Kiwamu, Nutsinee
As we were trying to relock the ifo after several locklosses due to high wind (50mph), we noticed the sideband signals wiggled a lot before another lockloss at DC_READOUT (wind speed ~35-40 mph). We found a coherence between POP18, POP19, POP_A_LF, AS90 and PRM, SRM, BS which indicates that the DRMI was unstable. The BS ISI Windy blends weren't turned on.
One of the two lock losses seemed to be associated with PRM saturation. We heard of the saturation alarm voice pointing PRM DAC in full lock mutiple times before the lockloss in NOMINAL_LOWNOISE. I am not sure if this is the direct cause, but as shown in the attached, PRM had been experiencing ~20 sec oscillation in longitudinal which used to be a big issue in the past (alog 19850). At that point wind was around ~40 mph on average. Also, I attach spectrum of each coil on the M3 stage. It is clear that the components below 0.1 Hz are using up the DAC range when wind is high.
Just as a check, I remade Kiwamu's plot for PRM, SRM, and MC2, with all the stages that are used for actuation.
At this point, the wind ine corner station varied between 3 and 13 m/s. The 30 mHz – 100 mHz BLRMSs were about 0.02 µm/s in the CS Z (consistent with sensor noise), 250 µm/s for EX X, and 250 µm/s for EY Y.
Since this time, we have increased the offloading of PRM and SRM to M1 by a factor of 2, but we probably need an even higher crossover in order avoid saturation during these times. It may have the added benefit of allowing us to stay locked during even windier times. Additionally, MC2 does not look like it needs any work on its crossovers in order to avoid saturation.
The above comment should say 0.25 µm/s for EX X and EY Y.