sheila.dwyer@LIGO.ORG - posted 23:25, Wednesday 19 December 2018 - last comment - 12:38, Thursday 20 December 2018(46084)
locking notes this evening
- 28 Hz instability:
- We found that the PRC2Y ASC loop was changing the loop shape of the PRCL loop, only after the interferometer sat at 20W for several minutes. This was the cause of the 28 Hz instabilities we have seen several times (4062).
- Hang moved the ASC gain changes and cut offs for SRC2, PRC1 and PRC2 to the end of 2W ASC engage state, so that these are on during the power up. We haven't seen the 28 Hz instability since.
- Rana measured the PRC2 loop, which has a very funny shape. We switched PRC2 actuation from PR2 to PR3 a few months ago. Rana suggested that there could be something wrong with the actuator (like a broken coil) that is causing large cross couplings.
- 3.2 Hz instability
- Since fixing the 28 Hz instability, we have been having trouble with an instability at 3.2 Hz which seems to be an ASC problem.
- We measured many things, including PRC2P which has a suspicious feature around 3Hz (but this loop should be stable).
- Rana edited the cut off filter for PRC2.
- Georgia saw that the 3.2 Hz instability seems to be more in pitch than yaw, it is especially visible in MICH P, PRC1 Y, SOFT P+Y, HARD P+Y
- Moving PRC2 to POP:
- When we powered up to 20W we lost lock when the PRC2Y error signal drifted away from zero and there was a large excursion in SRC1+ SRC2 Y (POP 90 increased, OPO18 dropped).
- Next lock we moved the PRC2 ASC loop to POPX, this is now in the guardian, this seems to have helped us to be more stable.
- In one of the locklosses, 1229307644 we saturated the ADC for MC2 trans before the lockloss. I lowered the whitening gain by 9dB, and added 9dB filters to keep the calibration the same as in 9716.
- We are also saturating the ADC for PWR_IN_AC_INMON on ASCIMC, which I think is on a diode in transmission of the bottom PSL periscope mirror.
This loop uses the POP WFS as a sensor and feeds back to PR2 angle. The PRC1 ASC loop uses POP QPD as a sensor and feedsback to PRM.
as you can see, this loop has multiple unity gain crossings. Hang previously has a -3dB notch for the 3 Hz part, but I think that's only a band-aid.
Best off to either lower the gain more or to put in a good compensation for the PR2-M3 TF. There was a compensation filter in that filter bank but its not quite right and its also not turned on by default.
On one lock we turned the loop gain down by 2 and things looked stable for awhile, but then we had a 3.5 Hz instability in cHard yaw, so unclear if this is a problem.
The second attachment shows the old/new filter. It is now more unconditionally stable. Checked yaw - it seems OK.
Georgia, Craig, Dan, Rana
Georgia noticed that there were ~3.5 Hz yaw oscillations in arm common mode ASC. Using the continuously running ASC sine waves, we looked at the CHARD/soft matrix and it was nearly bad: the CSOFT sensor was seeing almost purely cHard, whereas the cHard sensor had a mixture of both.
The Matrix initially had 3 RF sensors for cHard and only Transmons for cSoft. (no blending going on for yaw, although there was some pitch blend).
WE then added the CHARD_A input matrix elements to cSoft_B and ramped it in so as to cancel the cHard contribution to the cSoft loop. This worked well and as you can see in the attachment, the CSOFT spectrum no longer shows the 23 Hz cHARD line.
Next we will do the real 4x4 inversion to clean up PRC1 and PRC2 from the other loops.
We measured the PR2 M3 P2P sus TF. To do so we misaligned PRM, SRM, and ITMY, and then used AS_A_DC_PIT/YAW to measure the response to PR2 PIT motion. During the process we also engaged the DC centering loops to keep the beam onto the PD, yet we turned the DC gains of the centering loops down by a factor of 100 so that they should not affect the responses around sus resonances.
The measured response is shown in the first figure. From it we can also see that the PR2 P2Y coupling is actually quite small, indicating a good coil balancing.
We also fit the P2P response (up to a DC gain ) into zpk format. The fitting result is shown in the second plot and in the last .txt file we gave the zpk representation in the foton format (s-domain). This can be used for better plant inversion.
The yaw loop seemed fine and thus we did not measure them this time.