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AdV-DET (Commissioning)
gouaty, mwas, bersanetti, chiummo, menzione - 0:46 Friday 24 March 2017 (37016) Print this report
SWEB Z control test

Tonight we tested the longitudinal control of the SWEB bench while the ITF was in recombined mode. In order to check the performance of this loop to cancel microseismic noise, we excited the bench by injecting a line along Z at the top of the suspension.

The following data have been collected:

Line Frequency (Hz) Line Amplitude Loop status Loop gain Start time (utc) Duration (s)
0.1 600 OPEN   21:46:00 300
0.1 600 CLOSED 10 21:51:30 840
0.1 400 OPEN   21:39:30 360
0.1 400 CLOSED 10 22:06:20 240
0.1 400 CLOSED 50 22:15:30 180
0.1 400 OPEN   22:18:40 160
0.1 300 OPEN   22:22:00 90
0.1 300 CLOSED 50 22:24:20 140
0.1 200 OPEN   22:33:30 130
0.1 200 CLOSED 50 22:36:36 180
0.2 300 OPEN   22:51:00 60
0.2 300 CLOSED 50 22:52:00 100
0.3 300 OPEN   22:54:14 64
0.3 300 CLOSED 50 22:55:34 116

The gain was set in line 408 of the SWEB_LC process. For a gain equal to 70 or above it was found that the loop became unstable (growing oscillations).

Data will be analyzed later.

Comments to this report:
mwas - 8:37 Friday 24 March 2017 (37020) Print this report
Summary:
* A recombined interferometer is a good enough configuration to see the scattered light on SWEB (and presumably SNEB)
* Controlling the bench Z motion directly at the bench level can work

The bench control in Z loop is designed just to attenuate the micro-seismic motion (than then causes glitches above 10Hz through upconversion).
It subtracts the ground motion using geophones at the base of the tower, but these give a good signal above 0.2Hz. At 0.1Hz there is 40 deg phase difference, and is high-passed at 0.05Hz (information from Joris). So the loop is AC coupled, and has a resonant gain at 0.4Hz

Figure 1 shows the analysis of the last two injections at 0.3Hz, which are in the band where the loop has some gain.
The first half of the figure is with the loop off, and the second with the loop on. Clearly the predicted arches match the glitches in B8_DC during the whole time, and enabling the Z control loop reduces the motion, and the glitches go up to only 15Hz instead of 80Hz.

Figure 2 shows what is going on with the bench motion. In the first half the bench doesn't have the additional control on, and in the second half clearly the motion is reduced, while during all the time the injected motion at the top stage of the suspension stays about the same.

The next step would be to try the loop on a day with high micro-seismic, so the injection is done with the ground and not using the suspension. This would check if using the geophone to remove the ground motion works well enough, or if the mirror local controls signals have to be use instead. Looking at the predicted arches, the latter is slightly better (and used in Figure 1).
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