Hi Chris,


Your point is well taken. I have gathered some equipment together, and
will spend some time trying to get a transfer function of the device this
weekend. Personally, I think that the motion is way too small for the air
gap between the plates to contribute much. Damping is only one effect,
however. I have seen systems where the compression of the air in the gap
causes a large enough density change to alter the dielectric constant
significantly. Anyway, lets get a transfer function and see what it looks
like.


I intend to open the loop and drive the coil with a constant current.
That means that the displacement will only decrease by four orders of
magnitude over the proposed 0.1-10 Hz test so I may have to run two
tests, each of which will cover just one octave.


If I have time, I will try the same thing with the loop closed.


Incidentally, The electronics documentation has been sent to both Larry
and John.


No, I haven't yet read "Transfer Function
of an Ultra Low Frequency Vibration Isolation System" , but I have
it on order from the school library. Should get it early next
week.


Dave...


At 06:38 PM 3/13/03 -0500, you wrote:

Hi Dave,



      I have been reading through your excellent
text. Have you looked for any damping due to the capacitative sensor
plates, while in operation? This might be done by setting the pendulum
frequency to say 1 Hz, disconnecting the feedback and putting a small
step current through the force feedback coil. If there was no damping,
the system would oscillate for some time. Blair used a photodiode setup
for measuring the sensor characteristics. I use an optical differential
system on one of my sensors, but I use an under run filament bulb to give
a constant light. I get about 15 nm resolution. 


      The reason that I query this, is that one
limitation on the use of capacitative sensors which use changing plate
separations in air, is the gas pumping effect due to the moving plates. I
would expect a significant effect with a 2" square plate and a gap
of 10 thou. at 10 Hz. One way of reducing the problem is to drill an
array of holes in the central plate to let air flow more easily between
opposite sides of the plate. This need not greatly effect the capacity.



      The damping force F = 3.n.u.A^2 / 2.Pi.x^3
for circular flat electrodes according to Jones, where n is the viscosity
of air, u is the plate velocity, A is the plate area and x is the plate
separation. The damping falls off with the cube of the plate separation
and increases with the square of the Area, which suggests two ways of
controlling any damping. The problem that I can see is that any air
damping effects will be frequency and amplitude sensitive, which could
limit the maximum seismic frequency that you can observe. I am not sure
how much the force feedback loop reduces the amplitude of the armature
with your control system. 


      There seem to quite a lot of test results
/ design considerations given in 

"Transfer Function of an Ultra Low Frequency Vibration Isolation
System" 

Jiangfeng Liu, John Winterflood and David G Blair 

Rev. Sci. Instrum. 66 (5),  3216-3218, (May 1995) 

      I wondered if you had read it? 


      Regards, 


      Chris Chapman