Rana, Evan
Tonight, we took some transfer functions from the ETMX/ETMY ESDs to LSC DARM IN1, when controlled by ALS DIFF. Right now we only use ETMX as our DIFF actuator (as we had previously had some trouble using ETMY), but perhaps it is time to revisit this. Using both ESDs will double our range. Additionally, by properly balancing the relative drive strengths between the two ESDs, we can make ourselves more immune to cross-coupling between the ALS-controlled DARM and CARM loops.
For the excitation we drove each L3 TEST L filter bank with 40000 ct from 10 Hz to 40 Hz.
Evidently, ETMX and ETMY have opposite signs and their gains differ by 6 dB or so (at least below 20 Hz, where the coherence is OK). Since we're aiming for a DIFF loop with a UGF of 15 Hz or so, we don't care so much what's going on above 20 Hz, so long as it's not outrageous. So we (hopefully) should be able to implement a balanced ESD drive with little trouble by engaging DIFF with both ESDs (with fiducial output matrix gains), then turning on a line in DARM, and then adjusting the output matrix gains until the line is nulled.
This plot shows the ESD DAC range used by a single ETM to lock the ALS DIFF loop (with the UGF around 15 Hz I think).
The dip between 20 and 200 Hz comes from a filter called "NicLP" which has now been renamed to be descriptive. The RMS of 14000 counts is dominated by the bump at 15 Hz (due to poor phase margin).
We replaced this with another filter (RLP33) which is just a pair of poles and a pair of zeros. This gives us back 12 deg of phase at 15 Hz and only increases the RMS drive by <10%.
We then brought in the ETMY ESD and saw that it does indeed run stably and decreases the load needed on each ESD. It went unstable after ~10 sec due to some low frequency oscillation, but we think that is probably just the L2P being off on ETMY (or maybe that we have not turned on ETMY UIM or PUM drives). We should be able to get this going with ~1 hour of work tomorrow.