J. Garcia, J. Kissel
As a supplement to Garcia's analysis of the H2 SUS ITMY (QUAD) main chain and reaction chain transfer functions taken on 110825, after that QUAD had been suspended post-mate I attach a "pre-mate and post-mate" comparison for both chains. The Reaction Chain (R0 -- see 110825r0alltfs.pdf) shows a significant increase in stiffness in pitch (with the first mode at 0.58 Hz simply disappearing), along with other degrees of freedom less effected but still not passable. Although several things changed between the measurements (listed below), my suspicion is that the new cable routing -- the "as designed" cable routing -- is what's causing the badness. The hope is to get this diagnosed *before* we must remove the lower half for fibering, but is not a "fix before detaching lower half or die" as we still have to lace up the ESD cables, and do (at least one more) iteration(s) of cable arranging to find an ideal cable route that has minimal effect on the suspension dynamics. We can do preliminary lacing while the reaction chain is split from the main chain (so as to not do so with a delicate fibered main chain in the way), and route the remainder up the top mass after the re-mate.
The differences between the two measurements are
- 2011-08-12 (ORANGE)
- QUAD mounted to Solid Stack on Mechanical Test Stand (MTS) 1
- No stiffening elements are in place
- Satellite amplifiers still under "old" design (as per D0901284-v1)
- Cable Routing as shown in entry 1230, where the R0 L1/UIM and R0 L2/PUM cables, in their jump between the R0 TOP mass to CAGE, are clamped well at R0 TOP mass, but temporarily shoved in cubby holes at the corner of the cage
- 2011-08-25 (BLACK)
- QUAD mounted to BSC-ISI on MTS 2
- All stiffening elements are in place, including stiffening sleeve, lower structure side panels, and upper structure cross-braces.
- Satellite amplifiers have been modded, as per aLOG entry 1257, and described in G1100856
- Cable routing as shown .pdf attachement "cablerouting110825.pdf", where the R0 L1/UIM and R0 L2/PUM cables, are now routed "as designed," using the c-clamp cable bracket on the now-installed cross-brace.
Things to keep in mind while looking at the three traces in the R0 comparison:
- The model is a main chain model, it's merely to guide the eye
- The ORANGE curve represents an "already approved" cabled up reaction chain, a la entry 1217, and is therefore the nominal reference
The reason why I suspect cabling:
(1) Adding stiffening to the structure/cage should not affect the dynamics of the suspended portion of the QUAD. The only way I could envision this having an effect is if any protruding elements of the suspended portion (read CABLES) are [interfering with/rubbing against] the newly installed structure pieces. This would be most likely at the L2 PUM stage, where the tentacles of the quadrapuss OSEM cables have to loop around the back of the PUM, where there is now a cross-brace of the stiffening sleeve.
(2) The modified satellite amplifiers should have an overall, DC, (160k - 121k)/ 160k = 20% decrease in sensor gain, but otherwise no change in functionality. Given that the calibration on these measurements is at best 50%, we should see minimal change if at all. Further, and change seen would be on all degrees of freedom. They change definitely would not affect the dynamics of the suspension as seen.
(3) As with the stiffening elements, whether the QUAD is mounted to the Solid Stack or the BSC-ISI should make no difference to the suspended dynamics.
(4) The cable routing from the R0 TOP to the CAGE is significantly different. The new routing is more "aligned" to pitch -- the most different degree of freedom.
Remember, the cable philosophy:
When jumping between suspended isolation stages,
(1) the clamping on either side of the connection should be as close to the jump as possible
(2) the jump should be made with as few cables as possible
(3) the length of cable between the jump should be a loose S shape, and without touching anything around it (which is why (1) is important)
(4) the distance of the jump (from clamp point to clamp point) should be minimized
These points are only necessary between stage jumps. Cable routing on/within a mechanically connect (but not suspended) stage doesn't matter, and in fact should be tightly contained against the stage, such as to not interfere with other stages.
So, we should try the following:
- "Splitting up" the L1/UIM and L2/PUM cables, i.e. clamp them in separate locations (Bundles of cables are stiffer than single cables)
- Re-orienting the R0 TOP launching point such that the cables come out of the clamp parallel to the Transverse axis, as they were on the Solid Stack (I think this may relieve some of the "pitch pressure" if you will)
- Finding some way to clamp the cables on the *inside* of the cross brace, rather than on the outside. This would require some ingenuity, use of a different clamp, or a re-design of the current clamp. ()
Unfortunately, I think we got lucky with Betsy's awesome first attempt at this when the QUAD was mounted to the Solid Stack. I'm sure we'll need to iterate on some combination of all three of the above ideas, and add some new ones before the dynamics are restored. Not to mention we have to add a whole 'nother lacing cable -- the ESD extension cable (see blue trace in T1100327-v2).
