[Jeff B, Stuart A]

Jeff B posted Phase 1b M1-M1 TFs for PRM (HSTS) in LHO aLOG entry 4195. These recent TFs from 2012−09−06_2000, with damping OFF, show a Y to T coupling at 1.1 and 2.05 Hz. This coupling goes away with damping ON, as can be seen in the TFs from 2012-09-10_0900.

Jeff K had made some comments regarding the nature of this Y to T cross coupling in LHO aLOG entry 3831, outlining possible scenarios for how it could be caused. 

As a consequence, it was requested that Jeff B switched the "side" of the SD BOSEM [i.e. +T] to the opposite side of the PRM structure [-T], and accordingly change the sign of the SD BOSEM gain in the OSEM OUTPUT FILTERS, from -1 to +1.

Following these changes, repeat TFs were taken Friday afternoon for just the T DOF.

A comparison plot showing the TF for the T DOF both before and after swapping the side location of the BOSEM is available below. Pot Key:-

Blue Trace   = Model Prediction
Orange Trace = X1 PRM M1 (2012-09-06_2000) - SD BOSEM on as-built side
Black Trace  = X1 PRM M1 (2012−09−14_1700) - SD BOSEM on opposite side

It can be seen that swapping the side location of the BOSEM appears to have mitigated the Y to T cross coupling!  

n.b. all data, plots and scripts have been committed to the sus SVN as of this entry.