jeffrey.kissel@LIGO.ORG - posted 17:13, Friday 28 October 2016 - last comment - 10:34, Monday 31 October 2016(30970)
All V3 & R3 Modes Notched in All HSTS Suspensions -- Success at 27.5 Hz and 41 Hz.
J. Kissel With hints of improvement last night from Sheila (see LHO aLOG 30947) on reducing the most egregious ~41 Hz peak in DARM (which is known to be the HSTS roll modes, a.k.a R3), I launched a campaign of adding similar ~1 Hz wide notches to all HSTS suspensions' local damping loops (M1_DAMP) at both the highest roll mode and highest vertical modes (a.k.a V3 at ~27.5 Hz). The filter designs are V3 notch: FM8 "SB27.5" ellip("BandStop",4,1,60,27.05,28.05)gain(1.12202) [Input -- Always On; Output -- Ramp; Ramp Time -- 2 sec] R3 notch: FM9 "SB40.9" ellip("BandStop",4,1,60,40.4,41.3)gain(1.12202) [Input -- Always On; Output -- Ramp; Ramp Time -- 2 sec] where I've made sure that the lowest and highest frequency V3 modes (coincidentally both IMC mirrors: MC1 @ 27.38 Hz, MC2 @ 27.74 Hz) faLL within the stop band of the notch (and I've confirmed that this is true with Sheila's R3 notch design as well). Further, the V3 notch causes a phase loss of only 2.2 [deg] at 10 Hz, so it will not impact any of the damping loop's phase margins. To confirm, I've spot checked SR2's local damping open loop gain tfs just to be sure. Indeed all DOFs are still quite stable (and quite poorly tuned, as expected). During the campaign I found that PRM, MC1, and MC3's "ellip50" standard low-pass filters were not engaged, so I engaged them. I've greened up all ODC status lights, and then accepted all of the changes in the SDF system -- V3 filters ON, R3 filters ON, ellip50 filters ON, and Correct Damp State. While haven't had an uber-long lock stretch since I've turned on all of these notches, I was at least able to grab 5 averages of a 5 [mHz] BW ASD and compare against the long data set from last night. The notches appear to have done their job -- a large fraction of the modes have been squashed and don't show up in DARM anymore. Success! The SR2 damping loop open loop gain TF templates have been committed here: /ligo/svncommon/SusSVN/sus/trunk/HSTS/H1/SR2/SAGM1/Data/ 2016-10-28_2223_H1SUSSR2_M1_openloop_L_WhiteNoise.xml 2016-10-28_2223_H1SUSSR2_M1_openloop_P_WhiteNoise.xml 2016-10-28_2223_H1SUSSR2_M1_openloop_R_WhiteNoise.xml 2016-10-28_2223_H1SUSSR2_M1_openloop_T_WhiteNoise.xml 2016-10-28_2223_H1SUSSR2_M1_openloop_V_WhiteNoise.xml 2016-10-28_2223_H1SUSSR2_M1_openloop_Y_WhiteNoise.xml
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Following up with another extremely long lock stretch from over the weekend (new data starts at 2016-10-31 07:00 UTC), it looks like I've definitely killed several of the resonances, such that they don't substantially appear in the DARM noise. However, the MC2 V3 mode @ 27.7642 Hz and what remains of the PR2 R3 mode @ 40.935 Hz are still as bad as they were before. This likely means these modes are getting excited via non-local control. The ISC control is a likely culprit for MC2, because the V3 mode is abnormally high and may be out of range of a generic notch. Further the IMC cross-over is around 15 Hz, so notching these frequencies is more difficult (i.e. there's currently no notches for the M3 stage length control of the IMC). We'll continue to poke around looking for poorly notched loops.