jeffrey.kissel@LIGO.ORG - posted 10:31, Wednesday 17 April 2024 - last comment - 12:19, Wednesday 17 April 2024(77242)
Update on Fast Shutter Timing
J. Kissel During today's Systems Call, it came up that there was scant documentation on the timing performance of the existing fast shutter system. A quick look through the aLOG showed some results from the 2016 era including examples like LHO aLOGs 29062, 28958, and 26263. Koji's LHO:29062 is probably the most conclusive, quoting (repeated here for convenience): "The fast mechanical shutter start blocking the beam at 1.7ms, reaches half blocking at 1.85ms, complete block at 2.0ms." Here, I take some modern data with what digital signals stored in the frames: [65 kHz] H1:OMC-PI_DCPD_64KHZ_AHF_DQ OMC DCPDs (down stream of the OMC Cavity) [4096 Hz] H1:ISI-HAM6_BLND_GS13Z_DQ Vertical GS13s on the HAM6 ISI (nominally calibrated into 1 nm/s, but I've scaled it to arbitrarily show up demonstratively on the OMC DCPD plot) [2048 Hz] H1:ASC-OMC_A_NSUM_OUT_DQ OMC QPD A SUM (up-stream of the OMC Cavity, but still down-stream from the fast shutter) [2048 Hz] H1:ASC-OMC_B_NSUM_OUT_DQ OMC QPD B SUM (""") [2048 Hz] H1:ASC-AS_C_NSUM_OUT_DQ AS_C QPD SUM (up-stream of the fast shutter) [16 Hz] H1:SYS-MOTION_C_FASTSHUTTER_A_STATE Slow-channel record of the logical state of the shutter [16 Hz] H1:YSY-MOTION_C_SHUTTER_G_TRIGGER_VOLTS Slow-channel record of the trigger PD voltage (which is AS_C) With these channels, I show three recent lock-losses. It seems like all of the photo-diode signals show a very long "turn off time," (of order ~1 [sec]) and are pretty un-reliable at precision estimates of shutter timing. This is likely a linear combination of - For the OMC DCPDs at least, there's an *additional* hardware "shutter" where the cavity's length actuator -- the OMC PZT -- are quickly railed to unlock the cavity after a lock-loss trigger, - For the OMC DCPDs at least, there's a non-negligible cavity ring-down time, - For OMC DCPDs and all the QPDs, impulse response of the photo-diode's readout electronics and anti-aliasing filters, and/or the However, the "clunk" of the shutter on the HAM6-ISI is quite obvious and precise, so I use the start of the impulse (a few samples after the transition from "no motion" to "large fast negative start of the clunk's ring-down") that as an upper-bound estimate of when the shutter is completely closed. But, with the imprecision of the [16 Hz] channels -- 62 ms -- it's tough to get anywhere. In short -- what's stored in the frames are a pretty bad metrics for precision timing of the fast shutter system. But anyways -- the upper limit of the time between the change of the logical state of the trigger and the HAM6 ISI GS13s registering a large kick is - 08:23:20 UTC :: 0.3125 (+/-0.0625) and 0.341064 (+/-0.000244) = 0.02856 [sec] = 28.5 [ms]. - 13:01:26 UTC :: 0.125 and 0.1604 = 0.0354 [sec] = 35.4 [msec] - 23:39:28 UTC :: 0.3125 and 0.328857 = 0.016357 [sec] = 16.4 [msec] I'll keep looking for better ways to increase the precision on this statement, but for now, we should still role with Koji's results from LHO:29062
Images attached to this report
Comments related to this report
In G2201762 > O3a_O3b_summary.pdf Niko and I showed in O3b, there was a subset of ~25 locklosses where the light fell off H1:ASC-AS_C_NSUM_OUT_DQ on the order of 1ms, where this usually takes ~30ms. There was none of these locklosses in O3a and although the analysis is not currently online, I haven't noticed any of these 1ms fast lcoklosses in O4.
These O3b "Fast" locklosses often had the light fall off POPAIR (shutter now closed in NLN) and IMC_TRANS in the same 1ms period, sometimes with the FSS signals oscillated beforehand, showing these locklosses may have been correlated to issues with the PSL system that were resolved after O3.
Ah HA! Thanks Camilla!
Camilla's comment is only tangentially related, but is fruitful anyways!
The O3B study she points to is researching lock losses that are dubbed "Fast lock losses" by the collaboration because they happen in under 1 [msec], but as the plots on page two of her linked study show, light "disappears" from the interferometer because the IMC is unlocking, NOT because the fast shutter has triggered. Camilla thought they were related to my aLOG about the fast shutter because of conversations with Rana suggesting that "if these fast locklosses release all of their energy to the dark port prior to the fast shutter firing, then we need to know, and make the fast shutter faster!"
HOWEVER, this pointer reminds me of two more channels,
[2048 Hz] H1:ASC_A_DC_NSUM_OUT_DQ The "DC" or "QPD" component of WFS A (down-stream of the fast shutter, but upstream of the OMC)
[2048 Hz] H1:ASC_B_DC_NSUM_OUT_DQ The "DC" or "QPD" component of WFS B (down-stream of the fast shutter, but upstream of the OMC)
Apparently, whatever's going on with the OMC QPDs and the OMC DCPDs (also downstream of the fast shutter) which seem to take a lot longer for light to dissipate, *doesn't* happen with the AS WFSs.
So, these channels much more clearly show the cut-off of light. That being said, they corroborate well with the HAM6 ISI GS13s.
The uncertainty still remains, though, on when the shutter gets the firing signal -- i.e. when does the shutter's trigger PD get enough voltage above threshold to trigger the shutter.
For this we still only have the 16 Hz "SYS-MOTION" channels.
And thus my conclusions about timing remain.
Anyone know of a faster channel of the trigger PD?
Images attached to this comment
