PSN-L Email List Message

Subject: Re: seismometer performance
From: Brett Nordgren Brett3mr@.............
Date: Fri, 15 Feb 2008 22:40:57 -0500

At 09:13 PM 2/15/2008 -0500, you wrote:
>Hi all,
>   Brett, you did a beautiful job with your paper on force feedback. I can 
> almost understand it! One issue discussed but unresolved is how to handle 
> drift. Have you ever looked at the efforts of one of our PSN members, 
> Allan Coleman. He has built a number of force feedback sensors of both 
> horizontal and vertical types. His designs feature the use of motors to 
> recenter the pendulum.


Thanks for the kind words.  Yes, I had studied what Allan was doing, though 
I should go back and see what's new.  He's had some very interesting 
ideas.  I completely agree that some form of mechanical rebalancing process 
will be essential, extremely so in a vertical.  The real value of using 
feedback to aid in centering is to reduce how often you need to 
mechanically rebalance, and minimize zero point drift between adjustments.

If you think about it, any systematic mechanical rebalancing of the system 
is also feedback, though it sometimes involves a human in the feedback 
loop, and it will be a little hard to describe well with numbers.

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 

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.  Allan's got the right approach.

Thanks for Allan's links.  I'll go take a look.


>   His website is:
>   You can also access it from a link in:
>   Look for "Allan Coleman's seismometer designs".
>   Force rebalance is a necessity for network sensors. All need a flat 
> response that is known and stable. As for me, there is too much involved 
> circuitry to cope with. I choose to use conventional open loop sensors of 
> known period and damping, and then to make their response flat and 
> broadband using my inverse filter program, "WQFilter.exe". This utility 
> is available for download from
>PSN Station REM
>Locust Valley, NY
>The year's hottest artists on the red carpet at the Grammy Awards. Go to 
>AOL Music.

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