Randall,
One brief thought -- I''m familiar with dithering to solve frictional 
problems.  So why not dither the force feedback at the A/D sample rate?  
The friction non-linearity would generate harmonics of the dither 
frequency, but these would also be rejected by the synchronous sampling 
of the A/D (they would show up as a DC offset -- i.e., it would appear 
as if the seismometer had a tilt that was not there in reality.)  The 
question is, "Does the "fine structure nonlinearity" show up as random 
stiction/rachetting?"  I.e., if one was to dither the force feedback, 
would the result be a lifting of the noise spectrum floor and result in 
overwhelming broadband noise?

As an aside, Brett asked you to start a direct email of this discussion 
off the PSN list.  I would hope this doesn't happen.  I can't learn from 
a discussion I don't have contact with.  :-)
Regards,
Charles R. Patton

 Randall Peters wrote:
>     As an engineer you will appreciate something that is needed for the improvement
> of force balance instruments.  Friction-limited systems have to be dithered to
> overcome the adversities of the friction.  

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