A lot of work has been done on HAM3-ISI for the past month. I'm trying to summarize here what we know and which path we should follow.

Situation before the SEI meeting

. We see a high Q peak around ~0.65Hz in all the local sensors (GS13+CPS) and all the DOFs, except RZ.

. This peak is present only when the Z sensor correction is ON. It doesn't matter if the Z sensor correction is coming from HEPI or the ISI, the peak is present when one of them is ON (see here and here). It doesn't seem to have any link with the X,Y sensor correction.

. The peak seems non-stationary. First of all, the peak is not the same if the sensor correction is done with the ISI or HEPI (see first attachment). Second of all, the amplitude and frequency of the peak vary with time (see second attachment and here).

Progress made

. The problem doesn't come from the ground STS. We checked the electronic chain (swapping distribution chassis) and tried another ground instrument (see here). The problem is independant (see here).

. This is not mechanical/rubbing issue. We did a driven transfer function around this frequency with a perfect coherence/no sharp peak.

. The problem doesn't come from HEPI. We turned OFF the HEPI loops with no improvement (see here)

. We don't think it comes from a specific sensor. Everything looks fine wihtout sensor correction, plus the problem shows up in all the sensors (if it was , let's say, a CPS issue, it wouldn't appear in the GS13).

. It might come from the blend: by switching to a 750mHz blend, the peak seems to disappear. However, we know that the 750mHz blend is obsolete, so I wouldn't draw solid conclusions from that...

. It might come from the drive (see here)

New situation after the SEI meeting

Now it seems that the peak appears even when the sensor correction is OFF (see third attachment)! I don't know if it's a good or bad thing, but that's the first time we've seen that. The only thing I did this afternoon is switching blend filters on all DOFs and turning the Z sensor correction ON and OFF... The ground motion doesn't seem any different than usual.

Action item for Sunday

. Keep investigate to see if the sensor correction is the cause of this peak

. Implementation of a slighty different blend in Z to see if it makes a difference.

. PR2 and MC2 have a mode at 0.67Hz. Could it be a weird coupling between the sensor correction and suspension? We'll try to turn the damping loops ON and OFF to see if it makes a difference

. The ISI model has been restarted before, but we haven't tried to restart the actual computer