jeffrey.bartlett@LIGO.ORG - posted 11:04, Tuesday 18 September 2012 - last comment - 14:13, Wednesday 19 September 2012(4232)
H1-PRM Cross Coupling
There was a transverse to yaw cross coupling noted in the TFs for H1-PRM (orange trace). To isolate the problem we moved the side BOSEM to the other side of the suspension and re-tested. With the BOSEM on the opposite side, the cross coupling disappeared (black trace). Before moving the BOSEM back to its correct side position, I found the magnet/flag base to be misaligned. This was corrected and the BOSEM was re-centered to 50% light. Subsequent testing shows the cross coupling has been removed (purple trace). H1-MC3 and H1-PR2 also show the same cross coupling. We will check the side magnet/flag base alignment on these two suspensions and re-align as necessary.
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Comments related to this report
[Stuart A, Jeff K and Jeff B] Adjustment of the PRM (HSTS) side flag mounts rectified the Y to T cross coupling observed in previous measurements. But something else appears to have cropped up. The plots below (allhstss_2012-09-19_0940_All_Phase1b_PRMs_ALL_ZOOMED_TFs.pdf) show a comparison between the latest two complete data sets of un-damped M1-M1 TFs, with the most recent being the orange trace. It can be seen that these recent TFs exhibit signs off:- - Roll (slide 4) looks to show stronger vertical mode coupling at ~0.85Hz - Pitch (slide 5) looks to show stronger vertical mode coupling at ~0.85Hz Most likely, these are sensor-related cross couplings -- as in the usual imperfect subtraction of common mode signal (Vertical) in the differential signals (Roll and Pitch). But they were not so apparent in the 2012−09−06_2000 data, and we wish to be sure that they are not related to the re-alignment carried out on PRM to rectify the Y to T cross coupling. To demonstrate the significane of these Pitch and Roll cross couplings, we provide a plot showing similar couplings for ALL HSTSs from both sites (allhstss_2012-09-19_1040_All_Phase1b_HSTSs_ALL_ZOOMED_TFs.pdf). This demonstrates that compared to other HSTSs the feature in pitch (again the orange trace) is less of a concern. However, the feature in roll could be considered egregious. It should be noted that the most recent PRM TFs were taken during the middle of an active day. Therefore, the next data will be taken at a quieter time, when the common mode (Vertical) signal is reduced (or by just turning on the Vertical damping loops while taking the Pitch and Roll measurements with loops open). All plots and scripts have been committed to the sus SVN as of this entry.
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