3/24/2004 Broadband Seismometer Workshop, Lake Tahoe 1

Akiteru Takamori

Earthquake Research Institute, University of Tokyo

In Collaboration with:Alessandro Bertolini (Universita di’ Pisa)

Riccardo DeSalvo (California Institute of Technology)

Compact Tiltmeterwith

Folded Pendulum

3/24/2004 Broadband Seismometer Workshop, Lake Tahoe 2

Basic Idea

Ocean

Deep Borehole

Bi-Directional Optical LinkPower SupplyS ignal Delivery

Robust ConnectionSuitab le for Long DistanceEM Noise , Chemical Resistant

DriverActive Power Consu ming Comp.May Require Maintena nce

Instrument (Tiltmeter)Compact Low. Freq. Pend.Passive Components OnlyReliable, Maintenance FreeOperation

Introduction

3/24/2004 Broadband Seismometer Workshop, Lake Tahoe 3

Tiltmeter

Principle

− Sensing: Bob Displacement− Pendulum: Mechanical Amplifier

Pendulum Length : Amplification Gain

Key Technologies

PendulumTransducer

xx+dx

?l

θlx =d

0/1 ω∝l

⇒ Lower Resonant Freq. = Higher Gain

3/24/2004 Broadband Seismometer Workshop, Lake Tahoe 4

Example of Folded PendulumAccelerometer for GW Detector

Precise Monolithic Construction− Minimizes Cross-Coupling

Ultra-thin Flexures for Hinges− Reduce Residual Elastic Force

Key to Realize Low Resonant Freq.− Achieved Res. Freq. < 500 mHz− Machined by Electropolishing : ~ 30 mm− Flexures under Tension

Avoid Buckling

Key Technologies

20 cm

Normal Pendulum

Inverted Pendulum

Payload

Frame

Flexures / Hinges

20 cm

3/24/2004 Broadband Seismometer Workshop, Lake Tahoe 5

Precision Micro Machining

ElectropolishingRoutinely Used for Surface Treatment

− Reduce Surface Roughness to Submicron Level

Machining Precision: ~ 1 m− Optimization of Current and Density, Temperature, Flow in Electrolytic Liquid etc.

Key Technologies

30 ? m~170 ? m

3/24/2004 Broadband Seismometer Workshop, Lake Tahoe 6

New Tiltmeter Mechanics

CompactnessDimension: ~ few cm Cube

Key: Improving Flexures− Reduces Residual Elasticity− Reduced (< 30 m) Thickness

Target Res. Freq. : 500 mHz− Corresponds 1 m Pendulum Mechanical Gain: ~ 20 - 50

Key Technologies

Conceptual Design

3/24/2004 Broadband Seismometer Workshop, Lake Tahoe 7

Optical Transducer

Optical Lever / Fiber PhotointerrupterTarget Sensitivity: 10-10 m (cf. Laser IFO 10-11 m

)Light Source: Laser or LED?

− Power Loss in Fiber vs. Available Power− Coherent Length affects sensitivity?

Produces Linear Signal− Defines Unique Operating Point

Requires some R&D Effort− Intensity Modulation− Drift of Fiber Position?

Key Technologies

3/24/2004 Broadband Seismometer Workshop, Lake Tahoe 8

Optical Actuator

Wireless Electro-Magnetic ActuatorUtilizing Si Solar Cell

− Directly Connected to Voice CoilFully Passive Components near InstrumentElectro-Magnetic Interaction

− ~ few mA, mN Force Easily AchievableSufficient to Control Small FP

− Strong Dependency on Temperature

Requires External Calibration on Site− Photon Drive?

Key Technologies

3/24/2004 Broadband Seismometer Workshop, Lake Tahoe 9

Summary

Key Technologies and Scientific GainCompact & Precise Low Freq. Mechanics⇒ Allow Instrum. to Fit Tight Space (ex. Boreholes) Achieve High Mobility Realize Nearly Identical InstrmentsOptical Link⇒ Reliable Operation at Remote Sites / in Hazardous E

nvironmentOptical Transducer & Actuator⇒ Passive and Reliable Calibration on the Site: Photon Drive

Summary

Can be converted to Seismometer R&D.