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