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Entry  Wed Mar 6 18:14:33 2013, Manasa, Summary, LSC, Calibration of BS, ITMX and ITMY actuators MICHerrCalibration.pngMICHerrCalibration.pngMICHOLTF.pngactcalibITMBS.png
    Reply  Thu Mar 7 01:43:35 2013, yuta, Update, LSC, calibrated MI differential length spectra LSCMICHOLTF_MI.pngMImotion.png
    Reply  Fri Mar 8 02:17:04 2013, yuta, Update, LSC, calibration of PRM actuator calibREFL55Q.pngactcalibPRM.pngOLTF_PRCL.png
       Reply  Fri Mar 8 03:07:19 2013, yuta, Update, LSC, calibrated PRM-ITMY length spectra LSCPRCLOLTF_PRITMY.pngPRITMYmotion.png
Message ID: 8248     Entry time: Thu Mar 7 01:43:35 2013     In reply to: 8242
Author: yuta 
Type: Update 
Category: LSC 
Subject: calibrated MI differential length spectra 

Free swing MI differential length is 86 nm RMS and residual length when locked is 0.045 nm RMS(in-loop).
Looks very quiet. Comparison with PRMI is the next step.

Openloop transfer function:
  OLTF of simple MI lock using AS55_Q_ERR as error signal and ITMs as actuators is below.
  UGF ~ 90 Hz, phase margin ~ 40deg
  I added 16 Hz resonant gain to suppress bounce mode.

MI differential length spectra:
  Below. Calibration was done using calibrated AS55_Q_ERR and actuator response(elog #8242)

  Expected free swing is calculated using

x_free = (1+G)/G * A * fb

where G is openloop transfer function, A is actuator response, fb is feedback signal(C1:LSC_ITMX/Y_IN1) spectrum. I used A as simple pendulum with resonant frequency at 1 Hz, Q = 5. Since free swing RMS is dominated by this resonance, RMS depends on this Q assumption.

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