PSN-L Email List Message

Subject: Re: Homade Geophone
From: John Popelish jpopelish@........
Date: Thu, 04 Aug 2005 20:00:06 -0400

Geoff wrote:
> Has Anyone ever made a horizontal Geophone
> using the pendulum idea with the magnet
> attached to as the weight of a pendulum
> and the coil fixed to the earth and
> free to move in all horizontal directions ?
> I would be interested in knowing what you
> used to dampen the mass and what overall
> gain you needed on the amplifier.
> The idea here is to get first time of arrival
> only and not worry about direction of ground motion.

I haven't gotten around to commissioning it yet, but I have built a 
geophone type structure horizontal seismometer.  That is, the magnet 
structure produces radial field that passed through the entire 
solenoidal coil.

I have the magnet structure mounted on the pendulum and the coil 
mounted to the base.  This makes it a moving coil design, since, 
during a quake, the pendulum stands approximately still and the Earth 
and the base move.

I made the magnet structure with a symmetrical pair of magnet sets, so 
there is no net external magnetic fields.  This prevents changes of 
the Earth's magnetic field moving the pendulum.

The coil moves relative to the magnets only in one axis.

Damping is easy.  A resistor across the coil can do that, or an 
additional conductive sheet (aluminum or copper) mounted near the 
outside of the magnet structure can produce damping drag.

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