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

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

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 2

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

*IP = Inverted Pendulum

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 3

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:

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 4

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

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 5

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

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 6

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

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 7

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.)

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 8

Frequency Response to Seismic Motion

Active Isolation at micro seismic peak

No seismic reinjection above 5 Hz

Resonance is damped

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 9

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

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 10

Noise Budget @Kamioka in Stormy day

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

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 11

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).

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 12

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

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 13

END

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 14

Appendix

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 15

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

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 16

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.

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 17

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

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 18

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

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

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

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 20

Pole Plot

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 21

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)

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 22

Noise Budget @Kamioka in Normal day

Geophone Noise

Takanori SekiguchiItaly-Japan Workshop (19 April, 2013) 23

Noise Budget @Kamioka in Stormy day