Elli, Sheila

Using the OMC suspension as an actuator for HAM6 centering caused large motion of OMC, and added scattering noise to DARM (17273.)  For this reason we rearranged the centering loops to center only on AS A and not AS B (alog 17325). This is what livinsgton does, but they only use AS A WFS for feedback.  We had difficulty with our ASC on monday 17577, which we though was due to the beam nt being well centered on WFS B, and indeed reverting our HAM6 centering fixed the problem.  Now we would like to see if we can use only one of our AS WFS for control. 

One option would be to move D ETM control to WFS B, however we saw that there is a sign flip in the WFS B signal between the CARM offset where we first engage the D etm wfs and the signals on resonance, this was similar to what was seen at LLO.  Today we looked into switching the BS and SRM loops from WFS B to WFS A.  First Elli tuned the phase of the indivual segments to minimize the SRM signal seem in the Q phase.  We were able to switch the BS loops over to WFS A, we replace the input matrix element with a -1 for AS A 36 Q yaw and -0.5 for AS A 36 Q pit.  We could close the loops like this and they seemed stable, but the build up in AS 90 was lower than when we locked using AS B.  In full lock we looked at this again, we were able to switch YAW over to AS A using the same matrix element, but the zero crossing for the pitch error signal was at an alingment where the build up was not very good.  

Once the BS was switched over to AS A, we had a look at switching over the SRM feedback in DRMI.  The offsets in the error signals were much smaller after we had already switched the BS control to AS A than before, and we did close the loops.  However, something became misaligned which was fixed as soon as we opened the loop.  

Aslo, we have had at least two lock losses in the last two days that I think were caused by engaging the SRC2 loop.  This is the loop from AS_C to SR2, which has a term in the output matrix that also feeds it to SRM.  The control signal sent to SRM has caused SRM to saturate in the two attached screen shots.