aLIGO LHO Logbook

H1 SUS (AOS, ISC, SUS)

evan.hall@LIGO.ORG - posted 21:44, Tuesday 02 December 2014 - last comment - 10:29, Wednesday 03 December 2014(15399)

ITMX bounce mode damping

Kiwamu, Dave, Dan, Alexa, Sheila, Elli, Evan

We have been having trouble keeping ALS DIFF locked for the past few hours. Alexa finally figured out there was a large 10 Hz peak in DIFF error spectrum and the ESD drives.

We thought it might be either the ETMX or ETMY 9.8 Hz bounce modes, since these have previously given us trouble (LHO#15118, LHO#15247). However, by looking at oplev spectra, we determined it was actually ITMX and the frequency was 9.84 Hz.

So we copied over Matt’s ETMX damping filters (pass9.8, which has the sharp gain, and phase9.8, which allows us to adjust the phase) and adjusted the frequency to 9.84 Hz. Then I turned off the oplev filters and engaged pass9.8, and played with the gain (originally −0.05) until I got damping (the necessary gain was 300). The mode was originally a factor of 5–10 above the ITMX pitch oplev noise floor, and after 10 minutes we were able to push it into the noise floor. The use of phase9.8 was not necessary.

I have restored the ITMX pitch damping, along with a 9.84 Hz elliptic stopband filter stop9.84 which is modeled after an ETMX L1 filter.

Comments related to this report

daniel.hoak@LIGO.ORG - 21:58, Tuesday 02 December 2014 (15400)

Link

Attached is a fine-resolution spectrum of the ITM OpLev pitch signals. ITMX bounce mode = 9.8469 Hz, ITMY = 9.8315 Hz. Probably ITMY will be a problem someday, too.

The bounce mode peaks are very narrow - only about 2mHz wide. One theory (from Dave) is that there might be two modes very close to one another, and only one of them couples to the oplev feedback; the other stores energy and makes the mode difficult to damp. The list of suspension resonances says that the z2 and z3 modes are both around this frequency, but doesn't say if we expect them to be within 2mHz of each other.

Images attached to this comment

matthew.evans@LIGO.ORG - 05:29, Wednesday 03 December 2014 (15405)

Link

The phase9.8 filter was designed to be used with the standard OpLev damping to prevent excitation of the bounce mode, not with the Pass9.8 (used with the 0:50 filter to damp the mode after it gets excited).

keita.kawabe@LIGO.ORG - 08:37, Wednesday 03 December 2014 (15407)

Link

ITMX is supposed to be as bad as ETMY.

ETMY > ITMX >>>>>>>> ITMY > ETMX, in terms of bounce-to-length coupling.

https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=14876

norna.robertson@LIGO.ORG - 10:29, Wednesday 03 December 2014 (15409)

Link

I want to clarify something Dan wrote.
There is only one vertical resonance at ~ 9 Hz in the quad - it is the highest of the four vertical resonances, called modev4 in the list of resonances which can be seen at e.g.
https://awiki.ligo-wa.caltech.edu/aLIGO/Suspensions/OpsManual/QUAD/Models/20140304TMproductionTM

The reference to z2 and z3 describes the dominant motion in that mode i.e. which masses are moving and in which directions in that mode. The highest vertical resonance involves the penultimate and test masses moving out of phase with each other in vertical - hence the z2 ( which is PUM vertical motion) and z3 (test mass vertical motion).
The numbering is historical from our original MATLAB models, based on extending a triple model- masses are 0,1,2,3 from top to bottom.