[JeffK, Sheila, Jenne]
The bounce mode has been giving us extra trouble today. In the end, we used a very fine bandwidth spectrum to confirm that it really was ITMY's bounce mode (as the monitor bar graph was telling us). We tried out some different phases, and ended up with some settings that are now working. When I went to put the new settings into the guardian, I found that these exact filters had once been used for ITMY, but had been commented out and changed. I'm not sure how long ago this happened, but it was at least before Sheila made the gain=-1 filters, so that the numerical gain is positive for all test masses, since that filter wasn't included in the commented-out settings.
So. Why did the damping phase change some time ago, and why did it change back? Unknown.
J. Kissel, S. Dwyer First attachment -- a screen cap the currently functional Bounce and Roll mode damping settings that work as Jenne describes above. One potential cause Shiela and I explored: we're using DARM_CTRL as our error signal for the bounce mode damping. We've always assumed that the DARM open loop gain is large enough that the open loop gain of the bounce mode damping loop is unaffected by changes in the DARM filter, D, or the DARM plant, or sensing function, C. Given recent, uncompensated issues with SRC cavity detuning cause optical spring changes in the plant, the DARM open loop gain near 10 [Hz] has been questionably low. We've been struggling with interferometric SRC alignment signal, and therefore leaving SRC angular control essentially off recently. This implies the SRC detuning can be changing from lock-stretch to lock-stretch, which means the DARM optical plant is changing from lock-stretch to lock-stretch. And if the overall DARM open loop gain is low enough, then the phase impacts of this change in optical plant will change the phase necessary to damp the bounce modes. The second attachment compares Kiwamu's DARM OLG TFs taken on 2016-09-16 vs 2016-09-21, and indeed, the open loop gain magnitude is only ~4 to 5 at 10 [Hz], and once can clearly see substantial phase differences between the two measurements. The third attachment shows the loop math to aide the following proof of how the two loops are coupled: C = DARM IFO plant, or sensing function D = DARM filter bank A = ISC L to TST L DARM super-actuator TF V = Bounce mode damping filter alpha = TOP V to TST L actuation function xi = coupling from TST L to DARM Delta L_{res} = Delta L_{V} + Delta L_{ctrl} = Delta L_{V} + A D C Delta L_{res} d_{ctrl} = D C Delta L_{res} --> Delta L_{res} = d_{ctrl} / (D C) d_{ctrl} / (D C) = Delta L_{V} + A D C d_{ctrl} / (D C) d_{ctrl} = D C Delta L_{V} + A D C d_{ctrl} G_{darm} = - A D C (1 + G_{darm}) d_{ctrl} = D C Delta L_{V} d_{ctrl} / Delta L_{V} = D C / (1 + G_{darm}) --> G_{V} = alpha xi beta D C / (1 + G_{darm}) --> IF G_{darm} << 1, then G_{V} ~ alpha xi beta D C and we have dependence on the changes in the optical plant of DARM. --> IF G_{darm} >> 1, then G_{V} ~ alpha xi beta D C / G = alpha xi beta / A and we have the scenario we have always assumed was happening. Now that Jenne's just restored old damping, this is less of a suspicion, but I wanted to write it down so that we keep this sort of loop inter-dependence in mind.