B. Bland, J. Bartlett, T. Sadecki, J. Kissel
After noticing suspicious, what-appears-to-be, cross-coupling of Yaw (Y) modes (1.1 and 2.05 Hz) into the Transverse (T) driven transfer function of Garcia's a few days ago (see LHO aLOG 3769), Betsy and I plotted those results against other suspensions, just to confirm this is something unique to this suspension (remember, this guy has given us trouble after trouble already, see LHO aLOG 3714).
The first attachment (allhstss_2012-08-13_X1SUSMC3_ALL_ZOOMED_TFs.pdf) shows this comparison between
X1 SUS MC1 -- a phase 1b, metal mass, SUS; built by the same people, measured in the same place, in the same measurement phase. Doesn't show it (not thing common to this phase of an HSTSs life).
L1 SUS MC2 -- a phase 3b, glass mass, SUS; built by different people, measured on an ISI in-chamber. Doesn't show it (not a location thing, not a glass mass vs. metal mass thing, not a "who assembled it thing).
L1 SUS MC2 -- a phase 2b, glass mass, SUS; built originally by the same people, measured after glass swapping. Doesn't show it (not an "appears or goes away" through the phases thing).
X1 SUS MC3 -- the latest measurement of concern, DAMPING LOOPS OFF. This shows the cross-coupling.
X1 SUS MC3 -- the latest measurement of concern, DAMPING LOOPS ON. This does NOT show the cross-coupling.
Given this comparison, we came up with three scenarios that we could image that would case Y to couple to T, and have any other cross coupling show up in other degrees of freedom. They're similar to what's shown for P to V coupling in G1100865 (pg 4):
(1) The SD coil drive axis is mis-aligned with the SD magnet axis (though the axis are parallel to each other), creating a small lever arm causing the top mass to twist in Y when pushed in T
(2) The SD magnet is cocked (in such a way that the whole flag-magnet assembly looks like a "z" shape), such that the SD coil driver axis non-parallel with the the SD magnet axis (but the flag is still aligned), creating a small Yaw force vector when driving in T
(3) The TOP mass has its adjustment mass arranged very lop-sidedly (that's a word, right?), such that the T center of mass / Y moment of inertia is pushed forward or backward (i.e. in +/- L), which creates a small lever arm, twisting the mass, when forced along the mass center line, which is no longer in line with the CoM/MoI.
As such, we recommended Bartlett to make a corresponding assessment of those features. He confirmed the mass distribution was in no way abnormal, so that rules out (3). He did readjust the the SD OSEM to center it a bit better. However, he only used the slop inherent to the BOSEM mounting; there really is no adjustment possible to the tablecloth -- its location is rigidly define by the cage -- so wasn't able to move it much. Not to mention, moving the tablecloth would affect the alignment of all other TOP OSEMs. After, he took a quick T to T (only) transfer function which is attached (and calibrated straight from DTT, woo!), 2012-08-13_1245_X1SUSMC3_M1_T-T_TF.pdf. It shows no improvement. That rules out option (1) (queue sad trombone).
This doesn't rule out option (2) though...
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Conclusions:
- I'm not super concerned yet:
- The cross-coupling goes away with damping loops closed (the normally-used configuration).
- There are many more chances to attack this problem.
- These two twins (MC1 and MC3) have been sitting in the staging building dragging there feet for a long time.
SO -- given the absence of [further ideas/low-hanging solutions], Betsy and I've agreed that the best course forward is to just tackle this problem chamber-side, and let the assembly get on to more suspensions.
I still would like to see spectra from this guy though, to show that his lower stage OSEMs are functional...
