Hi Chris,

At 12:28 PM 2/16/2008 -0500, you wrote:
>In a message dated 2008/02/16, Brett3mr@............. writes:
>
>>If carried down to low enough frequency, integral feedback can go a long
>>way toward resisting instrument drift.  This is 'just' a matter of making
>>the instrument force / acceleration response approach zero at very low
>>frequencies.
>>
>>Unfortunately, there is dirty little secret about using R-C integral
>>feedback to resist drift error forces, and that is evident when the
>>integrator is 'straining' to cancel a fairly strong unbalance force, in
>>which event there will be a substantial voltage across the integrating
>>capacitor.  The cap. has a temperature coefficient of C which is of the
>>same order of magnitude as the Temp. Coeff. of a steel spring i.e. pretty
>>large.  Since in a feedback integrator the charge, Q in the cap. changes
>>relatively slowly and can be considered to be constant as a first
>>approximation, if its capacitance goes up with temperature, its voltage
>>goes down in proportion because of Q=CV thus introducing its own rather
>>large drift effect.  It will only work as expected if the system is already
>>reasonably well balanced mechanically making the voltage across the cap.
>>not too large.
>
>
>Hi Brett,
>
>        You have four drifts here. The change in the magnet strength with 
> temperature, the changes in the coil with temperature and the change in 
> the capacitor with temperature. The magnet strength is decreasing, the 
> coil area and resistance are increasing and the capacitance is decreasing 
> with increasing T. I don't know to what extent these can be chosen to 
> cancel? However, you only have to put the low pass frequency below the 
> minimum response frequency, but this could give problems with 1000 second 
> instruments.

Yes, I agree the forcing coil/magnet also introduce effects similar to what 
I was describing for the integrator capacitor; except that you can avoid 
the coil resistance issues by using a current driver.  It's obvious that 
you can't depend on electronic feedback to do all your centering, forever, 
but must occasionally turn a screw to unload the feedback loop.

Thanks for your thoughts,
Brett



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