craig.cahillane@LIGO.ORG - posted 15:05, Monday 18 November 2019 - last comment - 20:24, Monday 18 November 2019(53330)
TMS QPD input segments XB1 and YB2 extremely close to saturating in full lock
Attached ndscope shows all TMS QPD input segments during a powerup up to our lockloss at Nov 18 2019 22:21:30 UTC.
32000 cts is the maximum our ADCs can output.
Two channels are within 1000 cts of saturating:
H1:ASC-X_TR_B_SEG1_INMON (XB1)
H1:ASC-Y_TR_B_SEG2_INMON (YB2)
This is all consistent with what our PIT YAW outputs are telling us: our QPDs are poorly aligned.
XA XB YA YB
---------------------------------
PIT 0.40 0.54 0.48 0.57
YAW 0.66 0.84 -0.47 -0.71
EDIT: We just locked again and it appears that after some thermalization YB2 is 100% touching the saturation limit. See attachment 2.
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ADC overflows for the TMS QPDs YB upper left, XB upper right are overflowing during this lock. These do not seem to have an immediate effect on the interferometer: No locklosses, no immediately apparent glitches [although those might be happening, these QPDs are used for CHARD control]
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Looking at whether we can align the TMS at 2 watts and trust it will be well-aligned at 38 watts input (33.5 watts on back of PRM) Seems like yes, we can align red on the TMS QPDs at 2 watts and expect it to be okay for 38 watts, as long as it's after MOVE_SPOTS. During the guardian state MOVE_SPOTS, we adjust the spots on the ITMs and to avoid our point absorbers prior to going to full power. During this stage we move significantly on our TMS QPDs, especially in pitch. Not a lock killer, but enough to saturate a segment once we reach full power. After we've reached full power, the QPD alignment continues to drift but nowhere near as significantly as the MOVE_STOPS change. If we realign the TMS to center on the QPDs after MOVE_SPOTS, I suspect our saturations will go away.
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I checked the CHARD error and control signals during a time when the TRX and TRY B QPDs were saturated (now) and not saturated (early in the lock). The CHARD error signal consists of 5 PD signals for pitch, and 6 for yaw. TRY B is not one of those PDs. TRX B is. TRX B's signal is only slightly altered by the single segment saturation. (PDF 2) The normalized sum is falling as the TMS drifts further off this QPD. This causes the NSUM, PIT, and YAW ASDs to all decrease at AC. The saturated segment does not contribute at AC. Basically, our signal from this QPD is falling to zero, decreasing the overall gain of the loop. Ultimately, saturating in TRX B does not produce huge noise effects in CHARD due to the number of PDs contributing to CHARD's error signal, and the fact that TRX B's signal has not completely flatlined due to the other segments still contributing. There is a small increase in noise in pitch from 4 to 8 Hz. See PDF 1. There is effectively no increase in noise in yaw.
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