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Entry  Tue Aug 29 01:52:22 2017, gautam, Update, SUS, Test mass actuator calibration ALS_comparison.pdf
    Reply  Tue Aug 29 15:04:59 2017, gautam, Update, SUS, ETMY Oplev PIT loop gain changed ETMY_OLPIT.pdfGTR_comparison.pdf
    Reply  Wed Oct 18 17:34:09 2017, gautam, Update, SUS, ASDC 
Message ID: 13267     Entry time: Tue Aug 29 15:04:59 2017     In reply to: 13265
Author: gautam 
Type: Update 
Category: SUS 
Subject: ETMY Oplev PIT loop gain changed 

Last night, while we were working on the ALS, I noticed the GTRY spot moving around (in PITCH) on the CCD monitor in the control room at ~1-2Hz. The operating condition was that the arm was locked to the IR, and the PSL green shutter was closed, so that only the arm transmissions were visible on the CCD screens. There was no such noticable movement of the GTRX spot. When looking at the out-of-loop ALS nosie in this configuration (but now with the PSL green shutter open of course), the Y arm ALS noise at low frequencies was much worse than the X arm.

Today, I looked into this a little more. I first checked that the Y-endtable enclosure was closed off as usual (as I had done some tweaking to the green input pointing some days ago). There are various green ghost beams on the Y-endtable. When time permits, we should make an effort to cleanly dump these. But the enclosure was closed as usual.

Then I looked at the in-loop Oplev error signal spectra for the ITMY and ETMY Oplev loops. There was high coherence between ETMYP Oplev error signal and GTRY. So I took a loop transfer function measurement - the upper UGF was around 3.5Hz. I increased the loop gain such that the upper UGF was around 4.5Hz, with phase margin ~30degrees. Doing so visibly reduced the angular movement of the GTRY spot on the CCD. Attachment #1 shows the Oplev loop TF after the gain increase, while Attachment #2 compares the GTRX and GTRY spectra (DC value is approximately the same for both, around 0.4). GTRY still seems a bit noisier at low frequencies, but the out-of-loop ALS noise for the Y arm now lines up much more closely with its reference trace from a known good time. 

Quote:
 

Y-arm ALS wasn't so stellar tonight, especially at low frequencies. I can see the GTRY spot moving on the CCD monitor, so something is wonky. To be investigated.

 

Attachment 1: ETMY_OLPIT.pdf  9 kB  | Hide | Hide all
ETMY_OLPIT.pdf
Attachment 2: GTR_comparison.pdf  31 kB  | Hide | Hide all
GTR_comparison.pdf
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