Summary and discussion
Radar plots of the sensing matrix for PRCL, MICH, and SRCL are attached (where MICH is the usual beamsplitter drive). This was taken in the nominal O1 configuration.
Something is strange with the response in POP9; driving beamsplitter seems to result in the same response as driving PRM. Certainly we expect some amount of POP9I response from driving the beamsplitter, since this drives PRCL, but it seems strange that they are almost exactly equal.
For POP45, beamsplitter appears in both I and Q; again, this is not surprising. However, its contribution in I seems to dominate over the SRM contribution. This could potentially be bad, because we only do PRCL → SRCL subtraction (not MICH → SRCL subtraction).
Sensing matrix is as follows:
| PRM | BS | SRM | |
|---|---|---|---|
| POP9I (W/µm) | 3.1 | 3.1 | 0.01 |
| POP45Q (W/µm) | 0.21 | 1.2 | 0.0 |
| POP45I (W/µm) | −0.65 | −0.68 | 0.18 |
There is some ambiguity in the signs here that still needs to be resolved.
The magnitudes of the PRM ⇝ POP9I and SRM ⇝ POP45I elements agree fairly well with what is necessary to explain the open-loop transfer functions for PRCL and SRCL (3.6 W/µm and 0.13 W/µm, respectively). For MICH, there is some frequency-dependent residual between the model and the measurement (which has been observed before). The gain needed to make them match around the UGF is 0.6 W/µm.
Details
I added a 132.1 Hz notch to the PRCL, MICH, and SRCL SFMs. Then one by one I drove PRM, SRM, and BS with a 132.1 Hz line for a few minutes, using the digital oscillator just after the LSC SFMs.
I took the time series for POP9I/Q and POP45I/Q during this time, and demodulated them with 0.1 Hz low-passing.
All times are 2015–01–13 Z. The actuator calibrations are based on digital filters, the suspension models, and the suspension electronics.
-
PRM:
- Oscillator amplitude: 1000 ct
- Injection time: 16:19:40 to 16:23:40
- Calibration: −3.0×10−16 m/ct
-
SRM:
- Oscillator amplitude: 2000 ct
- Injection time: 16:25:35 to 16:30:35
- Calibration: −3.0×10−17 m/ct
-
BS:
- Oscillator amplitude: 20 ct
- Injection time: 16:33:05 to 16:37:05
- Calibration: 3.8×10−16 m/ct (including digital f2 inversion filters)
As for sensors, the calibrations are 2.2×108 ct/W for POP9 and and 2.3×108 ct/W for POP45, based on LHO#24959 plus an additional 30 dB of whitening gain.
In the above log, I was using a beamsplitter compliance that did not include the violin modes. Because we drive the beamsplitter from its middle stage, Shapiro effect noticeably increases the magnitude of the compliance even around 100 Hz, even though the first violin modes occur around 300 Hz.
The attached plot shows the corrected model, along with a new set of OLTFs taken on the 14th. Both PRCL and SRCL show some discrepancy below 10 Hz.
