(Sheila, Alexa)
Today we worked on the slow/tidal feedback to the ETMs. In doing so, we saw that the L2P for the M0 state was not very good at low frequency. Using the same method as was done for the BS (alog 14022), we found that the gain of ETMX L2P (with FM1, FM2 ON), should be changed from -1.0 to -4.0 at 0.01Hz. So far it appears that this is OK for high frequency. Meanwhile, we found that the gain of ETMY L2P (with FM1,FM2 ON) should be changed from -1 to 22 at 0.01Hz. We are not sure if this is okay at higher frequencies.
The slow/tidal feedback for both arms now works, and no longer causes a drift in the alignment. We made the following the changes to the feedback servo (this is all taken care of in the guardian already):
- ALS-X_ARM_GAIN: 210000 (old: -2100)
- ALS-Y_ARM_GAIN: -210000 (old: -2100)
- End station REFL PDH slow option is always disabled now; the slow feedback is engaged/disengaged by turning off/on ALS-X/Y_ARM input
- ALS-X/Y_ARM: we only have one boost on now
It appears that the DIFF PLL VCO is now in range, but maybe that's a coincidence with ground motion today....
One can probably bleed off the dc term (f<10mHz) by feeding back to HEPI. The path is all set up and the LSC signals are received by a HEPI ISC input filter. All that should be required is an integrator in this path.
We were having some problems with ALS DIFF earlier tonight, so we reverted the L2P gains back to -1. It is unclear at the moment if this was related at all.