Abstract:

I designed SR2 IDAMP Y filter and set new filter banks at FM3 of K1:VIS-SR2_IP_IDAMP_Y filter bank.
SR2 oscillation seems to be quiet but it is necessary to check the long-term stability of the loop for a while.

Detail

As reported in klog34104, SR2 yaw started to oscilate somehow from around the begining of June, which could affect the IFO stability.
To fix the oscillation, I first checked what is the problem.
Since 0.19Hz is not the resonant frequency of yaw, the issue is likely to be local control, especially cross-over of two controls.
SR2 have two controls that have mix the two different feedback pathes: IP_IDAMP_Y and IM_OLDAMP_Y.
To investigate which loop is problematic, I turned off the loops one by one.

Figure 1 shows the OpLev signals and filter condition when I tested.
Top graph shows the OpLev signals and the second graph shows IM_OLDAMP filter setting (ON:around 38160, OFF: around 38280).
The third and forth graph shows the IDAMP_Y and DCCTRL_Y conditions, respectively.
The inertial damping control turned off at t= 0, TM yaw motion became quiet, so the problem seems inertial damping.

Then, I checked the SR2 IDAMP_Y filters and found that the overall gain was different from SR3.
Figure 2 shows the SR3 IDAMP_Y filter design, which damps 0.32Hz peak and DC control below 10mHz.
Figure 3 shows the SR2 IDAMP_Y filter design, which controls below 1.5Hz.
To adjust the design concept of inertial damping to healthy suspensions (SR3), I implemented the new filter at FM3 (comp250609) of K1:VIS-SR2_IP_IDAMP_Y filter bank (fig4).
Figure 5 shows the design of OLTF with new filter, which achieves to damp 0.32Hz and DC control less than 10mHz.
Figure 6 shows the OLTF measurement results before (blue) and after (red) engaging new filter, which seems to be as we expected.

Figure 7 shows the spectrum before (blue) and after (red) the work.
For the reference, I also plotted the spectrum on May 26, which we thought healthy.
Since it will take a long time to confirm this modification makes suspensions healthy, we need to keep monitoring.

Final configuration of SR2 IDAMP filter is shown in fig8.