Marc, Yuhang
Following the End mirror OpLev check (entry 2412) and the measurement with the sensing matrix of entry 2411 we decided to use a 4x4 matrix to try to remove the coupling of unwanted degrees of freedom on the reconstructed signals from the 2 QPDs.
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We locked the FC and only close End mirror length control.
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Tune the phases : We found out that all the phases changed by ~40 degrees. So we used time series of all segments and maximized the I signals
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We sent a line at 2 Hz on Input and End pitch and yaw. Using the calibration of entry 1877 we injected a 3.78 mrad line for every dof. Namely
|
|
Pitch |
Yaw |
|
Input |
600 |
172 |
|
End |
727 |
141 |
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We measured the amplitude of the injected line (removing the background level noise) on each QPDs.
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We computed the driving matrix and had to modify the medm configuration (outmatrix.adl) to allow to use the 4x4 matrix
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|
|
QPD1 pitch |
QPD2 pitch |
QPD1 yaw |
QPD2 yaw |
|
Input pitch |
6 |
-5 |
-0.5 |
-1 |
|
End pitch |
-5 |
10 |
0.5 |
1 |
|
Input yaw |
1 |
-2 |
4 |
-3 |
|
End yaw |
2 |
-4 |
-4 |
12 |
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We checked each QPDs signals as in entry 2412 but coupling was still visible.
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We checked the demodulation phases and they seem to have moved by ~-40 degrees back to the previous phases...
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We checked possible reasons for this phase changes :
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We aligned the FC to another position : no phase changes
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We tried to put the optical bench to the ground (before putting the electronics inside of a clean room the optical table was connected to the nim racks ground) : no changes
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We putted back the 2x2 matrix of entry 2412 and tried to tuned the coupled coefficients by hand to remove coupling. It could approximatively work but during this tuning we could also see that the level of coupling increases or decreases sometimes without noticeable reasons...