K. Venkateswara

After the problems with BRS last week (15005), it was left off for the last ~five days. I tested the damper controller and it looked like the issue with vibration driving the BRS had cropped up again (14596). It wasn't clear why this problem had reappeared but since the foam sheet had worked previously, I went back to that configuration. With the foam sheet under it, the damper appears to be working fine. However another issue with it is that the zero position of the turn-table appears to change, seemingly randomly. It is possible that the stepper motor is occasionally getting stuck. As it's not servo'd this may explain the drift in the zero position. For the moment, I think it requires periodic checking to ensure its zero hasn't changed too much. If it changes by 90 degrees, it can drive the BRS in a positive feedback loop like last week. A servo loop is a better long term solution.

I have also modified the following filters:

1. BRS transfer function inversion filter to correctly account for the imaginary zero at 7.3 mHz (due to the finite d). I was using pairQ(7.3e-3,3) before whereas pair(7.3e-3,0) should be the right choice.

2. The tilt-subtraction filter has also been changed - the high-pass filter (2-pole) has been changed from 2.5 mhz to 4.3 mHz to correctly compensate for the T240 resonance.

The attached pdfs show the ASD for the BRS and the ground T240 in angle and displacement.