An Affordable Broadband Seismometer: The Capacitive...
Transcript of An Affordable Broadband Seismometer: The Capacitive...
1SSA‘99 Aaron Barzilai
An Affordable Broadband
Seismometer:The Capacitive
GeophonePoster F1
http://micromachine.stanford.edu/smssl/projects/Geophones/SSA99PosterFinal.pdf
2SSA‘99 Aaron Barzilai
Aaron Barzilai1, Tom VanZandt2,
Tom Pike2, Steve Manion2,
Tom Kenny1
1Dept. of Mechanical EngineeringStanford University
2Center for Space Microelectronics TechnologyJet Propulsion Laboratory
Contact: [email protected]
3SSA‘99 Aaron Barzilai
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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Objective
• Develop an Affordable, Robust Broadband Seismometer
• Enhance the Ability of Seismometer Arrays to Detect Low Frequency Signals
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Seismometer Information Flow
AccelerationInput Mechanical
System
RelativeMotion Electrical
System
OutputVoltage
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Cross-sectionSchematic
A Conventional Geophone:OYO Geospace 4.5 Hz GS-11D
Magnet
Cylinder
Coil
GeophoneHousing
LeafSpring
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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
Mec
hani
cal S
ensi
tivity
[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
Ele
ctric
al S
ensi
tivity
[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
Geo
phon
e S
ensi
tivity
[V/g
]
Frequency [Hz]
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Circuitry NoiseOutput Voltage
Noise Spectral Density
Geo100Ω
10kΩ
VO
Typical Circuit
100x Ampifier10-7
10-6
10-5
10-4
10-2 10-1 100 101 102
Out
put V
olta
ge N
SD
[V/√
Hz]
Frequency [Hz]
Peak CausedBy Resonance
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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[ ]
Sensitivity Vg[ ]
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
Res
olut
ion
[g/√
Hz]
Frequency [Hz]
Thermomechanical Limit
PredictedCircuitry Limit
MeasuredCircuitry 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 HardwareAdditionalHousing
MovingElectrode
FixedElectrodes
Insulation
33.02 mm43.18 mm
y
CircuitModel
C =εε0 A
a − y
C =εε0 A
a + y
a = Balanced Gap ≈ 250µm
A = Area = 6.0 ×10-4 m2
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
+VSQ
x12
-VSQ
DemodulatorVOUT
Reference
Input
VCEN
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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
12
⋅5.0 ×103(4.7 ×10−3s + 1)
47 ×103(5.0 ×10−4 s +1)
˙ X
VACCELERATION
High Frequency
Low Frequency
1.0 × 106
˙ X IN
1
37428
1
.026
VIF
V
33s
33s +1
High Pass
10
(10s + 1)
Integrate
VVELOCITY
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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
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101
102
10-2 100 102
Sen
sitiv
ity [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
Sen
sitiv
ity [V
/m/s
]
Frequency [Hz]
3dB pts
6.3 V/m/s
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Resolution Comparison
USGS LNM:Seismic Noise At Quietest Sites On Earth
Capacitive Geophone Measurement Limited byAmbientSeismic Noise
10-10
10-9
10-8
10-7
10-6
10-2 10-1 100 101 102
Res
olut
ion
[g/√
Hz]
Frequency [Hz]
ConventionalGeophone
USGS LowNoise Model
CMG-40T
CapacitiveGeophoneUpper Limit
Capacitive GeophoneThermomechanical Limit
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Conclusions• Need to collaborate with a vault
operator to take data at a quieter site• The Low Frequency Resolution of a
Geophone can be Improved by Adding Capacitive Detection
• A Capacitive Geophone is an affordable seismometer suitable for broadband use