We tried locking with the variable finesse MICH offset technique again today.
A daytime task tomorrow will be to figure out where we are in MICH and CARM offset spaces. This will require some thinking, and perhaps some modelling.
We were using the UGF servos and checking out their step resonses, and had the realization that we don't want the gain multiplication to happen before the offsets are applied, in the case of MICH and CARM. Otherwise, as the UGF servo adjusts the gain, the offset is changed. I think this is what ChrisW and I saw earlier on in the evening, when it seemed like the CARM offset spontaneously zoomed toward zero even though I didn't think I was touching any buttons or parameters. Anyhow, we no longer used the MICH and CARM UGF servos for the rest of the night. We need to think about where we want the offset to happen, and where we want the UGF servo multiplication to happen (maybe at the control point, with a very low bandwidth?) such that this is not an issue.
Also, I'm no longer sure that the sqrt(I^2 + Q^2) instead of the usual demodulation is going to work for the UGF servos (Q made this change the other day, after we had talked about it). When the numbers going into the I and Q servo banks are small (around 1e-5), the total UGF servo gets the answer wrong by a factor of 10 or so. If I made the "sin gain" and "cos gain" 1000 instead of the usual 1, the numbers were of the order 1e-2, and the servo worked like normal. So, I think we were perhaps running into some kind of numerical error somehow. We first noticed this when we lowered the DARM excitation by a factor of 10, and the servo no longer functioned. We should take out this non-linear math and go back to linear math tomorrow.
During the evening tomorrow, we should try locking the PRMI with a large MICH offset, and then leaving CARM and DARM on ALS, and seeing how far we can get. Is it possible to just jump over to RF signals, since we won't have to worry about the detuned cavity pole?
Tonight, the locking procedure was the same as usual, but stopping the carm_up script before it starts to lower the CARM offset at all. Only difference was that MICH triggered FMs were 2,3,7 rather than the usual 2,6,8.
So, assuming you have the IFO with CARM and DARM on ALS held at +3 CARM offset counts (which we think is about 3nm), and the PRMI is locked on REFL33I&Q with no offsets, here's what we did:
- PRCL UGF servo on
- MICH offset goes to -20
- MICH transitions to ASDC (0.27*ASDC, then normalize by POPDC)
- DARM UGF servo on
- CARM offset to 1 (arms about 0.25)
- CARM transition to SqrtInvTrans
- Lower CARM gain to 4
- CARM offset to 0.6 (arms about 1)
- DARM transition to DC transmission
- Increase MICH offset to about -650 or -670
- Lower CARM offset, see what happens
Something else to think about: Should we normalize our DC transmission signals by POPDC, so that the arm powers will change when we change the MICH offset (for a constant CARM offset)?
The best we got was holding for a few minutes at arm powers of 7.5, but since the MICH offset was large and the power recycling was low, this was perhaps pretty far. This is why we need some calibration action.
Also, earlier today I copied the CARM and DARM "slide" filter module screens so that we have the same thing for MICH. Now all 3 of these degrees of freedom have slider versions of the filter module screens, which are called from the ctrl_compact screen.