J. Kissel, H. Radkins I'm continuing to investigate why ITMY's pitch mode (at 0.55 Hz) continues to ring much longer than the other QUAD's upon lockloss (see and example attached as 2016-11-08_2151UTC_ITMY_Oplev_Bad_TimeSeries.png). Today, I grabbed the open loop gain transfer functions of the M0 DAMP L, the M0 DAMP P, and the L2 OLDAMP P local damping loops on both H1 SUS ETMY and H1 SUS ITMY, to show that the identical settings are producing identical results for QUADs that are essentially physically identical. Indeed, the open loop gain transfer functions of both suspensions look the same. We don't have too many lock loss data points yet since I made the change, but I did discover that the digital output limiter of all QUAD's L2 OLDAMP P bank were different (one as low as 5000, and one as high as 500000, and ITMY was in the middle at 10000). So, I've made all the limits the same, at 40000, in hopes that the problem is with the square-wave that ITMY gets sent during a lock loss. This change is NOT accepted in the SDF, because I'm unsure if it matters yet. Still unclear what is going on here... Hugh's taking a look at these same optical lever signals after every lock loss for the past month. Preliminary results show that this behavior is intermittent, but has definitely gotten worse in the past week or two. We hope he can trace this back to some event like a maintenance day to better try to address the problem.
All of the BSCs have AC coupled loops installed on St2 for RX/RY (somewhat detailed in my post 28907 for ETMX), maybe it would be helpful to try turning this on for ITMY? The GS-13s see a lot of this motion. Attached trend has both ITMs oplevs and GS-13 pitch for each ISI ST2. ITMY St2 clearly gets more kick, just like the ITMY optic. Second attached plot are some spectra of the ITMY GS13s, red is just before this lockloss and pink is during the lock loss. Green is the ST2 RX motion during a quiet time this morning, showing where the ac couple loop gets suppression. Most of the motion comes from .46 hz SUS resonance, and the AC loop doesn't do anything there, but the 2 and 3.5 hz motion should be suppressed some. The attached PDF shows some design plots for the ITMY loops from my work on this in August.
