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Entry  Tue Mar 24 19:41:57 2020, gautam, Update, Wiener, Seismic feedforward for MCL IMCseisFF.pdffilterComp.pdfoldFilter_v_proposed.pdfMCL_ff_performance.pdf
    Reply  Wed Apr 1 00:51:41 2020, gautam, Update, Wiener, Slightly improved MCL FF MC2_act_calib.pdfIIR_fit_to_FIR.pdfFIRvIIR.pdf
Message ID: 15282     Entry time: Tue Mar 24 19:41:57 2020     Reply to this: 15290
Author: gautam 
Type: Update 
Category: Wiener 
Subject: Seismic feedforward for MCL 


I think the feedforward filters used for stabilizing MCL with vertex seismometers would benefit from a retraining (last trained in Sep 2015). 


I wanted to re-familiarize myself with the seismic feedforward methodology. Getting good stabilization of the PRC angular motion as we have been able to in the past will be a big help for lock acquisition. But remotely, it is easier to work with the IMC length feedforward (IMC is locked more often than the PRC). So I collected 2 hours of data from early Sunday morning and went through the set of steps (partially).

Attachment #1 shows the performance of a first attempt.

  • 1 hour of data was used as a training set, and another hour to validate the trained filter.
  • All the data was downsampled to 64 Hz.
  • The number of FIR filter taps was 32 seconds * 64 Hz. 
  • Going through some old elogs, there were a number of suggestions from various people about how the training should be done
    • There was a suggestion that pre-filtering the target signal by the (inverse) actuator TF (i.e. TF from MC2 drive to MCL) is beneficial, presumably because it gives the Wiener filter fitting fewer parameters to fit.
    • There was also suggestions that some frequency-dependent weighting of the target signal should be done (e.g. by bandpassing MCL between 0.1 Hz - 10 Hz) to emphasize subtraction in this band.
    • For this particular example, in my limited paramter space exploration, I found that neither of these measures had particularly significant impact.
  • In any case, the time-domain FIR filtering seems to approach the theoretical best possible performance (based on coherence information). 
  • I have not yet checked what the theoretical limit on subtraction will be based on the seismometer noise ASD.

Attachment #2 shows a comparison between the filter used in Attachment #1 and the filters currently loaded into the OAF system. 

  • In the band where significant subtraction is possible, there is some difference in the shape of the filter.
  • Why should this have changed? I guess there are multiple possibilities - seismometer recentering, signal chain changes, ...

Attachment #3 is the asd after implementing a time domain Wiener filter, while Attachment #4 is an actual measurement from earlier today - it's not quite as good as Attachment #3 would have me expect but that might also be due to the time of the day. 

Conclusions and next steps:

On the basis of Attachments #3 and #4, I'd say it's worth it to complete the remaining steps for online implementation: FIR to IIR fitting and conversion to sos coefficients that Foton likes (prefereably all in python). Once I've verified that this works, I'll see if I can get some data for the motion on the POP QPD with the PRMI locked on carrier. That'll be the target signal for the PRC angular FF training. Probably can't hurt to have this implemented for the arms as well.

While this set of steps follows the traditional approach, it'd be interesting if someone wants to try Gabriele's code which I think directly gives a z-domain representation and has been very successful at the sites.

* The y-axes on the spectra are labelled in um/rtHz but I don't actually know if the calibration has been updated anytime recently. As I type this, I'm also reminded that I have to check what the whitening situation is on the Pentek board that digitizes MCL.

Attachment 1: IMCseisFF.pdf  238 kB  Uploaded Tue Mar 24 20:48:59 2020  | Hide | Hide all
Attachment 2: filterComp.pdf  153 kB  Uploaded Tue Mar 24 20:58:17 2020  | Hide | Hide all
Attachment 3: oldFilter_v_proposed.pdf  206 kB  Uploaded Tue Mar 24 21:00:27 2020  | Hide | Hide all
Attachment 4: MCL_ff_performance.pdf  32 kB  Uploaded Tue Mar 24 21:00:40 2020  | Hide | Hide all
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