Phase 3b top mass (M1-M1) damped and undamped transfer functions on the beamsplitter under vaccum have been measured two days ago with the ISI floating, using the automated matlab script.
The attached pdf are showing in order the results for :
(1) Comparison between M1-M1 measured damped (red) and modeled (blue) transfer function for the 6 DOF
(2) Comparison between M1-M1 measured undamped (red) and modeled (blue) transfer function for the 6 DOF
(3) Comparison between M1-M1 phase 3a undamped (in air april 22nd 2013, orange curve), phase 3b undamped (in vacuum July 30th 2013, blue curve), phase 3b damped (in vacuum July 30th 2013, pink curve), and undamped modeled transfer function (in blue)
(4) Comparison between M1-M1 vertical DOF hanford beamsplitter phase 3b (in vacuum, July 30th 2013, orange curve), and livingston beamsplitter phase 3a (in air, April 16th 2013 black curve) and undamped modeled transfer function (in blue)
Most of measurements are in good agreement with the model. Although, the 1.70 Hz longitudinal mode cross coupling into the vertical degree of freedom, cf 3rd page of (1) and (2)
When looking at the comparison between phase 3a and phase 3b for the vertical DOF (3rd page of (3)), it appears that the cross coupling was not present in April when the suspension was in air.
Livingston beamsplitter seems to also have cross couplings in their vertical degree of freedom, but with roll and pitch mode instead of longitudinal (cf (4))
After discution with J.Kissel, the cross coupling might come from the top mass touching an earthquake stop. This would be due to the sag of the suspension after the air have been evacuated from the chamber.
BS (in chamber, in vacuum, ISI damped (both stages), suspension undamped, no offsets) vertical degree of freedom transfer function has been re-measured yesterday, and isn't showing any signs of coupling anymore (blue curve of attached plot). In order to understand why an extra resonnance was present previously, a series of tests has been carried out, to try reproducing the coupling.
1) First, a vertical offset (-200000 cts) has been applied to the top mass osems, to push down the suspension by ~1mm, and a transfer function has been taken from 1.5 to 2Hz (green curve of attached plot). The cross coupling was still not present
2) The active control of the ISI has been turned off with the vertical offset still on, and an other tf has been taken from 1.5Hz to 2Hz (Red curve). This time, the cross coupling reappeared
3) To reproduce the theory, the offset has been turned off, with the ISI still undamped, and a last tf from 1.5Hz to 2Hz has been taken, still showing the cross coupling, but with a really poor coherence.
Conclusion : When the ISI is floating, and the BS is being excited in the vertical degree of freedom, stage 2 (Rx dof) of the ISI resonates @ ~1.6Hz which then excites the third longitudinal mode (~1.69Hz) of the suspension, creating the cross coupling between vertical and longitudinal we were seeing in our transfer functions