takanori sekiguchi external review alignment control servo for type-b/type-bp (oplev/wfs) 1 t....
TRANSCRIPT
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
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Actuator responses
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YawCompensation with zeros at 1.2 Hz Servo
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Yaw
Open Loop Total Servo
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PitchCompensation with zeros at 0.7 Hz Servo Precise
Notch with Q~1e3,Depending on actual Q
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Pitch
Open Loop Total Servo
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Pitch noise
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Control succeeded in prototype
Pitch Yaw
Red: Open loop gain, Blue: Closed-loop actuator response
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Servo Filter design in prototype
Pitch YawNotch Q=50
* Need to compensate low pass in oplev driver @15 Hz
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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.
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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?