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 period.

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 data.

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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