[Jeff K, Stuart A]

IIET: #11730

ITM QUAD suspensions at LLO have exhibited sharp ring-ups at 3.3 Hz from ASC pitch drive resulting in IFO lock-losses (see for example LLO aLOG entries: 40926, 40953, 40972 and 41501), as a work-around LLO currently employs a "TWO_LEGGED_HARDASC" scheme reducing ASC HARD drive from 4 QUADs to 2 QUADs i.e. just feeding back to the ETMs (see LLO aLOG entry 41502). The 3.3 Hz feature corresponds to the QUAD R3 Mode, which when looking at QUAD L2 P to P Damped TFs (see allquads_2019-01-30_AllSUSQUAD_Phase3b_L2_Damped_ALL_TFs.pdf below) which we have confirmed is also prominent in LHO QUAD suspensions too. However, at present, the 3.3 Hz R3 modes are not being rung-up at LHO.

Given there has been no change since O2 in the LLO QUAD local damping scheme, which demonstrates equally ineffective damping of the 3.3 Hz R3 mode as the LHO QUAD local damping scheme, thus, we believe there has been a change in the global control at LLO. This was not an issue at LLO during O2, so we suspect that changes in the ASC loops to handle higher IFO power have exacerbated the 3.3 Hz ring-up problem.

We found that the 3.3 Hz R3 mode is not well modeled in the QUAD dynamical model (see L1ITMX_Model_L2_PtoP below) which also suggests that the R3 mode should be completely damped n.b. we imported L1 ITMX live damping filters.

Therefore, we are now attempting to adjust QUAD dynamical model parameters to better fit the measured QUAD TF data. Failing this approach, we may seek to resurrect the QUAD model fitting code T1100163.