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Entry  Thu Apr 7 11:47:22 2022, Yehonathan, Update, BHD, Initial BHD modeling: AS - LO mode matching 
    Reply  Thu Apr 7 21:15:04 2022, Yehonathan, Update, BHD, Initial BHD modeling: AS - LO mode matching 
       Reply  Mon May 16 16:13:01 2022, Yehonathan, Update, BHD, Initial BHD modeling: Damped suspension model Damped_SUS.pngETMY_SUSPOS_GOL.pdfSUS_Response.png
          Reply  Thu Jun 16 17:40:15 2022, Yehonathan, Update, BHD, Comparison of MICH OLTF model with measurement MICH_Model_Measurement_Comparison.pdf
             Reply  Fri Jun 17 12:05:32 2022, Yehonathan, Update, BHD, Comparison of MICH OLTF model with measurement 
                Reply  Fri Jun 17 13:07:08 2022, Koji, Update, BHD, Comparison of MICH OLTF model with measurement 
Message ID: 16923     Entry time: Thu Jun 16 17:40:15 2022     In reply to: 16858     Reply to this: 16927
Author: Yehonathan 
Type: Update 
Category: BHD 
Subject: Comparison of MICH OLTF model with measurement 

I made some progress in modeling MICH loop.

Putting all the LSC and SUS filters together with the MICH Finesse model I constructed an OLTF model and plot it with the measurement done by Paco and Yuta in this elog (attachment 1).

There are 2 unknown numbers that I had to adjust in order to fit the model with the measurement:

1. The SUS damping loop gain (found to be ~ 2.22), which seems to vary wildly from SUS to SUS.

2. The coil driver gain (found to be 45), which I should measure.

I find coil_driver_gain*SUS_damping_filter_gain by increasing it until the SUS damping loop becomes unstable.

The coil driver gain I find by making the measurement and model overlap.

However, there is one outstanding discrepancy between the measurement and the model: Paco and Yuta measure the MICH calibration to be 1.3e9 cts/m while my model shows it to be 1.3e10 cts/m, an order of magnitude larger.

Details

The model can be summarized with these lines of code (I assume that the product of the ADCs(DACs) and and whitening(dewhitening) filters is unity):

BS2AS55 = TFs["AS_f2"]["BS"]

PD_responsivity = 1e3*0.8 #V/W
ADC_TF = 3276.8 #cts/V
demod_gain = 6.77 #According to https://wiki-40m.ligo.caltech.edu/Electronics/LSC_demoddulators
whitening_gain = 10**(24/20) #24 dB
BS2MICH = BS2AS55*PD_responsivity*demod_gain*whitening_gain*ADC_TF

DAC_TF = 6.285e-4 #V/cts, elog 16161

coil_TF = 0.016 #Newton/Ampere per coil, elog 15846 

coil_R = 20e3 #Ohm

actuation_TF = DAC_TF*coil_TF/coil_R


OLTF = (BS2MICH*MICH_ctrl_cmplx*-6*0.5 + OSEM_filters_cmplx*OSEM_TF*2.22)*coil_filters_cmplx*actuation_TF*SUS_cmplx*45

  • BS2AS55 is the optical plant, calculated with Finesse
  • MICH_ctrl_cmplx is the MICH control filter with gain of -6
  • 0.5 factor comes from the LSC output matrix
  • OSEM_TF is the product of the OSEMs input filters and damping loop filters.
  • coil_filters_cmplx are the coil filters
  • SUS_cmplx is the suspension transfer function (w0 = 1Hz, Q=200)
Attachment 1: MICH_Model_Measurement_Comparison.pdf  18 kB  Uploaded Thu Jun 16 18:45:48 2022  | Hide | Hide all
MICH_Model_Measurement_Comparison.pdf
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