Takanori SekiguchiExternal Review 1

Alignment control servofor type-B/type-Bp

(OpLev/WFS)

T. Sekiguchi

Takanori SekiguchiExternal Review 2

Overview• OSEM damping controls are not allowed to be switched on during

the observation phase, in terms of control noise couplings

• Thence alignment control should rely on oplev and WFS. Oplev is used during/just after the lock acquisition phase and then gradually the control will be shifted to controls using WFS.

• We want to use actuators on IM since they have larger range. Another possibility is having hierarchical controls using IM&TM actuators.

• To achieve requirement on noise coupling from WFS, UGF should be set around 1 Hz.

Takanori SekiguchiExternal Review 3

Control using IM actuators

• Put two zeros at a certain frequency to compensate 180o phase delay. Then the response looks like normal pendulum’s.

• Design servo filters, having enough gains below 0.1 Hz and low gains above 10 Hz to mitigate noise coupling

Takanori SekiguchiExternal Review 4

Actuator responses

Takanori SekiguchiExternal Review 5

YawCompensation with zeros at 1.2 Hz Servo

Takanori SekiguchiExternal Review 6

Yaw

Open Loop Total Servo

Takanori SekiguchiExternal Review 7

PitchCompensation with zeros at 0.7 Hz Servo Precise

Notch with Q~1e3,Depending on actual Q

Takanori SekiguchiExternal Review 8

Pitch

Open Loop Total Servo

Takanori SekiguchiExternal Review 9

Pitch noise

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Control succeeded in prototype

Pitch Yaw

Red: Open loop gain, Blue: Closed-loop actuator response

Takanori SekiguchiExternal Review 11

Servo Filter design in prototype

Pitch YawNotch Q=50

* Need to compensate low pass in oplev driver @15 Hz

Takanori SekiguchiExternal Review 12

Problem• Feedback using servo with high-Q notch filter is not robust, since

the control gets unstable with slight change of the system.

• In prototype, the control gets unstable when TM is locked to a photo-sensor (which mimics the signal of interferometer)

• We can change notch frequency after lock acquisition, maybe… It’s not a smart solution, though.

• RM pitch resonance Q should be reduced.

Takanori SekiguchiExternal Review 13

Q reduction of RM pitch mode• In actual system, RM pitch mode has lower frequency (~3.5 Hz),

due to material change from tungsten to steel and thickness reduction from 0.6 mm to 0.55 mm.

• This makes situation even worse.

• How to reduce Q?