Sheila, Rana

We saw the dIzumi/dBallmer instability happening again tonight at the 19->23 W transition. Since this was happening in the cSoft DoF, we started debugging it and made some changes to see if we can damp the instability with a more rational loop than using oplevs or ISS.

Balancing the input Matrix:

We drove lines in cSoft and dSoft and then adjusted the relative gains for the X & Y TransMons so that the crosscoupling was < 15% (as measured at ~8 Hz). Confusingly, we only adjusted the matrix elements for dSoft. The existing cSoft matrix elements already gave a good decoupling. Before the adjustment we saw (via step response) that big DC steps in cSoft made steps in dSoft. After the matrix change this behavior is gone. The step responses seem well decoupled. 

After much struggle, we were able to measure the soft loop shape. After turning up the gain from -0.05 to -1.14, we have a ~30 sec time constant and the UGF is a little below 100 mHz. We are able to see that the mismatch between the radiation pressure modified soft mode resonance and the digital compensator (FM10) is what makes cSoft go unstable if we turn up the gain further. Tomorrow we make a better filter and then see if we can damp the angular instability with an angular feedback loop.