PSN-L Email List Message

Subject: Re: fine structure nonlinearity vs dithering
From: Brett Nordgren Brett3mr@.............
Date: Fri, 08 Feb 2008 19:16:47 -0500


I'm very glad to hear that you're interested in following the 
discussion.  My only concern had been that we were taking up bandwidth on 
stuff that might not have been of interest to all that many folks.  I'll 
happilly continue posting my comments, though will also be happy to take 
this elsewhere if there's a sense that we should.

In reply to your comments, I don't yet understand how the nonlinearity acts 
and how it should mathematically be treated.  In a flat to velocity design, 
if it acts at all like regular linear spring-mass damping, it would be 
completely overwhelmed by the feedback.

         The amount of linear velocity damping of a spring-mass
         has virtually no effect on the response of a flat-to velocity
         feedback instrument.

The way spring-mass damping acts in feedback designs with other responses 
(flat to acceleration?) might well be different.


FYI, below is what I'd written to Randall,
He may want to post his reply.

>I assume that you don't believe that feedback is worthless in the 
>implemtation of seismic sensors, but you obviously feel that there are 
>some deficiencies in the presently available feedback instruments.  The 
>first question that comes to mind, is what effects would you expect to see 
>in the performance of those instruments which would relate to the effects 
>you describe?  Can one come up with some experimental design to highlight 
>the areas in which they fall short?
>I'm reluctant at this point to throw the baby out with the bath water and 
>accept that linear analysis is of no value in feedback seismometer design, 
>but I am quite willing to accept that there are areas that have not been 
>adequately explored in both theory and experiment.  For one thing, I need 
>to understand to what degree you feel the anomalous behavior you describe 
>extends to designs which aren't attempting to push the state of the 
>art.  Just when would these effects begin to be felt as you tried to 
>extend performance?

At 11:50 AM 2/8/2008 -0800, you wrote:
>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.  :-)
>Charles R. Patton


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