PSN-L Email List Message

Subject: resistive damping
From: S-T Morrissey sean@...........
Date: Tue, 9 Nov 1999 12:18:39 -0600 (CST)



Marchal,

Taking into mind what Jim H. has pointed out, resistive damping is
a much simpler approach than mechanical eddy-current damping in that
it does not require any additional construction on your seismometer.

But, as Jim points out, it does require a strongly coupled coil-magnet
arrangement, and it also requires that the resistance of the coil
be low enough to pass the damping current. Some long-period seismometers
were equipped with 100 000 ohm coils to produce lots of output voltage,
but had to be provided with separate 500 ohm coils for damping.

So how do you use resistive damping? You can plug parameters into
the formulas, or you can experiment. The damping resistor is simply
connected across the output terminals of the signal coil. If you
use a speaker coil/magnet with a 4 ohm coil, the damping resistor
has to be connected right at the coil, since the external wiring
resistance will be more than the 4 ohms needed to damp the seis.

To see if your coil/magnet is strong enough to do the job, simply
short it at the output terminal. Introduce a small offset and watch
the mass move: it should be very sluggish, almost like it was dragging.

Then you can replace the short at the output with trial resistors and
again observe the effect. I would start with a resistor about equal in 
value to the resistance of the coil, which you should measure. If the coil
is 500 ohms, I would try values from 300 to 1500 ohms in about 100
to 200 ohm steps, or use a variable resistor.

With proper damping, the mass should at first move quickly about half 
the distance it will eventually travel, then slow down and stop and 
move backwards just ever so slightly. You can observe this with the 
seismometer pointer or in the output of your digitizer. A very slight 
(about 3%) overshoot will indicate critical damping, which provides
the most output but a very narrow-band response. Less overshoot will
be slightly overdamped, with a broader response (a 15 second seis will
more easily record 60 second waves), but at a reduced output, which 
should be no problem with additional amplifier gain..

If you cannot get critical damping with even very low resistance, like
100 ohms, either the coil resistance is too high from using too much
very fine wire, or the winding doesn't adequately fill the air gap of
the magnet. The air gap must be predominately filled with turns of
the coil and not the form for the coil or lots of clearance. The coil
must be wound slowly turn-beside-turn, not scramble wound, to get the
maximum number of turns inside the gap. And any form used must take
up a minimum of space and fit closely to the magnet, preferably with
clearances of less than 1 mm.

I will later describe a method I use to wind a very compact coil
on a temporary form that allows only windings of the coil to fill
the magnet gap.

Regards,
Sean-Thomas

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