PSN-L Email List Message

Subject: Lehman notes
From: S-T Morrissey sean@...........
Date: Wed, 12 Jan 2000 01:04:42 -0600 (CST)

Ted and  Jim and co.

Regarding the boom/mass/coil/hinge questions.

Symmetry is very important in the design of the boom. The geometric
center of the mass and of the coil(s) should be carefully aligned 
with boom axis. This is important since the damping force of the 
coil should act on the center of mass of the mass and boom; otherwise 
torques about the boom axis will result. For perfect symmetry, a
"dummy" coil form or equalavent mass is often installed on the opposite 
side of the mass from the main signal coil. Sometimes tis is an
independent calibration coil and magnet, or a second signal coil.

A boom suspension made with an inverted Y wire yoke, with the split
lower ends attached to the widest portions of the mass, will help
control any rotations and keep the coil aligned in the magnet gap.
At the upper mast end the Y wire goes through a saddle that is 
attached to the mast top by a fine wire as the upper pivot.

The magnet MUST be mounted to the frame if good resolution and 
low noise are expected.  Some attempts at a moving magnet seis
were made, but even with massive shielding, like the 1/4" steel tank
that is the cover for the Johnson-Mattheson seis, geomagnetic and
cultural magnetics (from steel boot shanks to hunting knives) were 
still a problem.

I would certainly go the extra effort of using a flat flexure or
taught wire for the boom pivot, rather than any bearing device.
The mast end of the boom must fork and pass the mast so the taught
wire pulls from the back.  An example is shown on my web site in the 
diagram of the classic Sprengnether long-period horizontal.



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Larry Cochrane <cochrane@..............>