Hi Dave-  You got my attention- Nice work!- Do you have any records
(seismograms) posted that we can look at?
Jim O'Donnell - UNLV  
On Sun, 16 Mar 2003 11:57:47 -0500 "David H. Youden" 
writes:
> All,
> 
> I got a chance yesterday to run some tests on my instrument.
> 
> Here's what I did:
> 
> I opened the loop and drove the force balance coil directly with a 
> HP 
> function generator ( Model 3310B). I placed a 1000 ohm resistor in 
> series 
> with the coil. I took my input signal from across the coil so I had 
> a 
> sensitivity of 1000 volts/amp. The output was taken at the output of 
> the 
> cap gauge circuit, after a 2 pole 35 Hz filter that would have been 
> 
> inconvenient to remove. The output device was a HP 54603B memory 
> scope. The 
> driven mass is pretty close to 500 grams.
> 
> I attempted to take phase and P-P position data at constant 
> acceleration 
> from 0.1 Hz to 10 Hz while holding the current through the coil at 
> 94 
> microamps, i. e. 94 millivolts across 1000 ohms. Why that value? 
> Because it 
> was the maximum that I could use at 0.1 Hz and still maintain 
> linearity of 
> the cap gauge signal.
> 
> Results:
> 
> The force balance coil/magnet combination have a transfer function 
> of 0.26 
> Newtons/Amp.
> 
> I lost the signal in the noise at about 6 Hz. Obviously, I could go 
> higher 
> with a larger input signal.
> 
> The phase between acceleration and displacement was less than 20 
> degrees 
> throughout the range.
> 
> The P-P displacement was about 60 nanometers at 6 Hz.
> 
> The laws of physics still apply, as the P-P position signal was 
> inversely 
> proportional to the square of the frequency.
> 
> There was no sign of roll-off due to damping from the cap gauge, but 
> then 
> the amplitudes were pretty low at the higher frequencies.
> 
> Comment: My low pass filters are set to roll off at 5 Hz so data 
> beyond 
> that point is academic.
> 
> Dave ...
> 
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