B. Weaver, J. Oberling
After this morning's PSL trip there was a 0.5W increase in IMC-PWR_IN, observed ~15 minutes after reaching NOMINAL_LOW_NOISE. We started digging in the Guardian code and rediscoverd that the ISS diffracted power gets adjusted by a software PID loop located in the IMC_LOCK guardian; this adjustment is done via changing the ISS 2nd loop reference signal. At the start of a lock stretch this PID loop is necessary to keep the ISS diffracted power around the predetermined nominal value of 8%. However today we witnessed that when the diffracted power trended towards the upper threshold (set in the IMC_LOCK guardian code) of 10%, the IMC_LOCK turned on the PID loop, thus increasing the measured IMC-PWR_IN by ~0.5W.
Looking back 16 days we discovered 2 more cases of the IMC_LOCK guardian turning on the ISS PID loop during a NOMINAL_LOW_NOISE (and observing) lock stretch. Attached is the 16 day trend and the 2 examples. The 2nd example was 30 hours into last week's 46 hour lock stretch. Also note, in the 16 day trend, the continued power discrepancy from lock to lock.
Looking at the 16 day trend, by eye it seems the ISS diffracted power is hugging the upper threshold of 10% more often then not, hovering around 9%. This might be normal acceptable behavior, however we were surprised to see that a PID loop was increasing power in the middle of an observing lock. Will follow up with commissioners.
What do the in- and out-of-loop signals for the outer loop look like during these events? (I.e., SUM14 and SUM58.)
B. Weaver, J. Kissel, Not sure what Evan wants with the "SUM14" and "SUM58" channels, so I found what channels are stored in the frames for the ISS, and used my vague knowledge of how the ISS SECOND LOOP works, and chose H1:PSL-ISS_SECONDLOOP_SUM14_REL_OUT_DQ H1:PSL-ISS_SECONDLOOP_SUM58_REL_OUT_DQ and reproduced Betsy's plots with trends of those channels in addition to the original channels. I'll leave it to Evan to interpret the results, 'cause we don't know how. Let us know if you need anything different, Evan (or on a different scale, or whatever).
