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VIS (BS)
takafumi.ushiba - 10:32 Monday 01 August 2022 (21662) Print this report
Hierarchical angular control of BS for stronger damping

Abstract:
For reducing the residual motion of BS, especially yaw, I implemented hierachical angular control of BS with high bandwidth (~5Hz).
New filters seem working well and the residual motion is less than 1 urad for both pitch and yaw.

Detail:
BS large residual motion sometimes disturbed our interferometer work, so I started to solve the problem.
Since the actuator range of BS is smaller than SRs, we cannot use the same high-bandwidth control filters for yaw like SR3.
So, I decided to implement hierachical control by using TM and IM.

Cross-over frequency between TM and IM controls are designed as 0.5 Hz.
UGF of controls are set as 5 Hz.

Figure 1 shows the spectra of residual motion of BS.
As you can see, residual RMS was reduced by more than 10 times.
Now, RMS values are less than 1 urad, which is enough to start initial commissioning.

Note:
Thanks to hierachical control, TM actuator output is now less than several thousands, which is about 10% of full actuator range.
So, this control can probably work even in the louder condition like one of the flanges are opened.

Images attached to this report
Comments to this report:
shinji.miyoki - 15:48 Monday 01 August 2022 (21663) Print this report

Wow, amazing effect!

The fluctuation of the beam to OMC is better to be checked again although other mirrors' motion should be stabilized.

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