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Entry  Wed Aug 17 07:35:48 2022, yuta, Bureaucracy, General, My wish list for IFO commissioning 
    Reply  Fri Aug 19 14:46:32 2022, Anchal, Update, SUS, Open loop transfer function measurements for local damping loops of BHD optics 
       Reply  Sat Aug 20 20:26:10 2022, Anchal, Update, SUS, Open loop transfer function measurements for local damping loops of Core optics ALL_CORE_SUS_OLTF_2022-08-20-merged.pdf
          Reply  Mon Aug 22 14:36:49 2022, rana, Update, SUS, Open loop transfer function measurements for local damping loops of Core optics 
Message ID: 17092     Entry time: Fri Aug 19 14:46:32 2022     In reply to: 17085     Reply to this: 17096
Author: Anchal 
Type: Update 
Category: SUS 
Subject: Open loop transfer function measurements for local damping loops of BHD optics 

[Anchal, Tega]

As a first step to characterize all the local damping loops, we ran an open loop transfer function measurement test for all BHD optics, taking transfer function using band-limited (0.3 Hz to 10 Hz) gaussian noise injection at error points in different degrees of freedom. Plots are in the git repo. I'll make them lighter and post here.

We have also saved coherence of excitation at the IN1 test points of different degrees of freedom that may be later used to determine the cross-coupling in the system.

The test ran automatically using measSUSOLTF.py script. The script can run the test parallelly on all suspensions in principle, but not in practice because the cdsutils.getdata apparently has a limitation on how many real-time channels (we think it is 8 maximum) one can read simultaneously. We can get around this by defining these test points at DQ channels but that will probably upset the rtcds model as well. Maybe the thing to do is to separate the c1su2 model into two models handling 3 and 4 suspensions. But we are not sure if the limitation is due to fb or DAQ network (which will persist even if we reduce the number of testpoints on one model) or due to load on a single core of FE machines.

The data is measured and stored here. We can do periodic tests and update data here.


Next steps:

  • Run the test for old optics as well.
  • Fit the OLTF model with the measured data, and divide by the digital filter transfer function to obtain the plant transfer function for each loop.
  • Set maximum noise allowed in the local damping loop for each degree of freedom, and criteria for Q of the loop.
  • Adjust gains and or loop shape to reach the requirements on all the suspensions in a quantitative manner.
  • (optional) Add a BLRMS calculation stream in SUS models for monitoring loop performance and in-loop noise levels in the suspensions.
  • More frequency resolution, please. (KA)
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