Stefan, Kiwamu
We did the following things tonight:
- We measured the BS, PRM and PR2 actuator transfer functions again and convinced ourself that they are not crazy. The BS actuator was found to have the opposite sign and 10 times bigger gain at 20 Hz.
- As a preparation for PRMI locking, we started using RELAIR_A_RF45. We found that it is 5 times smaller than RF9 in terms of the PRX optical gain. We could easily close the PRX/Y loop with RF45_I.
- Because we were not sure if we could close the MICH loop with BS, we went back to PRY to verify the behavior of the BS actuator in action. The idea is to optimize the LSC filters for MICH loop beforehand using PRY.
-We discovered that there was a length to pitch coupling in BS which caused 5-10 urad excursion in its angle. This was big enough to unlock the PRY loop.
- POPAIR_B_RF18 demod phase = 80 deg
Our message is that we need to put some more efforts on the BS actuator.
A comment on the attached plot:
They are the transfer functions from the suspension input to REFLAIR_RF9. The vertical axis is not calibrated to any meaningful units. The BS transfer function is artificially sign-flipped to see how much the phase gets delayed. Also we took a ratio of BS/PRM to figure out what kind of extra filters we need to close a loop with BS. You can see that the PRM and BS actuator responses are different and therefore we are working on BS.