K. Venkateswara

At UWash, I have built a simple turn-table with two 1.0 kg brass weights controlled by a stepper motor. The angular position of the table is controlled in a 0-90 degree range by a 0-5 V signal applied to the stepper motor controller (consisting of an Adafruit Shield on top of an Arduino Uno). Pictures are shown on page 1 of the attached pdf. The gears and support structure were laser-cut out of acrylic (with our very own 400-W CO2 laser-cutter).

When placed under the beam-balance, the brass weights apply a purely gravitational torque on the balance depending on the angular position. To damp the balance, I take the autocollimator output (angular position of the beam-balance), differentiate it once and band-pass between 3-33 mHz and use it as the control signal after appropriate gaining. The result is shown on page 2.

This setup should work very well with the BRS at ETMX. We could have the damping controlled by the Front-End so that if the beam-balance does get rung up by people in the vicinity it could be remotely damped and the damping turned off once the amplitude is small. Preliminary results indicate that it could even be left on permanently with Q set to 50-100.