I noticed recently that there is a 1ish-hz peak on the HAM5. Digging through the summary pages it looks like it may have appeared around the 17th of Oct. It's not visible on the asds on the summary pages  that day, but if you look at the spectrogram, it shows up about 16:00 UTC. It's clearly there on the asds for the next day. 

Digging a bit deeper, it's on the HEPI and ST0 L4Cs as well, first attached plot. Worrying this might be like the HAM3 peak, I check the HAM2,3,4 & 5 HEPI L4Cs, and HAM3 HEPI doesn't show the ISIs 1 hz peak (second plot) , so I think it's a problem with the HEPI on HAM5, not the ISI. The HAM5 peak doesn't seem stationary, when I looked at midnight last night the peak was small on the HEPI L4Cs,  it was bigger a few hours ago, when Jenne and Chiara were trying PRCL to ISI offload, and now it's smaller again. 

I worry that something has changed the HEPI plant some. The 3rd attached plot shows the design plot for the X loop, the orange is the calculated open loop gain, and there is a 180 degree phase crossing at about 1hz, where the HEPI has gain of about 3. If something has reduced the gain of the plant, that could make this loop close to unstable. Most of the other loops are like this. 

If loop stability is the problem, it would be relatively straightforward to modify the boosts to get more phase margin at 1hz. I played around a bit in foton, the boost is just a pair of poles at .01hz and a pair of zeros at 1 hz, and some small tweaks were enough to get back 20 degrees of phase at 1 hz. Fourth attached plot, the dark blue line is the current filter, the pink(?) line is with at tweak to the boost. I haven't installed this.

If a chance comes up, or during a maintenance day, I'd like to get some open loop measurements on HAM5 HEPI. It would also be good to look at doing some other tests like linearity, range of motion or...

For the operators:

If HAM5 HEPI trips, please give me a call. Likely whatever has happened to HEPI has gotten worse, and it won't be stable. I can either come in and try to patch it up, or talk through the filter modifications over the phone.