Gabriele, Sheila, Alexa, Evan
Summary
We have engaged the DHARD WFS Y (and P) at 3 Hz on resonance with a reduced oplev damping gain in the ETMs. Again, to start off we closed the DHARD Y WFS with 3 Hz BW at 50pm CARM offset. Since this loop is also stable at low BW, we will leave it in the low BW configuration at this point, so that we are at a 3 Hz BW on resonance.
Details
We had tried engaging the new DHARD Y loops as described in LHO#17006. However, we quickly found that this configuration was unstable. So, we removed the partial plant inversion FM6 and took a plant TF. We found that the plant that Gabriele had measured with the oplevs was slighlty different than the @50pm plant (see Gabriele's comment). We adjusted FM6 accordingly to compenstate for the peaks seen between 1 and 5Hz. FM6 is now zpk([-0.3303+i*15.1459;-0.3303-i*15.1459;-1.9027+i*16.6711;-1.9027-i*16.6711; -0.2672+i*19.227;-0.2672-i*19.227],[-0.6659+i*18.7;-0.6659-i*18.7;-0.509+i*11.519; -0.509-i*11.519;-1.0404+i*15.4844; -1.0404-i*15.4844], 1)gain(0.469248).
To close the loop at low BW at 50pm CARM offset, we engage FM2, FM3, FM4, FM6, FM9 with a gain of 30. FM6 is described above, and the remaining filters are the same as in LHO#17006. With a gain of 360, this gives a UGF of 3 Hz and a phase margin of 36 deg.
On resoncance with a gain of 30, we measured that the UGF is 3.5 Hz with a phase margin of 36 deg.
This is in the guardian now.
In the first attached plot the blue circles show the measured DHARD plant transfer function, at 50 pm CARM offset. The red trace is a fit, which matches quite well the measurement. To be able to run the loop with a 3 Hz bandwidth and a simple controller like the one we used for pitch, we had to compensate for the two higher pole/zero pairs.
The second plot compares the DHARD plant measured today at 50pm using the ASC signals, with the one I measured on Saturday using only ETMY and its optical lever. They are clearly quite different. It's unclear to me why this happens. It can be that ETMX and ETMY are significantly different, and when driving DHARD we are using the sum of the two.
Sheila, Gabriele, Evan
We are on dc readout with the following loops locked (pitch and yaw):
- ASA45Q → dETM
- REFLA9I + REFLB9I → cETM
- REFLA9I − REFLB9I → IM4
- POPB → PRM
- −0.66 REFLA9I + REFLA45I → PR2
- ASB36Q → BS
dETM is high bandwidth (~3 Hz), as is BS. cETM is lower bandwidth (probably by a factor of 10 or so) because we found it was injecting noise into the DARM spectrum up to ~50 Hz. PRM is very low bandwidth (more than 30 s time constant; this is probably too long). IM4 and PR2 are something like 100 mHz or less.
The CHARD P,Y WFS have the same filters engaged as for the DHARD P, Y WFS respectively. The gains for CHARD (P,Y), are (-20, -40). If we want a 3 Hz BW, the open loop we took last night indicated we were about 10dB too low.
Here is an estimate of DAC noise propagated forward to the ETM ESDs. I've used Peter's recent DAC noise model, an ETM ESD force coefficient of 2×10−10 N/V2, a bias of 380 V on each ETM, and some hints from Jeff about the DAC → ESD signal chain.
Evidently this is somehow an overestimate, but the shape and magnitude are roughly in agreement with the spectrum between 50 and 100 Hz.
As a quick test of whether DAC noise is really a limiting source here, we could try ramping down the ETMY bias during full lock (since we're not using the ETMY ESD).
Also, Nic and Jamie have inquired about the uptick in the ASD above a few kilohertz. The noise there seems to be largely uncorrelated between the two DCPDs (see attachment), which seems to suggest that it's still shot noise. (Based on measurements that Dan and I took of the DCPD dark noise, I believe this feature is too big to be explained by excess noise in the DCPDs or their signal chain.)
