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VIS (PR2)
masayuki.nakano - 19:10 Tuesday 31 October 2017 (3602) Print this report
PR2 and PR3 oplev damping improvement

I designed new damping filter for type Bp suspensions. It damped all motion more by factor of 5-10 in RMS than previous filters.

TMs are damped by IM OSEMs below ~0.1 Hz and by TM OSEMs above. The UGF is 6Hz for YAW and ~2Hz for Pitch.
I attached below plots

  1. Transfer function of the servo filter.: It's used for all suspensions for YAW and PITCH and placed on FM3 of K1:VIS-PR2/3_TM_DAMP.
  2. Open loop transfer function of the damping loop. It's measured at K1:VIS-PR2/3_TM_OPLEV_TILT_IN1/2. I attached only one of PR2 and ones of PR3 is similar with that.
  3. Power spectrums of oplev signals.

Concerns and next work

  • Even there are enough phase margin, I could not increase the servo gain for pitch damping. A peak appeared at around 4Hz after increasing by factor of 3.
  • The coils should be balanced. I saw some coupling with yaw and .pitch
  • I also attached the spectrum of the PD signal placed in Xend. Around 1Hz the PR3 mirror motion causes the fluctuation of PD signal. (There are no coherence with other mirrors.) The motion should be damped more around that region.
Images attached to this report
Comments to this report:
ayaka.shoda - 13:20 Wednesday 01 November 2017 (3607) Print this report

There is a resonance of the RM in pitch at around 4 Hz, which you cannot see well with the oplev signals.
I will attach the simulated and measured transfer functions of TMP and IMP.

Just for curiocity, can you find coherence between IM OSEM signals and EX PD signals?
In case you can see coherence, it would be efficient to increase the control gain of IM loop. Another good point for increasing IM control loop is that the OSEM noise level is better than that of Oplev.

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