No suprise that the M1 offloading of PRM length actuation caused a large 3.4 Hz peak.
With the IFO down, I measured the PRM length lock actuation in three configurations:
- only M3, no offloading to M1
- M3 + M1 offloading with filters used in NLN and nominal gain of -0.02
- M3 + M1 offloading with filters used in NLN and reduced gain of -0.01, the gain that we showed yesterday to reduce the 3.4 Hz peak in PRCL
Look at the attached plot: when offloading to M1, we create a huge peak at 3.3 Hz in the PRM actuation, and the trasnsfer function is very different from what we get with M3 only. Reducing the gain by a factor of two improves the situation just a little bit.
So PRM offloading need some work.
I took measurements of the M1 and M3 actuations, we can use them to design a new offload.
I wonder if the other HSTS are in the same situation: SRM has a very similar offload filters (with gain of -0.01)
Tagging ISC and CSWG. For the record, Shiela recently designed offloading for the filter cavity control -- see LHO:66092. Maybe this is a useful starting point. The first aLOG I found on PRM offload design is from 2015, LHO:21084; this might be the last time it was reviewed. She's conveyed to be several times that she's been confused / disappointed in the HSTS matlab model's ability to support the efforts of design -- it might be worth a look there too. You may find a tagged version of the HSTS model in hstsmodelproduction-rev4067_ssmake3MBf-rev1891_hstsopt_metal-rev2039_released-2013-01-31.mat. Information to calibrate that modeled transfer function collection from fundamental units (e.g. m/N, or rad/N.m) can be found in both the controls design description for the HXTS, T1000061, and summarized in the controls design summary table, G1100968. A reminder that H1's PRM is driven still by 18-bit DACs, so the calibration is 20 / 2^18 [V/ct]. I have not had the time to digest Sheila's scripts from LHO:66092 or earlier to understand where her mysteries and source of complaints lie. I've at least been able to verify the top mass sensor and actuator calibration to within a factor of 1.5x through my damping loop designs, e.g. LHO:65310, designed by ^/trunk/HSTS/Common/FilterDesign/Scripts/design_damping_HSTS_20221110_H1SRM_ActualHSTSLeverArms.m.
In particular, for FC2 SUS control, Sheila had to add in a 1.3Hz phase wiggle (FC2, M1 stage, FM5 "wig1.3Hz") to make our M1/M3 crossover stable. But, from our FC2 M1/M3 crossover measurement, notice that beyond 1.3Hz, FC2 indeed seems to have a second instability at about 3.4 Hz, which has ~2x gain clearance and no phase margin from becoming unstable.
I took a quick (just 3 averages) measurement on SRM (as requested by Gabriele) and I didn't see any 3,4Hz peak with M3+M1 offloading and nominal gain of -0.01. At the next target of opportunity I will check other HSTS.
