Sudarshan, Kiwamu, (WP#5569)
During the maintenance period in this morning, we looked at a few things in order to improve the ISS robustness.
In summary, the 2nd loop ISS is not back in a great shape yet. There will be some times where it needs the manual engagement.
(1) The open loop measurement suggested that the UGF is at 10 kHz with a phase margin of about 50 deg. No crazy thing was found.
(2) The PID loop seems to have been suffering by unidentified extra offset, which explains the behavior we saw yesterday (alog 22863). I have edited the IMC_LOCK guardian so that it servos to a better locking point where the transient is less in the diffraction power.
(3) The IMC seems to be adding extra intensity noise below 1 Hz. This is the main reason why the PID loop does not converge simply because it is too high.
Change in the PID locking point
After a number of the 2nd loop engagement tests, I confirmed that it was the PID loop which pulled the diffraction power to a small value (alog 22870). This happens even without any additional gain or boosts which are engaged in the CLOSE_ISS loop. The reason of this bad behavior was found to be due to a different (unidentified) offset in the SECONDLOOP_SINGAL. I do not know why it changed. Originally the guardian was supposed to servo SECONDLOOP_SIGNAL to 0.7 which had been fine in the past in terms of transient kick to the first loop. However, as reported in the recent alogs, this locking point became bad in the sense that it gave a too large kick and eventually unlocked both 1st and 2nd loops. I experimentally adjusted the offset point which ended up with -0.4. I have edited the guardian accordingly and checked in to the SVN.
I then tested the manual engagement with the new PID locking point multiple times (Note, I did it manually because the PID loop was not able to converge due to large motion in the IMC). I confirmed that it did not pull the diffraction to a small value. Though it often lifts up the diffraction up to about 10% which is acceptable.
Extra intensity noise from IMC
This is something we already knew (alog 22482), but intensity noise became higher at low frequencies as the light goes through the IMC. This is likely due to some static misalignment somewhere (or perhaps very large offset in the length control) in the IMC. In order to identify which optics are contributing most, I looked at several channels for SUS-MC1, MC2 and MC3, and HAMs 2&3 table motion. However, sadly, all three MC optics and both HAM tables seem to equally contribute to the intensity fluctuation seen in MC2_TRANS and ISS PD array below 1 Hz according to coherence. The contribution from yaw tend to be larger than that of pitch according to the coherence, but they all are above a coherence of 0.85 or so below 1 Hz. It is difficult to say what optics are contributing most.
We need to study why the IMC motion is so high these days.
For those who will engage the 2nd loop in the future
The manual engagement now works fine. As usual, when the 2nd loop does not engage on its own for more than 5 minutes or so, please follow the instruction for the manual engagement (alog 2249). Also, when the manual attemp fails (i.e. the deffraction power is pulled by more than 5 % when manually engaging), one can prevent a lockloss by manually disengaging the second loop input (the very top red button in the 2nd loop ISS screen). Subsequently one can make another attempt once the diffraction power settles.
