PSN-L Email List Message

Subject: Re: Setting magnetic damping level
From: "GMV" gmvoeth@...........
Date: Sun, 22 Feb 2009 06:53:33 -0700

Larry is certainly the one to talk to.
he builds these things.
All I know is what a Geophysicist named
Willis Jacobs from the usgs ( retired ) told me and that is you want
significant damping like if you drop the mass
the signal on the opposite side of zero will
be only 1/10 the amplitude of the the initial
signal on the positive side.
I have heard you can pulse the geophone with a
tiny voltage and watch its response.
When you apply the voltage ( microvolts?)
it will raise the mass ever so slightly
it is when you remove the voltage that
gives you the impulse to watch.

In my personal opinion you want the damping at
1.414 and the Q at 1/1.414 if the electronics support
that or you want very heavy damping that straightens
the slope to like 2 HZ then use a straight forward
amplifier to both equalize and make up the losses.
If your Q is greater than 0.7071 area it seems
the mass will not stop oscillating fast enough to
satisfy the geophysics peoples who are really
the ones who know how to read the graphs.
Matching the geophone to the electronics
is kind of the secret to decent pictures and
its not as easy or cheap as it seems it should be.


----- Original Message ----- 
From: "Robert O. Green" 
Sent: Saturday, February 21, 2009 6:12 PM
Subject: Setting magnetic damping level

>I am a novice and working on a small vertical seismometer.  I have 
> Larry's amplifier and A/D.  My current approach to assessing the damping 
> is to move the coil some distance from the magnet and watch the wave 
> form following a 1 mm drop of the arm.
> Are there other approaches?
> How should the properly damped waveform look?
> I have moved the VS under the house into the crawl space for now.  This 
> is far better than my shaking house.
> Robert
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