Something happened at about 10:30PM (11-12-2014 6:30 UTC) and the 363Hz oscillation of the end Y PZT mirror(s?) went totally unstable.
Daniel and Alexa are heading to EY to swap the controller.
In the attached right, references are from 24 hours ago, current traces are as of now. The trend shows the green QPD and the VEA temperature for 3 days.
Turning the controller off and on brought the phase margin back to ~1º (from maybe ~0.01°). Needs a more permanent fix.
After ~2 hours, it started oscillating again. Alexa and I went back and jammed a piece of damping material between the lower and upper stages of the PZT flex mount. This seemed to have damped the resonance to a level similar to the one observed in EX.
Here is a picture of the foam installed under PZT2 (2 inch). It was difficult to get the foam all the way into the gap; so we left it installed on one side of the gab -- this might drift over time. I have also attached the new spectra of the EY and EX QPDs as of now.
The attached plots show the oscillating state (page 1), the "stable" state (page 2) from before the damper, and the damped state. The damped EY version most closly resembles the undamped EX version.
We also made sure that the low bandwidth jumpers were at the correct position in the controller. Looking at the controller (see attached) we found the two capacitors which are likely to be used by the integrator. The bandwidth could possibly be further decreased by increasing C5/C6. The current values are 2 µF each.
