PSN-L Email List Message

Subject: Re: Folded Pendulum Seismometer
From: "David H. Youden" dyouden@.........
Date: Sat, 15 Mar 2003 07:18:52 -0500

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.


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?


      Chris Chapman

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