Hi Chris,

I just had a couple of comments on your thread with Dave.

>>Regarding creep effects -- When a spring is first installed in an 
>>instrument there will be "pops" related to what is probably 
>>dislocation effects in the spring material . Their frequency will 
>>gradually reduce in time. The solution is to bake the assembly, 
>>with the spring at its operational stress, at ~ 160 C for several 
>>hours. This will essentially eliminate the effect.
>
>I wish it were that simple. Heat treating the spring under tension 
>does greatly reduce the noise, but it doesn't eliminate it.

I'm sure you're correct.  What Dave meant was just that after baking 
we can no longer see that type of noise in the output signal.

>You also have noise from the feedback circuit. Differentiating a 
>signal is a noisy process.

That's one I don't agree with.  Are you implying, then, that if you 
integrate a signal you can remove noise?  Don't think so, and 
differentiating, done properly, shouldn't add noise either.  That 
would seem  to imply that if you differentiate a signal, then 
re-integrate it, it will now contain additional noise just because of 
the differentiation process.  I believe differentiation simply 
accentuates frequency components in the signal which are typically 
noisy.  In a feedback seismometer, the derivative feedback is 
strongest at the high frequencies where it acts to most strongly 
cancel/offset high frequency noise.  The more high frequency noise 
the derivative feedback branch passes, the more it will get reduced 
in the instrument output.

>It could be a distinct advantage to combine active damping with 
>passive damping, which is quieter.

Our approach has been to seek the maximum Q possible in the passive 
spring-mass system in order to keep as far away as possible from any 
1/MTQ effects (though that's not likely to be an issue).  Normally, 
air damping from the displacement-sensor plates is the limiting 
factor on what Q we can get.  Then we independently design the 
feedback to shape the low corner of the instrument's velocity 
response to provide something like critical damping.  The two Q's are 
pretty much separate issues.

Regards,
Brett 


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