Geoff,

Absolutely you can do that.  You have just described something like 
the basic idea behind force-balance instruments, the only way they 
make commercial broadband seismometers today.

In such a vertical instrument, the spring's main job is to generally 
support the mass near the desired level and to not be too stiff so 
that the feedback circuit can do a good job of controlling the mass. 
(Actually that's a little harder to make than it sounds.)

The best instruments use moving-plate capacitor sensors to detect any 
microscopic movement of the mass and a coil-magnet arrangement to 
push the mass back towards its center position.

One of the big features of this arrangement is that the mass stays 
extremely well centered regardless of slow changes in temperature, 
atmospheric pressure, spring creep, etc.  Also you can control the 
instrument frequency response curve quite acurately, setting the low 
and high-frequency corners of a flat velocity response simply by 
selecting circuit component values.  A side-effect of this 
arrangement is that the output becomes quite linear in its response 
to ground motion.

Regards,
Brett


At 11:24 AM 8/8/2011, you wrote:
>I'm Not Trying to be a smartass but,
>ShaZam ??? !!!
>
>Is there any way to make the magnetic thing be linear
>like possibly using a Hall effect sensor an electro magnet and a pid loop
>like a automobile speed control ?
>
>I am possibly thinking of using magnetic repulsion/attraction  to
>reduce the pull of gravity and thus increase the
>period of a vertical geophone ?
>
>You need an acceleration in the opposite direction
>not quite as powerful as the Earths ?
>Does not matter what causes the acceleration in
>the opposite direction (UP) ?
>
>Any ideas in how to do such a thing ?
>
>Regards,
>geoff
>
>
>-----Original Message----- From: Christopher Chapman
>Sent: Saturday, August 06, 2011 6:12 PM
>To: psnlist@..............
>Subject: Re: Springs and Gravity or Magnitism
>
>
>
>
>Subject: Re: Springs and Gravity or Magnitism
>
>
>Subject: Re: Springs and Gravity or Magnitism  Hello Chris,  It is 
>my guess by what you say it may be possible to create a spring with 
>just about any relationship like fitting the curve of force like any 
>graph of a conic section ? . Hi Geoff, Probably up to cubic over 
>small ranges..I understand magnetic attraction to be log base two ? 
>Double the distance and divide the force by four ? Same with 
>electrostatic ??   No, It's more complicated. You have two poles on 
>every magnet. The force between any two poles is inverse square law, 
>but the force on a magnet involves both poles. So there may be a 
>rotational as well as a linear forces. Regards,   Chris
>
>
>
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