We have been having some locklosses during the TURN_ON_BS_STAGE2 step. Looking at a couple of events, there are transients seen in the BS st2 CPS during this step from engaging the isolation loop boosts. The boost filters have ramp times, some of them were 5 seconds, probably related after this alog, but the RX RY loops still had 2 second ramps. All of these are set to 5 seconds now, this seems much smoother from the ISI CPS. Otherwise, these filters have not been changed since 2020. I also tried using the "Always On" option in foton, this went poorly. And tripped the ISI.
First attached plot is one of the previous transitions, middle row are the SEI_BS guardian state transition and the ISC_LOCK state. Top and bottom rows are the St2 CPS, RX/RY on the top, X/Y on the bottom. It seems like the probably the ramp of 2 seconds for RX/RY is too fast, and this causes the X/Y to move around a bit through tilt to horizontal coupling. Second attached plot is the DRMI lock just now and the CPS transition is much smoother.
Nothing else has changed on these filters or loops in many months, I don't know why we would be having more problems now, but maybe we're just paying more attention.
This is, of course, exactly the opposite of what I did back in 2019, when I was decreasing ramp times to get the boost engaged as quickly as possible. Shorter ramps seemed to fix locklosses at this state back then. But we were turning the stage 2 loops completely off to acquire DRMI at the time, and I wasn't using the St2 RX/RY loops then either. Now we only turn off the boosts, which leaves the dc isolation loops mostly engaged.
I've counted six lock attempts since loading the new ramp time settings in the boost filters and I don't see any kicks in the St2 CPS when engaging the boost filters. I also don't see strings of lock losses during this step like before. Seems like that means it at least didn't make things worse.