Cataloging the many ways in which we are breaking lock or failing to lock since Friday, we found this one:
Sitting quietly at 10W DC readout, there was a slow ring up of a pitch instability in several ASC signals. Perhaps its time we went back to seriously controlling the ASC in the hard/soft basis instead of the optic basis in which its currently done. The frequency is ~1 Hz and the time constant is ~1 min. It would be great if someone can look at the signs of the fluctuations in the OL and figure out if this was dHard or cHard or whatever.
In the attached plot, I've plotted the OpLev pit signals during the time of this ringup (0702 UTC on 7/19). The frequency is 1 Hz. It appears with the same sign and similar magnitudes in all TMs except ITMX (there's a little 1 Hz signal in ITMX, but much smaller).
- Do we believe the calibration of these channels at the 30% level?
- Do we believe the sign of these channels?
- If the signs are self consistent, it seems to me that this is a Soft mode, Common arm fluctuations. But its weird for it to be at such a high frequency I think.
- Why does it take so long to ring up? If its due to Sidles-Sigg alone, I would guess that the Q would be lower (because of local damping). But if its a radiation pressure resonance and we have poor gain margin in our cSOFT loop, then it might could be.
Evan, Matt, Rana
We again saw the pitch instability tonight. We tried reducing it in a few ways, but the only successful way was to turn off the SRCL FF.
It appears that at higher powers, the SRCL_FF provides a feedback path for the pitch signals to get back to the Arms (since SRCL_FF drives the ITMs; and both of them as of Thursday). i.e. cSOFT has a secondary feedback path that includes some length<->angle couplings and produces a high Q unstable resonance. I don't understand how this works and I have never heard of this kind of instability before. But we repeatedly were able to see it ringup and ringdown by enabling SRCLFF.
To enable use of SRCL_FF, we've put a high pass filter into the SRCL_FF. This cuts off the SRCL_FF gain below a few Hz while preserving the phase above 10 Hz (where we want the subtraction to be effective). HP filter Bode plot attached.
