Stefan, Sheila
Today we had difficulty locking DRMI, we tried PRMI and found we had difficulty locking that as well. Then I found that I couldn't even keep MICH DARK locked. Stefan was able to keep it locked by turning the gain up to -1000, removing the limiter from the BS M3 ISCINF once it locks, and adding a boost (FM4 in LSC_MICH). I compared the control signal to what we saw on decmber 3rd at a time when DRMI was locked, and see that we have about a facotr of ten higher RMS, all due to motion below 0.2 Hz. I then tried turning off different parts of sensor correction, first just the ISI XY, then also the HEPI Z. turning off HEPI Z brought the RMS back down to the old value. For now I'm leaving it all off.
BS M2 DRIVEALIGN L2Y gain changed from -1 to -0.8. This significantly improved the BS DC balancing.
K. Venkateswara
Looks like Z sensor correction is producing excessive Rz (Yaw) motion of Stage 1 at all three CS BSCs, but particularly so at the Beamsplitter. Attached plot shows the ASD of the CPS_RZ from the same period. The REFs were taken when Z sensor correction (to HEPI) was ON for all three, and the normal traces were with it OFF. While the sensor correction reduces motion at ~0.15 Hz, it is unexpectedly inducing excess RZ in all three, but much more so in BS. The coherence plot on Page 2 shows that RZ of the BS was the reason the MICH was difficult to control.
I don't have an immediate answer but will investigate some more.
I think one solution to the above problem is to turn on Z to Rz subtraction and enable the Rz loop. Jim and I tried this last week briefly and the result appeared agreeable. So I tried it again at ITMX and the results are shown in the attached file. First page shows the CPS_RZ, second shows the T240_RZ and the third shows the OpLev_Yaw. Green curve is with no sensor correction, blue is with Z sensor correction to HEPI and the red is with Z sensor correction to HEPI and Z to Rz subtraction at Stage 1.
I will also try this at BS and ITMY chambers.