aLIGO LHO Logbook

H1 ISC

Link

sheila.dwyer@LIGO.ORG - posted 23:14, Monday 22 October 2018 - last comment - 23:12, Tuesday 23 October 2018(44740)

SRC ASC, LSC feedforward

Sheila, Hang, Craig, Georgia, Danny, Thomas

Hang lowered the gain in MICH P to avoid the possibility that it was unstable at 1.1Hz 44739. We later realized that the locklosses we've been having where MICH P rings up at 1.1 Hz were actually caused by the SRCLFF, not by ASC
We left the SRC ASC on as we increased the power, and this seems fine so it is now in the guardian.
We also added the ISS second loop back into the guardian.
In the low noise length control state there were some gain changes that didn't make sense when we looked at the OLG measurements, so we have commented them out. This leaves us with a SRCL ugf of 35 Hz, PRCL at 60 Hz ,and MICH at 9.5 Hz.
We engaged the MICH FF and it still seems to be doing a good job of reducing the coherence.
We found that the SRCL FF we have is unstable, we made the measurements to redo the fitting. We saved templates in userapps/lsc/h1/templates Craig is working on fitting and has a filter that is ready to try
We ran Gabriele's A2L script, it worked and reduced the coherence with ASC.
We lost lock for an unknown (fast) reason at 3:07 UTC, 4:15 UTC, 5:36 UTC (may have been a small EQ) and 6:09 UTC
ETMY mode1 gets rung up in about 1 out of every 5 locklosses. I am not sure why this mode in particular should be rung up, it is not at the edge of any of our notches. The filter used for damping is narrower, so it might ring longer on locklosses than the wider filters do.

Comments related to this report

craig.cahillane@LIGO.ORG - 23:35, Monday 22 October 2018 (44744)ISC

Link

The new SRCL FF is implemented in FM4 of SRCLFF1, named "Oct22".  It corresponds to the dashed green line in the plot below.

Sheila created two dtt templates in /opt/rtcds/userapps/release/lsc/h1/templates/ which make the DARM err/ SRCL out and DARM err/ SRCL FF measurements.  My notebook in /ligo/home/craig.cahillane/Git/Feedforward/SRCL2DARMfeedforward.ipynb reads in the latest results in those templates using dtt2hdf then fits them with iirrational.  Some tweaking of the fit was required.  Thanks to Hang for the useful pyctrl library for easy communication with iirrational and foton.

If you want to use this notebook, you first have to source my anaconda environment to get iirrational and dtt2hdf:
source /ligo/home/craig.cahillane/bashScripts/activateAnaconda.sh
jupyter notebook /ligo/home/craig.cahillane/Git/Feedforward/SRCL2DARMfeedforward.ipynb

Non-image files attached to this comment

20181022_SRCL_FF_iirrationalFit.pdf

craig.cahillane@LIGO.ORG - 02:08, Tuesday 23 October 2018 (44745)

Link

The filter "Oct22", in conjunction with the "AC" filter already in the SRCLFF1 bank, was unstable. 

The AC filter was supposed to AC couple the SRCL feedforward.  It consisted of two zeros at 0 Hz and two res g poles with low Q at around 1.0 Hz.  I tried pushing the AC cutoff up to 10 Hz, but I still had an instability at 3.5 Hz.
This instability was caused by the Oct22 filter adding too much gain and phase to the loop, giving a phase crossover right at 3.5 Hz with gain > 1.  (pic 1)

So I created the "Oct23" filter and a new "AC" filter by hand.  It's not the best fit in the world, but it's not unstable.

EDIT: I enabled SRCLFF1 in the LOWNOISE_LENGTH_CONTROL guardian.  The filter module gain should be 1.0, not -1.0, for "Oct23".  Changed in lscparams.

Images attached to this comment

Non-image files attached to this comment

20181023_SRCL_FF_iirrationalFit.pdf

gabriele.vajente@LIGO.ORG - 12:08, Tuesday 23 October 2018 (44751)

Link

It looks like when we are in NLN, the L3 ISCINF input to the ITMs (coming from the LSC feedforward) is the same, but the actual control signal sent to the L2 actuators is different in ITMX and ITMY. This might explain part of the instability issue: the SRCL feedforward is supposed to be a purely differential drive, and tus orthogonal to SRCL. But the ITM driving unbalance can cause a common drive, which feeds back to SRCL.

It seems that the difference in L2 driving in the two ITMs is a residual mistake from the ESD low noise transition. We'll fix it and measure again the feedforward paths.

Images attached to this comment

sheila.dwyer@LIGO.ORG - 23:12, Tuesday 23 October 2018 (44773)

Link

The differences in filter states and gains that are residual from the ESD transition are only in L3, so shouldn't impact the feedforward. I don't know why the drives to the 2 ITMs are different.