Craig, Hang, Georgia, Sheila
We've been having trouble locking, probably because of the wind and a small EQ, and perhaps because the IMC UGF was too high. We decided to use some single bounce time to investigate why we can't engage the OMC ASC. We were able to engage it when we were doing OMC scans just over a week ago. We haven't been able to lock the OMC because we rail the OM3 +OMC suspensions if we try to engage the QPD loops.
Hang and I noticed that the picomotor for AS_A was accidentally moved on August 7th. Even though the settings were set back, we suspect that the pointing onto the QPD may not have been restored because of the hysteresis of the picomotor. On August 7th the AS_C pico was moved for a better SR2 pointing through the OFI.
Craig used his script that restores optics to a given time using the top mass witnesses to restore SR2, OM1,2,3 and the OMC to their alignments from July 31st at 22:53 UTC, which is the time of one of our single bounce scans. We saw that this brought the OMC QPDs closer to their lock point, and pico'd AS_A to center the beam on the diode. AS_B pico has not moved since Thomas Vo adjusted it in June so we didn't touch that. Once AS_A was centered here we closed the AS centering loops, and pico's AS_A while it was in loop to bring the OMC ASC error signals to zero.
After this was done we were able to close the OMC APD loops without a problem, and then walked SR2 back to it's more recent alignment. The OMs are still not railed.
Dear Sheila, what you say about the Picomotors rings true. Calum and I did some investigations into the Picomotor hysteresis. The method by which the Picomotor achieves motion relates to a stick-slip phenomenon. As each Picomotor is preloaded with a certain force, there is an intrinsic tendency for a step in one direction to be unequal in motion to a step in the opposite direction. In addition, this highly friction dependent phenomenon is rife with hysteresis. Simple summary is that if you step N numbers of steps for a Picomotor, you will never know exactly how to reverse this action without a secondary measure of motion.
Past experience shows that a backward motion may be no more than 80% of the forward motion. So the resulting alignment error can be as large as 20% of the full motion.