Dan, Sheila
OP Lev Damping on ETMs
We have turned the op lev damping for the ETMs off completely after the interferometer is locked. This is now in the gaurdian. The attached spectra show the witness sensors with oplev damping on (dashed lines) and off (solid lines). We knew these loops were not good, but this clearly shows it. The bottom right plot shows the DHARD WFS error signal with the oplev damping on and off.
DHARD WFS
The DHARD loop, which was designed based on measurements made at a carm offset of about 50 picometers with high op lev damping gain (see alog 16501 comment) , needs to be redesigned to take into account changes in the plant on resoannce and with the op lev damping off or reduced. The attached plots show the change in the transfer function, the first one shows the open loop gain in the situation where the loop was designed, the transfer function on resonance, and a partial measurement of the TF with oplev damping gain reduced. You can see this loop becomes unstable, which explains why we have had to reduce the gain several times in the last few weeks. After turning off the op lev damping, we attempted to take some measurements which could be used to design a new loop, these are shown in the last screen shot and can be found in my directory under /Alingment/DHARD_WFS_NO_OPLEV_WHITE_NOISE.xml While the cohernce is not good at some frequencies, this should be better than what we have now.
Attachments:
1) Spectra showing that op Lev damping was imposing noise, and is now off
2) Swept sine measurements showing that DHARD PIT becomes unstable as we go on resonance and reduce Op Lev gains.
3) swept sine measurements of DHARD Pit plants on resonance and with reduced op lev gain
4) noise injection measurements of DHARD plant with op lev damping off.
Other locking notes from tonight:
- The ETMY violin modes were higher tonight than Friday by a factor of several. I made +/-60deg filters in the ETMY_L2_DAMP_MODE2 filter bank and toyed with the gain and phases. With a gain of 500k and no phase shift the peak height decreased by ~20% in half an hour. So, perhaps some progress.
- On Friday there was some coherence above 100Hz with the OMC ASC loops while the dither was on, I installed some more aggressive butterworth rolloff filters into the dither error signal filter banks. Also I added some poles and zeros to the POSX filter bank to counteract a somewhat poor job of plant inversion for this d.o.f. Tonight the dither alignment was robust, but the IFO itself was quite noisy, so it's not clear whether this was a measureable improvement over Friday.
- Progress was made on the OMC autolocker code. The challenge here is that there is no high-frequency readback of PZT2, only the DC monitor channel is recorded at >16Hz. There is some lag between the PZT2 output and the DC monitor readback, so while the code can find the mode (by sweeping the cavity), the voltage it expects the mode to be at isn't the same as the PZT slider value. This needs some work, more fast DQ channels in the OMC model would be helpful.
We made it to low noise a couple of times, but the pitch motion at the AS port was so bad that the DC lock was not very stable.