A few weeks back Oli put in an alog about IMC and SRM M3 saturations during earthquake lock losses. In April of 2023 Gabriele had redesigned the M1-M3 offload, reducing the gain of the offload somewhat to get rid of 3-ish hz instabilities in the corner cavities. This may have contributed to the SRM saturations that Oli found, so we want to try adding some low frequency gain back into the SRM offloading.

Sheila wrote out the math for me for the stability of both the SRCL loop and the stability of the M1-M3 crossover, so I have been looking at ways to increase the low frequency gain without affecting the stability above 1hz. The open look gain for SRCL looks like :

SRCL_OLG =  SRCL_sens * SRCL_FIlter * (SRM_M3_PLANT+SRM_M1_LOCK_FILTER * SRM_M1_PLANT);

The SRM M1-M3 offloading looks like:

SRM_OFFLOAD_OLG = SRM_M1_LOCK_FILTER * SRM_M1_PLANT * SRCL_sens * SRCL_Filter / (1-SRM_M3_PLANT * SRCL_sens * SRCL_FILTER);

Both of these are (or behave like) open loop gains (g), so the suppression/gain peaking can be show by looking at the 1/(1-g) for each.

I made a 50mhz boost filter for this and tried it during the commissioning window this morning. Bode plots for the boost (red), boost *m1 lock filters(blue) and the nominal M1 lock filter(green) are shown in the first image).  The affect on M3 drives and SRCL are shown in the second image asds, live traces are with the new boost, refs are without. There is good reduction below 100mhz, but there is gain peaking from the M1-M3 offloading at .2-.4hz. which might bleed into the secondary microseism during the winter. I'm working on a filter with similar gain, but less gain peaking in a region that won't affect the overall rms of the m3 drive as much. I will try installing and testing during maintenance tomorrow.