40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
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Entry  Tue Mar 21 15:13:44 2017, gautam, Update, IMC, IMC input beam mode matching IMG_7220.JPGIMC_ModeMatch.pdfsingleLensSensitivity.pdfsensitivity.pdfIMCmodeMatch.m
    Reply  Tue Mar 21 21:59:48 2017, gautam, Update, IMC, IMC input beam mode matching IMG_3654.JPGIMG_3655.JPG
       Reply  Wed Mar 22 00:33:00 2017, gautam, Update, IMC, IMC length offset nulling offsetInvestigation.pdfoffset_summing_amp.pdf
          Reply  Wed Mar 22 16:58:25 2017, gautam, Update, IMC, WFS sensing matrix measurements 
             Reply  Thu Mar 23 01:44:53 2017, gautam, Update, IMC, WFS sensing matrix measurements IMC_WFS_segment_TF.pdfIMC_WFS_channels_TF.pdfTFsummary.pdfIMC_WFS_170322.xlsx.zip
                Reply  Thu Mar 23 08:43:11 2017, rana, Update, IMC, WFS sensing matrix measurements 
                   Reply  Thu Mar 23 23:38:58 2017, gautam, Update, IMC, MC SUS damping gains stepped down 
                      Reply  Fri Mar 24 11:26:57 2017, gautam, Update, IMC, MC SUS damping gains restored 
                         Reply  Fri Mar 24 19:04:18 2017, gautam, Update, IMC, Seismic feedforward and WFS MCL_comparison.pdfseis_comparison.pdf
Message ID: 12906     Entry time: Fri Mar 24 19:04:18 2017     In reply to: 12904
Author: gautam 
Type: Update 
Category: IMC 
Subject: Seismic feedforward and WFS 

[valera, gautam]

On Wednesday at the meeting, we were discussing why we aren't able to achieve more seismic feedforward subtraction in MCL. We spent some time thinking about this yesterday, and this elog is meant to be a summary of the stuff we tried. 

  1. We let the WFS loops run for a while and settle, and then turned the input gain down to zero so that the integrators held the outputs to the suspension at a "good" alignment. If the WFS loop bandwidth is ~0.1 Hz, then they aren't helping us at 1Hz anyways. We then looked at coherence between the seismometer signals in this state compared to when the WFS loops were running, and noticed negligible difference. It doesn't seem like the WFS loops are injecting noise into MCL at ~1Hz.
  2. We decided agains implementing the WFS sensing matrix I measured on Wednesday evening, as we found that the relative magnitudes of the matrix elements are virtually the same as in Koji's measurement back in December 2016. But looking at matrix elements like MC1P->WFS1P compared to MC3P->WFS1P - there is a difference of a factor of ~3. Why should there be? The response should be completely symmetric to MC1 and MC3?
  3. While looking at the OSEM channels (i.e. SUSPIT_IN1_DQ, SUSYAW_IN1_DQ etc) for each of the MC optics, we noticed a dramatic difference between MC1 (factor of ~10 higher) and the other two MC optics.
  4. Looking at coherence between MCL and the seismometer channels, we felt that there is less coherence at low frequencies (1Hz and lower) now than there was back in January when I took a measurement. However, there was coherence between the OSEM signals and the seismometers - so it doesn't look like the seismometer is to blame. To make an apples-to-apples comparison, I compared the MCL and Seismometer channel spectra from January to now (for the latter, at two different settings of the damping loop gains on the MC suspensions), and also the maximum predicted achievable subtraction (using EricQs frequency domain multicoherence tool). The two changes I can think of since January are that the MC1 satellite box has been interchanged with the SRM satellite box, and the IMC servo gains have been reallocated since the RF upgrade. My findings are summarized in attachments #1 and #2.

The seismometer spectra look similar enough to be explained by time of day variations, so perhaps the culprit is MC1. The ambient MCL spectrum is almost an order of magnitude higher above 4Hz now, with the nominal damping loop gains, as compared to back in January. I think the damping loops on MC1 need to be tweaked.


Attachment 1: MCL_comparison.pdf  570 kB  Uploaded Fri Mar 24 20:04:36 2017  | Hide | Hide all
Attachment 2: seis_comparison.pdf  312 kB  Uploaded Fri Mar 24 20:06:30 2017  | Hide | Hide all
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