40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  40m Log  Not logged in ELOG logo
Entry  Thu Nov 17 15:17:16 2016, gautam, Update, IMC, MCL Feedback MCLerror.pdfMCLtest.pngYarmCtrl.pdf
    Reply  Wed Nov 23 01:13:02 2016, gautam, Update, IMC, MCL Feedback MCL_plant.pdfOLG.pdfMC_armSpectra_X.pdfMC_armSpectra_Y.pdf
       Reply  Wed Nov 23 15:08:56 2016, rana, Update, IMC, MCL Feedback 
          Reply  Fri Feb 3 00:36:52 2017, gautam, Update, IMC, MCL Feedback - framing the problem 
             Reply  Fri Feb 3 11:40:09 2017, rana, Update, IMC, MCL Feedback - framing the problem 
                Reply  Mon Feb 6 17:03:41 2017, gautam, Update, IMC, MCL Feedback - simulink model updated mc40_v1.pdf
                   Reply  Mon Feb 6 18:20:08 2017, Koji, Update, IMC, MCL Feedback - simulink model updated 
                      Reply  Wed Feb 8 19:13:02 2017, gautam, Update, IMC, MCL Feedback - TF measurements 
                         Reply  Thu Feb 9 23:35:34 2017, gautam, Update, IMC, MCL Feedback - TF measurements mc40_v1.pdfCMboard_OLTF_fit.pdfFSSFast_OLTF_fit.pdfPCdrive_OLTF_measured.pdfdata.zip
Message ID: 12635     Entry time: Wed Nov 23 01:13:02 2016     In reply to: 12623     Reply to this: 12637
Author: gautam 
Type: Update 
Category: IMC 
Subject: MCL Feedback 

I wanted to get a clearer idea of the FSS servo and the various boxes in the signal chain and so Lydia and I poked around the IOO rack and the PSL table - I will post a diagram here tomorrow.

We then wanted to characterize the existing loop. It occurred to me later in the evening to measure the plant itself to verify the model shape used to construct the invP filter in the feedback path. I made the measurement with a unity gain control path, and I think there may be an extra zero @10Hz in the model.

Earlier in the evening, we measured the OLG of the MCL loop using the usual IN1/IN2 prescription, in which above 10Hz, the measurement and FOTON disagree, which is not surprising given Attachment #1.

I didn't play around with the loop shape too much tonight, but we did perform some trials using the existing loop, taking into account some things I realized since my previous attempts. The summary of the performanceof the existing loop is:

  • Below 1Hz, MCL loop injects noise to the arm control signal. I need to think about why this is, but perhaps it is IMC sensing noise?
  • Between 1-4Hz, the MCL loop suppresses the arm control signal
  • Between 4-10Hz (and also between 60-100Hz for the Xarm), the MCL loop injects noise. Earlier in the evening, we had noticed that there was a bump in the X arm control signal between 60-100Hz (which was absent in the Y arm control signal). Koji later helped me diagnose this as too low loop gain, this has since been rectified, but the HF noise of the X arm remains somewhat higher than the Y arm.

All of the above is summarized in the below plots - this behaviour is (not surprisingly) in line with what Den observed back when he put these in.



The eventual goal here is to figure out if we can get an adaptive feedback loop working in this path, which can take into account prevailing environmental conditions and optimally shape the servo to make the arms follow the laser frequency more closely at low frequencies (i.e. minimize the effect of the noise injected by IMC length fluctuations at low frequency). But first we need to make a robust 'static' feedback path that doesn't inject control noise at higher frequencies, I need to think a little more about this and work out the loop algebra to figure out how to best do this...

Attachment 1: MCL_plant.pdf  54 kB  | Hide | Hide all
Attachment 2: OLG.pdf  46 kB  | Hide | Hide all
Attachment 3: MC_armSpectra_X.pdf  20 kB  Uploaded Wed Nov 23 11:55:28 2016  | Hide | Hide all
Attachment 4: MC_armSpectra_Y.pdf  19 kB  Uploaded Wed Nov 23 11:55:34 2016  | Hide | Hide all
ELOG V3.1.3-