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Entry  Wed May 30 15:51:22 2012, yuta, Update, IOO, MC beam spot oscillation specMC3_onoff_localdamping.pdf
    Reply  Wed May 30 19:27:38 2012, yuta, Update, IOO, MC beam spot oscillation 
       Reply  Wed May 30 20:12:15 2012, rana, Update, IOO, MC beam spot oscillation 
          Reply  Thu May 31 10:31:19 2012, Jamie, Update, IOO, MC beam spot oscillation 
Message ID: 6715     Entry time: Wed May 30 15:51:22 2012     Reply to this: 6718
Author: yuta 
Type: Update 
Category: IOO 
Subject: MC beam spot oscillation 

[Koji, Suresh, Jenne, Yuta]

Background:
  We noticed that the beam spots on MC mirrors are oscillating in ~ 1 Hz yesterday. It means MC mirrors are actually oscillating. This was observable even if the WFS servo is off.

What we did:
  1. By measuring the spectra of OSEM sensor outputs, we found that MC3 is the one that is oscillating.

  2.  Oscillation at ~ 1 Hz only happened when the local damping using OSEMs are on (see Attachment 1; REF is when the damping is on).

  3.  We found that this oscillation came from insufficiency in phase margin in SUSPOS loop. So, we increased the gain, C1:SUS-MC3_SUSPOS_GAIN, from 95 to 200. It helped a little, but oscillation is still there.

  4.  We measured openloop transferfunctions of SUSPOS, SUSPIT, SUSYAW, SUSSIDE loop, and concluded that diagonalization some how went wrong. The amplitude of the oscillation (peak height in the OSEM spectra) changed by pushing the MC SUS connectors.

Plan:
  - Fix the connectors so that we don't have to push them any more.
  - Redo the diagonalization of the MC suspensions.

Attachment 1: specMC3_onoff_localdamping.pdf  54 kB  | Hide | Hide all
specMC3_onoff_localdamping.pdf
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