Akiteru Takamori

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Compact Tiltmeter with Folded Pendulum
Akiteru Takamori Earthquake Research Institute,
University of Tokyo In Collaboration
with Alessandro Bertolini (Universita di
Pisa) Riccardo DeSalvo (California Institute of
Technology)
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Motivation
Introduction
Provide Instruments Suitable for

• Observation in Frontier Region
• Deep Underground (Boreholes)
• Stable Environment without Surface Perturbations
• Seafloor
• Observation near Subduction Boundaries
• Array Observation
• Higher Spatial Resolution
• Mobile Observation

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Basic Concept
Introduction

• Problems in Conventional Tiltmeters
• Water Tube Tiltmeter
• Requires Large Room
• Immobile
• Electrolytic Tiltmeter
• Unstable in Long Term
• Large Drift Related to Liquid Content

? Overcome these problems by using Compact
Solid State Instrment
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Basic Concept
Introduction
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Tiltmeter
Key Technologies

• Principle
• Very Traditional
• Sensing Bob Displacement
• Pendulum Mechanical Amplifier
• Pendulum Length Amplification
  Gain

? Lower Resonant Freq. Higher Gain
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Low Frequency Mechanics
Key Technologies

• Folded Pendulum (FP)
• Normal Pendulum Inverted Pendulum
• Kinematics Ideally Dominated by Gravitational
  Forces
• Low Resonant Frequency
• Tuned by Changing Load Distribution
• Limitation Set by Residual Elasticity in
  Pivots/Hinges
• Position Control Required
• To Keep System in Linear Region

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Low Frequency Mechanics
Key Technologies
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Example of Folded Pendulum
Key Technologies

• Accelerometer for Astrophysical GW Research
• Ultra-thin Flexures for Hinges
• Reduce Residual Elastic Force
• Key to Realize Low Resonant Freq.
• Achieved Res. Freq. lt 500 mHz
• Machined by Electropolishing 30 mm
• Flexures under Tension
• Avoid Buckling

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Example of Folded Pendulum
Key Technologies
Inverted Pendulum
Frame
Payload
Flexures / Hinges
Normal Pendulum
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Precision Micro Machining
Key Technologies

• Electropolishing
• Routinely Used for Surface Treatment
• Reduce Surface Roughness to Submicron Level
• Machining Precision 1 m m
• Optimization of Current and Density, Temperature,
  
• Flow of Electrolytic Liquid etc.

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New Tiltmeter Mechanics
Key Technologies

• Compactness
• Dimension few cm Cube
• Key Improving Flexures
• Reduces Residual Elasticity
• lt 30 m m Thickness
• Modified Shape
• Target Res. Freq. 500 mHz
• Corresponds 1 m Pendulum
• Mechanical Gain few to several tens

Inverted Pendulum
10 m m Membranes
Conceptual Design
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Optical Transducer
Key Technologies

• Optical Fiber Sensor
• Optical Lever / Reflective Displacement Sensor
• Target Sensitivity 10-10 m (cf. Laser IFO 10-11
  m )
• Produces Linear Signal
• Defines Unique Operating Point
• Requires some RD Effort
• Intensity Modulation
• Drift of Fiber Position?

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Optical Transducer
Key Technologies

• Optical Lever

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Optical Transducer
Key Technologies

• Reflective Displacement Sensor

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Optical Actuator
Key Technologies

• Wireless Electro-Magnetic Actuator
• Utilizing Si Solar Cell
• Modulation in Light Intensity
• NO Active Component near Mechanics
• Electro-Magnetic Interaction
• few mA, mN Force Easily Achievable
• Sufficient to Control Small FP
• Strong Dependency on Temperature
• Requires External Calibration on Site

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Optical Actuator
Key Technologies

• Wireless Electro-Magnetic Actuator

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Optical Calibrator
Key Technologies

• Photon Drive
• Recoil Effect in Photon Reflection
• Momentum Conservation
• NO Active Component
• Radiation Pressure Force
• Force Amplitude
• 7 nN _at_ 1 W Light i.e. 0.1 mrad for 10 g, 200
  mHz Pend.
• Likely Too Weak to Control Proof Mass on its
  own
• Purely Intrinsic Process
• Induced Force Intrinsically Known
• ? Useful for Accurate Calibration on Site

P Intensity of Light W c Speed of Light m/s
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Optical Calibrator
Key Technologies

• Photon Drive

P Intensity of Light W c Speed of Light m/s
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Summary
Summary

• Key Technologies and Scientific Gain
• Compact Precise Low Freq. Mechanics
• ? Allow Instrum. to Fit Tight Space (ex.
  Boreholes)
• Achieve High Mobility
• Realize Nearly Identical Instrments
• Optical Link
• ? Reliable Operation at Remote Sites / in
  Hazardous Environment
• Optical Transducer Actuator
• ? Passive and Reliable
• Provide Accurate Calibration on Site Photon
  Drive

Can be converted to Seismometer RD.