Jeff K and I saw that Phase 3b (in-vacuum) TFs had not yet been taken for the OMC and OMs suspensions, therefore, I've taken a complete set this morning, prior to next weeks planned vent of HAM6, for reference and acceptance purposes. I've taken undamped DTT TFs, with the HAM-ISI isolated and damped via Guardian. All suspension alignment offsets remained ON throughout the measurement. However, it soon became evident that OM1 was struggling in all DOFs, see snapshots of OM1 TFs below, the red trace is a good reference, the black trace raises some concerns. I turned OFF the alignment offsets for OM1, but the issue persisted. I then noticed that the ISC signal path is biassing OM1 and saturating the DAC output. Turning OFF the ISC path restored OM1s performance. A 7 day trend for OM1 is attached below, apart from some transient glitches, the alignment offsets have remained steady. However, the ISC bias has transitioned between +/-4000 counts. The ISC_LOCK Guardian state is also included, noting that an index >500 indicates the IFO is locked with DC readout. There was also a minor issue obtaining V-V DOF TFs on the OMC suspension, which resulted in the incorrect scaling when compared to the model (all other DOFs were fine). After investigating the signal chain, I discovered a gain of -500 in the TEST_FILTERS bank. I returned this to the nominal gain of +1, which rectified the problem. The OMC and OMs TF measurements have been compared with previous Phase 3a (in-air) measurements, as well with identical suspensions at LLO and are available below. Summary: Transfer functions for both OMC and OM suspensions are consistent with previous measurements, and similar suspensions at LLO (with no biases applied to OM1). However, the HAM6 vent (and swap on the OM1 optic) may provide an ideal opportunity to offload some OM1 alignment, if it's deemed necessary. All data, scripts and plots have been committed to the sus svn as of this entry.
When the IFO unlocks, the HAM6 DC centering loops rail, and the histories aren't cleared until a point in the DRMI Guardian just before the ASC is turned on. So, in the morning after a night of IFO locking, there is usually a large, bogus ISC signal being sent to the HAM6 tip-tilts. This is why Stuart observed a huge bias on OM1 that was probably causing either rubbing or nonlinearity from large values in the coil drives.
The attached plot shows 1 hour of typical OM1 signals during low-noise operations (GRD-ISC_LOCK_STATE > 500). The LOCK inputs are small (~hundreds of counts), and the COILOUTs are well within the +/-32k range. The UR and LL COILOUTs are larger (~11k counts), but Stuart says we shouldn't worry until we reach +/-24k, or 75% of the DAC range.
Some of the inputs from the OSEMs are large (UR is -19k counts), but this reflects a particularly large open-light level and not an OSEM whose flag is about to come out of the barrel.
So, there's no evidence that OM1 is rubbing or saturating during normal operations. OM2 has larger DC alignment offsets than we would prefer (~16k in COILOUT), but this is within the linear range.
