TJ asked me about the BRS this morning, as it was tripping an alarm he had set. When I opened the overview, the numbers were rather large and swinging rapidly (over a minute it was going from -40000 cts to +40000 cts, the transition was a couple seconds. The attached dataviewer trend shows it has bee doing this since the 21st, starting about 15:00 UTC. No clue what happened.
A little bit more supporting data: I attach a time series of the drift monitor (the DC comparison between the autocollimator's fringe patterns for the balance mirror and reference mirror; in military green; H1:ISI-GND_BRS_ETMX_DRIFTMON) and control signal for the gravitational damping motor (in dark red; H1:ISI-GND_BRS_ETMX_DAMPCTRLMON). I suspect the suspension has drifted enough that fringe patterns have crossed causing the output of the sensor to go non-linear. This then rung up the suspension after the damper turned on thinking that the SUS had rung up. Then we get into a nasty feedback loop. Looks like the temperature in the XVEA has remained stable over the weekend (and for several days prior), so I doubt we can blame the temperature swings for the cause of the drift. For now, Jim and TJ have turned off the control software to let the suspension cool off. We'll do a more invasive investigation tomorrow morning.
I've attached a couple of plots showing the BRS_DRIFTMON channels. First one shows the 1-hr period when the BRS data started to grow large. It looks like some 'disturbance' set it off and then the damper took it away with a positive feedback loop because it was in the incorrect position.
The next plot shows the same channel over 3 days, starting from 2 days before the disturbance. It looks like the mean position was quite stable ( except for the odd missing dataset). Once the positive feedback loop got started, it drove the turn table harder and harder, generating heat and causing the mean position to shift slowly. Once things settle down, the mean position should return to ~ -2.9k counts. This is well within the linear range of BRS.
Due to the large amplitudes BRS reached before the software could be turned off, I'm not sure it will be sufficiently damped so it could be restarted tomorrow.
