PSN-L Email List Message
Subject: Re: Homade Geophone
From: "Charles R. Patton" charles.r.patton@........
Date: Thu, 04 Aug 2005 20:14:49 -0700
I like the idea of optical shutters/slits. Back to Roger Baker's
optical laser setups. Of course, when I mentioned single pole, I didn't
mean a monopole, I just meant the N or S end of a bar magnet set so
that the end is pointing up into the bob with the coils orthogonal.
That's what comes of being too brief. As you point out, it is much more
practical to mount the coils at some distance from each other on the bob
and parallel to the ground, and then use a horseshoe magnet or multiple
poles to excite both sides of the coil -- double your output, double
your pleasure! The only down side, is you might want to balance the
weight about the torsion rotation point so that linear gusts of air
don't turn into torsion oscillations. If you really wanted to get
trick, you could use a pair of coils per axis, and set them up for
hum-bucking, while the magnets (two) are set up for aiding. Little or
no shielding required as long as the noise source isn't too near (causes
non-uniform field and poor cancellation.)
>In a message dated 05/08/2005, charles.r.patton@........ writes:
>This has two problems.
>1) Geometrically the difficulty is to have an output for any direction
>movement. It would be easier to have two coils set for orthogonal output then do
>the vectorial addition in the PC (which would take two A/D channels also) .
>The PC could also give direction at the same time.
>It can be done with analog multipliers also if you're a bit of a
> However, it is fairly easy to make an orthogonal sensor using a square
>end mirror and four 8 sq mm photo diodes. I can get a noise level of about 15
>nano metres. Maybe a square hole in an optical shutter with a light on one
>side and the photo diodes on the other?
>2) Having the magnet on the boom makes you very subject to variable
>magnetic fields all around, so then it becomes were you measuring a
>magnetic field variation or an earth movement.
> Unless the field is enclosed, you will pick up a lot of AC power
>interference. Even with just a soft iron weight, you will pick up some unwanted
>So I would suggest you set up your Foucault pendulum with two orthogonal
>coils standing up, and put the single magnet pole on the floor pointing directly
> into the bottom of the bob.
> ?? My magnets all come with two poles. You could use two rectangular
>coils and two sets of N+S NdFeB bar magnets pairs, at right angles on the end of
>the bob. Maybe with soft iron plates top and bottom and mild steel bolts to
>enclose the fields?
> I would prefer to put the coils on the end of a brass bob weight and sit
>the magnets and the soft iron plates comfortably on the ground.
> Chris Chapman
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