Peter, Kiwamu, Evan
In the process of trying to realign the IMC, we found that the LF OSEM on the top stage of MC3 is seemingly nonresponsive. Additionally, the left-hand OSEMs on the M2 and M3 stages have very few counts coming from their shadow sensors.
For M1 LF, the input readback is railed low, and does not respond even when the MC3 alignment slider is "moved" many hundreds of microradians in yaw. RT, in contrast, shows a clear response. The voltage and current readbacks for LF are similarly nonresponsive. It seems that they acquired their present values around yesterday at 11:20 AM local time.
Peter and I looked at the HAM2 driver chassis, but did not see anything immediately wrong. Then we went to the satellite boxes. Peter found that one of the cables for the MC3 top stage was not screwed in and was hanging slightly. Fixing this did not change the behavior of LF, however.
It seems that this problem can be traced back to the ADC/DAC chassis which drives the LF OSEM. It is putting out -25 V all the time.
By physically disconnecting the DAC drive to the T3, LF, RT, and SD on MC3 (thereby removing the -25 V being sent to LF), Kiwamu and I were able to see that the OSEM shadow sensors all gave reasonable numbers of counts (several thousands at least), including LF and the middle/bottom stage OSEMs.
We restarted the MC3 model, but this did not fix the issue.
As a background task over the past few weeks I've been working on a Matlab script that will provide the capability to test all suspension actuator channels. The aim would be to have it available to run on a weekly/bi-weekly basis, during a maintenance window by Detector Engineers or Operators, so that we can be more pro-active in our search for OSEM channels that may have become unresponsive. The test itself is very straightforward (just applying a ramping positive/negative bias to the OSEM and monitoring the sensors response), but the logistics of running it on so many channels and maintaining a measurement list, are somewhat more of a challenge. This script is currently being tested at LLO, and after completing my survey of actuators at LLO I aim to release it for LHO ASAP.