Rob, Lisa, Matt Evan Sheila Patrick

Tonight we were able to get back to locking on RF, but were unable to engage the soft loops.  (we were stable with all other ASC on)  We spent some time trying to understand the problem. It seems possible that 90MHz centering will help aleviate this problem.  

We watched all the top mass control signals as we turned on CSOFT P, with the SRC1 loops open and the AS centering on DC.  We saw that all test masses move as expected, but so do the other optics, including PR3, PRM, SR3, and BS.   The second attached screenshot shows how the OMs are driven by the centering loops, which see the CSOFT change.  We hypothesize that this is how CSOFT couples to the BS and other loops, and also see this signal in the 90MHz centering signals wich are out of loop here.  

We looked a little bit at using 90 MHz centering, but see some problems.  

For one, the 90MHz signals are not normalized by the sum, they are normalized by the input power.  It seems like we should be using the signals normalized by the sum, and that this would be a fairly simple model change replacing the input power normalization with the sum.  If we normalize this way we can simply use 1s in the input matrix and the centering loop gains should stay the same.  

The sum on ASARF90 is negative, so this centering loop might have a sign flip compared to the DC centering.  

We measured the MICH ASC looop to do a comparison between the gain with 90MHz centering on and with DC centering on.  While doing this we turned off the centering loops, and saw that the MICH loop was not stable this way.  This probably means that the MICH ASC loop stability somehow depends on the centering loop, and that we need to make sure we keep the centering loop BW the same when we change sensors.  

For the first people to come in the morning to work on locking, some good steps would be:

1) Add normalization of the 90 MHz centering to the ASC models. 

2) Measure the MICH ASC loops (templates available /ligo/home/evan.hall/Public/Templates/MichPitchSweep.xml and MichYawSweep.xml ) with the DC centering on.

3)measure the centering loop OLGs, for DC centering and 90 MHz centering.  

4) measure the MICH ASC OLGs with the 90 MHz centering on.  Make sure the gain is the same as for DC centering.