On 9/28/2012 5:41 PM, Brett Nordgren wrote:

....
> Magnetically supported verticals are going to suffer more from 
> variation in support force with temperature than those using springs.
> These numbers x 1g give the variation in apparent vertical 
> acceleration due to temperature change.
>
> NdFeB magnet  -1200 ppm/degC  (most commonly used)
> SmCo magnet  -300 ppm/degC (more $$ and not as strong)
> 17-7PH stainless spring -240ppm/degC   (we use)
> Special thermally compensated spring alloys (weaker and $$$) 
> <10ppm/degC   maybe

For a "reference design" in the following discussion, consider this 
"gravitometer" device from Scientific American - it has been discussed 
before on this list:

http://www.scientificamerican.com/article.cfm?id=detecting-extraterrestria

Basically it's a force-feedback vertical with permanent-magnet 
suspension and temperature regulation. I've wanted to build something 
like this for a long time, and I did some research on temperature 
coefficients of different permanent magnet types (it turns out the 
ceramic magnets used in the device have the WORST coefficient), and 
found Alnico (I forget if it's "Alnico 5" or "Alnico 8") to have the 
lowest of standard available types. But even more, I found that it's 
possible to make permanent magnets with (near) zero temperature 
coefficient, and indeed very large ones are used in some particle 
accelerators. But as one might expect, it's a specialty item and surely 
very expensive, and I didn't find a source selling any such magnets. 
Perhaps for professional-grade instruments this may be worth looking into.

Here's an abstract on such a magnet:
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=5087293&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F4915369%2F5087117%2F05087293.pdf%3Farnumber%3D5087293
Setting the temperature at a point where the temperature coefficient 
flattens out (where the integral is zero) would provide the least change 
with temperature and the best stability of the field. This is similar to 
how quartz crystal ovens are designed.

But what about electromagnets? The magnetic field of an electromagnet is 
determined by the number of turns (and their geometry), and the current 
through the coil. A coil for both the "stator" (fixed element used to 
provide force opposing gravity) and the "rotor" (the moving arm) would 
provide a force proportional to the product of the currents in the 
stator and rotor coils. Both currents would be regulated, and the other 
modulated by the position sense signal to maintain a fixed position.

Probably the biggest disadvantage would be the heat produced by the 
coils. This could be stabilized by putting the mechanism at the top of a 
vessel to reduce thermal currents. Very low frequency waves would cause 
the coil dissipation to change slightly with the force-feedback current 
changing to keep the arm in place, but this could be compensated by 
heat-generating resistors whose power input is changed complementarily 
to the coils to keep the total power into the system constant.
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