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MIF (ITF Control)
masayuki.nakano - 4:50 Tuesday 19 November 2019 (11628) Print this report
Effort for the stable lock.
FPMI got much stable now. 
By when I leave, it already locked for 2 hour.
 
Details:
  • I implemented alignment control with using dither and the TMS QPDs. The control scheme is as follows:
     PR3: Dithered at 19.2 Hz(P) and 11.1 Hz(Y), and demodulate the GRX TRANS. 
     PR2: Dithered at 13.3 Hz(P) and 16.8 Hz(Y), and demodulate the IRX TRANS. 
     IMMT2: Dithered at 9.1 Hz(P) and 7.2 Hz(Y), and demodulate the IRX TRANS.
     ITMX, ETMX: The DC  QPD signal at TMSX is fed back.
     Yarm, BS: No control
  • As shown in the attached figure, PR2 and IMMT2 oplev moving a lot. This indicates three hypotheses:
      1. PR2 oplev itself is moving
      2. IMC output beam is moving and the IMMT2 and the PR2 follow it.
      3. IMMT1 is moving and the IMMT2 and the PR2 follow it.
    The dv plot shows IMMT1 oplev signal, IMMT TRANS QPD signal, and PR2, IMMT2 oplev signals. IMMT TRANS QPDs show that 3 is not likely. We are not sure about IMMT1, but it does not seem to move so much although the noise is huge. I feel 1 is most likely and it can explain our unstable FPMI. 
  • The other oplevs are surprisingly stable in DC. We can trust them at least within a couple of hours.
  • Yarm is not controlled in angle. I implemented dither controls into BS and ETMY, and it does work with the single arm. However, this loop seems unstable with the FPMI. We can try more about it if we need it, but I don't think it's worthful to spend time on that. 
Next:
  • Still, the guardian is not good enough and we need to tweak the resonance scan by hand. I think we can solve it by using additional digital CARM with aux. signal such as TRANS, or normalized REFL.
  • (Yarm ASC implementation)
  • ASC with FPMI
  • MICH control noise reduction.
  •  
Images attached to this report
Comments to this report:
tomotada.akutsu - 6:45 Tuesday 19 November 2019 (11630) Print this report

PR2 tilt oplev P and Y started crazy when (or, after) the X arm engaged resonance... 

By the way, have this trend-oscillation also been obeserved when you did not do the dithering of PR2? (for example, the dithering freq is too high for PR2???)

In case this happens every time the cavity is aligned, this might be due to some ghost beams by the light source of the oplev itself, for example + unfortunately (as you may know, no cares for those ghosts there, as far as I know), although such big changes were not observred in the trend of tilt-SUM and len-SUM; even a fraction of light might be enough to rise "pitch and yaw signals" even though it won't change the SUM so much.

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