air flow is laminar, and measurements that I
have made on similar, far more expensive systems indicate no change in
the noise at the picometer level when the bearings are turned on. The
fall to the floor is caused by tilt, as these things are, as you can
imagine, extremely sensitive to tilt.
David H. Youden wrote:
I have been thinking ( a dangerous thing, to be sure) about a seismometer
that has no natural frequency. Perhaps (probably) this idea is not new,
but never-the-less, here it is:
Imagine, if you will, a stack of, say 1" diameter aluminum tubes . .
A day without math is like a day without sunshine.
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