Download - Inverted Pendulum Control for KAGRA Seismic Attenuation System

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 1

Inverted Pendulum Control for KAGRA Seismic Attenuation SystemD2, Institute for Cosmic Ray ResearchTakanori Sekiguchi


Contents• Introduction of IP controls• IP control model and simulation• Current status of IP control experiment• Summary

*IP = Inverted Pendulum


Suspension Local ControlsTarget:• Damping of mechanical resonances• Drift control at low frequencies

Purpose:• Reduction of RMS motions for lock acquisition• Quick recovery after large excursion (e.g. EQ)• Stable operation of the interferometer

RMS displacement 0.1 μm

RMS velocity 0.1 μm/sec

RMS yaw and pitch angle 0.1 μrad

Damping time of resonances ~ 1 minute

Rough idea of the requirement from MIF:


IP Local Control• Top stage X, Y, Yaw motions are controlled.• Drift control of IP & active damping of resonances

(below 1 Hz)

Sensitivity of sensors


Starting Point• Starting from 1-D suspension model with simplified system• Check controllability with combined sensor (LVDT & geophone)

≡

Simple suspension model Pre-isolator Prototype in Kashiwa


Control Model

(Calibration of the sensors are included in “suspension” block)

• Geophone senses top stage velocity• LVDT senses relative displacement between top & ground• Geophone for damping (>0.1 Hz), LVDT for drift control (<0.1 Hz)

Sensor Noise


Filter Design

Chebychev filter for steep cut-off around micro seismic peak

High pass filter to reject glowing-up noise at low frequencies

Gain boost at micro seismic peak

Open-loop transfer function • Crossover frequency: 0.03 Hz• Unity gain frequency: 0.8 Hz

(phase margin: 60 deg.)


Frequency Response to Seismic Motion

Active Isolation at micro seismic peak

No seismic reinjection above 5 Hz

Resonance is damped


Noise Budget @Kamioka in Normal day

RMS dis.: 1.5x10-6 2x10-6 m (@0.01 Hz)RMS vel.: 1.5x10-7 2x10-7 m/s

Geophone Noise


Noise Budget @Kamioka in Stormy day

RMS dis.: 2x10-5 6x10-6 m (@0.01 Hz)RMS vel.: 2x10-6 5x10-7 m/s


Summary• We investigate IP controls with combined vibration sensors

(LVDTs and geophones).

• Sensor noise (especially, geophone) is dominant with quiet environment in Kamioka mine.

• Low frequency vibration (<10 mHz) should be stabilized by other ways (global control).


Current Status of Pre-Isolator Prototype in Kashiwa• IP is currently tuned at 80 mHz.• LVDTs and geophones are installed and calibrated.• X, Y, θ motions constructed by LVDTs and geophones

resemble very well.

Measured Y displacement by LVDT & geophoneWith excitation from virtual Y actuator

• Next step: Apply X control with combined sensors


END


Appendix


Why Local Controls Are Necessary?• Multi-suspension system has many mechanical

resonances to be damped.

• Low frequency oscillators are sensitive to disturbance like temperature change, and drift easily.

Local controls are required for lock acquisition and stable operation of the interferometer


RequirementFor Lock Acquisition:• Small RMS velocity and rotation angle of the mirror:

RMS velocity ~0.1 μm/sec

RMS yaw and pitch angle ~0.1 μrad

Rough idea of the requirement:

• Short damping time of the mechanical resonances (within ~min.)

During Operation:• Actuation forces on the mirrors must be within the actuator range.

(e.g. ~0.1 μm displacement level is allowed for test masses)

• 10 times smaller local control noises than other fundamental noises in the observation band.


Control TopologyTop Stage• LVDT: Drift control• Geophone: Damping of pendulum

modes

Intermediate Mass• Damping of residual resonances• Alignment control

Optical Lever• Damping angular resonances?• DC alignment signal


Control TopologyX, Y• Damp: GEO, OSEM• DC: LVDT

Z• Damp: LVDT, OSEM• DC: LVDTs on GAS

Pitch, Yaw• Damp: OSEM• DC: Oplev

Roll• Damp: OSEM


Resonances of Pendulum modes• Resonances at low frequencies contribute to RMS• Damped by magnetic damper, but not perfectly

Simulated Mechanical TF of Type-A SAS

Pendulum mode @micro seismic peak


Pole Plot


Pole Plot [variable gain]• Too small gain unstable by LVDT control (~0.2 Hz)• Too much gain unstable by geophone control (~0.01 Hz)

Pole plot with variable gain of geophone control (gain 0 to 5, Blue: gain = 1)


Noise Budget @Kamioka in Normal day

Geophone Noise


Noise Budget @Kamioka in Stormy day

Download - Inverted Pendulum Control for KAGRA Seismic Attenuation System

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