Summary:
- The PRFPMI can be controlled by a mix of ALS and RF signals and circualting arm powers > 100 can be maintained for several tens of minutes at a stretch.
- The complete RF handoff still cannot be realized - I need to study the AO path crossover more carefully to understand what exactly is wrong and what needs to be done to rectify the problem.
Measurements:
Over the last couple of days, I've been trying to see if I can measure the phase advance due to the AO path - however, I've been unable to do so for any combination of CM board IN1 gain and MC Servo board IN2 gain I've tried. Yesterday, I tried to understand the loop shapes I was measuring a little more, and already, I think I can't explain some features.
Attachment #1 shows the TF measured (using SR785, and the EXC_A bank of the CM board) when the CM Slow path has been engaged.
- All CARM control in this state is digital.
- For the CM Slow path, the digital filter includes a pole at 700 Hz, pole at 5 kHz and zero at 120 Hz (the latter two for coupled cavity pole compensation).
- In this conditions, the arm powers are somewhat stable at ~150, but still there are fluctuations of the order of 50%.
- The "buzzing" as the arms rapidly go in and out of resonance is no longer present though.
- The UGF of the hybrid REFL11+ALS loop is ~200 Hz, with ~45 deg of phase margin.
- Turning off the MC2 violin filters gives some phase back. But I don't really understand the flattening of the TF gain between ~250-500 Hz.
Attachment #2 shows error signal spectra for the in-loop PRFPMI DoFs, for a few different conditions.
- Engaging the REFL11 digital path smooths out the excess noise in the ~30-50 Hz band, which is consistent with the fact that the arm powers stabilize somewhat.
- However, there is some gain peaking around ~400 Hz.
- This is in turn imprinted on the vertex DoFs, making the whole system's stability marginal.
I believe that a stable crossover is hopeless under these conditions.
Next steps:
- Account for the measured OLTF, understand where the flattening in the few hundred Hz region is coming from.
- Repeat the high BW POY experiments, but with the simulated coupled cavity pole - maybe this will be a closer simulation to the PRFPMI transition.
|