an affordable broadband seismometer : the capacitive geophone aaron barzilai 1 , tom vanzandt 2 ,
DESCRIPTION
An Affordable Broadband Seismometer : The Capacitive Geophone Aaron Barzilai 1 , Tom VanZandt 2 , Tom Pike 2 , Steve Manion 2 , Tom Kenny 1. 1 Dept. of Mechanical Engineering Stanford University. 2 Center for Space Microelectronics Technology Jet Propulsion Laboratory. - PowerPoint PPT PresentationTRANSCRIPT
1Aaron Barzilai
An Affordable Broadband Seismometer : The Capacitive Geophone
Aaron Barzilai1, Tom VanZandt2,
Tom Pike2, Steve Manion2,
Tom Kenny1
1Dept. of Mechanical EngineeringStanford University
2Center for Space Microelectronics TechnologyJet Propulsion Laboratory
2Aaron Barzilai
An Affordable Broadband Seismometer : The Capacitive Geophone
Aaron Barzilai4, Tom VanZandt2,
Tom Pike3, Steve Manion2,
Tom Kenny1
1Dept. of Mechanical EngineeringStanford University
2SiWave, Inc.3Imperial College
4CapitalOne, Inc.
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Cross-sectionSchematic
A Conventional Geophone:OYO Geospace 4.5 Hz GS-11D
MagnetCylinder
CoilGeophoneHousingLeafSpring
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Mechanical Sensitivity
• Acceleration Causes Relative Motion Between the Coil and the Housing
• Constant Sensitivity Below the Resonant Frequency
10-6
10-5
10-4
10-3
10-2
10-2 10-1 100 101 102
Mechanical Sensitivity [m/(m/s
2)]
Frequency [Hz]
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Electrical Sensitivity
• Inductively Measure Motion of the Coil Relative to the Magnetic Field
• Output Voltage Proportional to the Proof Mass Velocity
102
103
104
105
106
107
10-2 10-1 100 101 102
Electrical Sensitivity [V/m]
Frequency [Hz]
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Total Sensitivity
• At Low Frequency, Measurement of Proof Mass Velocity Reduces Sensitivity
• At High Frequency, Mechanical System Reduces Sensitivity
100
101
102
103
104
10-2 10-1 100 101 102
Geophone Sensitivity [V/g]
Frequency [Hz]
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Conventional Geophone Resolution
• Poor Resolution at Low Frequency Exacerbated by Reduced Sensitivity
• Resolution not at Thermomechanical Limit
Resolution gHz[ ] =
Noise VHz[ ]
SensitivityVg[ ]
Barzilai et al., “Technique for Measurement of the Noise of a Sensor in the Presence of Large Background Signals,” Rev. Sci. Instrum., July 1998, Vol. 69, Num. 7, pp. 2767-2772
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-2 10-1 100 101 102
Resolution [g/
√]Hz
[ ]Frequency Hz
Thermomechanical Limit
Predicted Circuitry Limit
Measured Circuitry Limit
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An Improved Seismometer: A Capacitive Geophone
• Use a Commercial, Off The Shelf Geophone as the Mechanical System
• Improve Low Frequency Sensitivity by Capacitively Measuring Proof Mass Displacement with only Simple, External Modifications
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Capacitive Hardware
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Capacitive HardwareAdditionalHousingMovingElectrodeFixedElectrodes
Insulation33.02 mm43.18 mm
yCircuitModel
C =εε0Aa−y
C=εε0Aa+ y
a= Balanced Gap≈250μm
A=Area=6.0 ×10-4m2
CNOMINAL≈21pF
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Electrical System Overview
VCEN is a Square Wave at the Same Frequency as VSQ
with Amplitude Modulated by y. VCEN is Demodulated to Produce an Output that is Proportional to the Displacement of the Proof Mass.
y+VSQx12
-VSQDemodulatorVOUTReferenceInput
VCEN
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Acceleration Sensitivity
• Constant Sensitivity At Low Frequency since Output is Proportional to Proof Mass Displacement
• Bandwidth from 83 sec to 50 Hz Defined by Controller
100
101
102
10-2 100 102
Sensitivity [V/g]
Frequency [Hz]
3dB pts
63 V/g
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Velocity Sensitivity
• Velocity Output Produced by Integrator Circuit
• Bandwidth from 45 sec to 50 Hz Defined by Controller and Integrator
100
101
10-2 100 102
Sensitivity [V/m/s]
Frequency [Hz]
3dB pts
6.3 V/m/s
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M 7.3 Near Australia 11/26/99 Recorded in Ground Floor Lab
in the Durand Building at Stanford.
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Conclusions
• The Low Frequency Resolution of a Geophone can be Improved by Adding Capacitive Detection
• Real Instrument Packaging needed to reach full potential performance of this approach.
• A Capacitive Geophone is an affordable seismometer suitable for broadband use
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Acknowledgements
This work was supported by the Center for Space Microelectronics Technology, Jet Propulsion Laboratory, California Institute of Technology, and is sponsored by the NASA Office of Space Access and Technology. We also acknowledge the NSF Career Award(ECS-9502046), the Charles Lee Powell Foundation, and the Terman Fellowship.
Special thanks to Marcos Alvarez at PASSCAL and Larry Cochrane of the PSN for their advice and assistance.
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Full Circuit Diagram Part I
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Full Circuit Diagram Part II
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Block Diagram1
s2 + 23s+ 645
1
37428
1
.026
˙ ̇ X
1
300s +1
X
VIF˙ ̇ X
1
2⋅5.0 ×103(4.7×10−3s+1)47×103(5.0 ×10−4s+1)
˙ ̇ X
VACCELERATION
High Frequency
Low Frequency
1.0 ×106˙ ̇ X IN
1
37428
1
.026
VIF
V
33s
33s +1
High Pass10
(10s +1)
Integrate
VVELOCITY