Thanks everybody for all their comments. It should take me a while yet
to parse all that information.
The Inyo looks like it might fit inside a pressure cooker? That might
help to isolate it from barometric pressure variation.
**********
I=92ll try to expand on how I derived the transfer function:

1.) Neither the ground nor the mass is stationary from the perspective
of an inertial frame.
2.) The only forces that can act on the mass are from the spring and
from the feedback transducer.
3.) Both the spring force and the feedback transducer force depend
only on the distance between the ground and mass (and derivative and
integral of that distance).

Those statements gave me this equation of motion for the mass:
X(s) is the mass position from an inertial frame.
Y(s) is the mass position from the intertial frame.
	F =3D ma		(Newton=92s Law)
	F =3D F(s)[Y(s) =96 X(s)]	(From  2,3)
So:	M * s ^ 2 * X(s) =3D F(s) [Y(s) =96 X(s)]

Then it=92s just algebra to get the transfer function.

Now F(s) =3D K_m  +  K_p  +  K_i / s  +  s * K_d

Where K_m is the mechanical spring constant, K_p, K_i, and K_d are the
constants of the PID controller. For example, K_p =3D Q*G/R_p where Q is
the position sensor sensitivity in V/m, G is N/m, and R_p is the
proportional feedback resistor. Likewise, K_d=3D G*Q*C, and K_i=3DQ*G/(T
*R_i) where T is the integrator time constant.

**********
My method for including the back EMF of the force transducer relies on
the principle of superposition. Basically you put a voltage source in
series with the force transducer whose voltage is proportional to
velocity, where the constant of proportionality is the generator
constant of the transducer. Then you calculate the current induced by
that voltage by short circuiting the position sensor output and
integrator outputs to ground. I will try to post a schematic to my
website to better describe how this is done. I=92ve also got a MATLAB
file I can post. The change with back EMF is basically negligible in
amplitude. Negligible would be an understatement in fact.

Cheers,
Matt
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