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Entry  Mon Jan 12 18:11:25 2015, ericq, Update, ASC, Test Mass -> Transmon QPD TFs measured Jan12_singleArmSensing.pdfJan12_singleArmSensing.xml.zip
    Reply  Thu Jan 15 18:57:41 2015, ericq, Update, ASC, Transmon QPD loops live loopDesign.pdf
       Reply  Fri Jan 16 04:14:38 2015, ericq, Update, CDS, Unexpected CDS behavior digitalProblem.pdf
Message ID: 10908     Entry time: Thu Jan 15 18:57:41 2015     In reply to: 10886     Reply to this: 10912
Author: ericq 
Type: Update 
Category: ASC 
Subject: Transmon QPD loops live 

I've measured the sensing for each of the arms, by using our calibrated oplevs, in terms of QPD counts per micron. It is:

ETMY: QPD PIT / OPLEV PIT =   22.0 count/urad
      QPD YAW / OPLEV YAW =   17.1 count/urad
ITMY: QPD PIT / OPLEV PIT =   -6.0 count/urad
      QPD YAW / OPLEV YAW =    5.9 count/urad
ETMX: QPD PIT / OPLEV PIT =   16.6 count/urad
      QPD YAW / OPLEV YAW =   -9.3 count/urad
ITMX: QPD PIT / OPLEV PIT =    4.0 count/urad
      QPD YAW / OPLEV YAW =   -6.0 count/urad

In the absence of a lens, the QPD would be significantly more sensitive to cavity axis translation than tilt, and thus about equally sensitive to ITM and ETM angle. However, there are lenses on the end tables. I didn't go out and look at them, but found some elogs from Annalisa that mentioned 1m focal length lenses. Back-of-the-envelope calculations convince me that this can plausibly lead to the above sensitivity ratios.

I used these quantities to come up with an actuation matrix for the ASC loops, and measured the effective plant seen by the FM, fitted it to some poles( looks like zpk([],-2*pi*[1.47+3.67i,1.47-3.67i],160); ), and designed a control servo. Here is the designed loop:

The servo works on both arms, both DoFs. A DTT measurement agrees with the designed loop shape, up to a few degrees, which are probably due to the CDS delay. The RMS of the QPD error signals goes down by about 20dB, and are currently dominated by the bounce mode, so maybe we can try to sneak in some resonant gain...?

Once we confirm that they work when locking, we can write up and down lines into the locking scripts...

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