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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.