Transcript Improved Low Frequency Performance of a Geophone

An Affordable Broadband Seismometer : The
Capacitive Geophone
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Aaron Barzilai , Tom VanZandt ,
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Tom Pike , Steve Manion ,
Tom Kenny1
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Aaron Barzilai
Dept. of Mechanical Engineering
Stanford University
Center for Space Microelectronics Technology
Jet Propulsion Laboratory
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An Affordable Broadband Seismometer : The
Capacitive Geophone
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Aaron Barzilai , Tom VanZandt ,
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Tom Pike , Steve Manion ,
Tom Kenny1
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Dept. of Mechanical Engineering
Stanford University
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Imperial College
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Aaron Barzilai
SiWave, Inc.
CapitalOne, Inc.
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A Conventional Geophone:
OYO Geospace 4.5 Hz GS-11D
Schematic
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Cross-section
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Mechanical Sensitivity [m/(m/s)]
Mechanical Sensitivity
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• Acceleration Causes
Relative Motion
Between the Coil and
the Housing
10-2
10-3
10-4
10-5
10-6 -2
10
10-1
100
101
Frequency [Hz]
102
• Constant Sensitivity
Below the Resonant
Frequency
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Electrical Sensitivity [V/m]
Electrical Sensitivity
• Inductively Measure
Motion of the Coil
Relative to the
Magnetic Field
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105
104
103
102 -2
10
10-1
100
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Frequency [Hz]
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• Output Voltage
Proportional to the
Proof Mass Velocity
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Geophone Sensitivity [V/g]
Total Sensitivity
• At Low Frequency,
Measurement of
Proof Mass Velocity
Reduces Sensitivity
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103
102
101
100 -2
10
10-1
100
101
Frequency [Hz]
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• At High Frequency,
Mechanical System
Reduces Sensitivity
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Conventional Geophone Resolution
Resolution
 
g
Hz
10-4
Resolution [g/ Hz]
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10-6
10-8
Measured
Circuitry Limit
Predicted
Circuitry Limit
10-9
-10
10
Thermomechanical Limit
10-2 10-1
100
101
Frequency [Hz]

• Poor Resolution at
Low Frequency
Exacerbated by
Reduced Sensitivity
-5
10-7
Noise VHz 
=
Sensitivity Vg
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• Resolution not at
Thermomechanical
Limit
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
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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 Hardware
Circuit
Model
C
0 A
C
0 A
Additional
Housing
Fixed
Electrodes
Moving
Electrode
y
ay
Insulation
ay
a  Balanced Gap  250m
A = Area = 6.0  10-4 m 2
33.02 mm
43.18 mm
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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.
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Acceleration Sensitivity
• Constant Sensitivity
At Low Frequency
since Output is
Proportional to Proof
Mass Displacement
Sensitivity [V/g]
102
63 V/g
3dB pts
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100
-2
10
0
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Frequency [Hz]
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• Bandwidth from
83 sec to 50 Hz
Defined by Controller
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Sensitivity [V/m/s]
Velocity Sensitivity
• Velocity Output
Produced by Integrator
Circuit
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6.3 V/m/s
3dB pts
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10-2
100
Frequency [Hz]
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• Bandwidth from
45 sec to 50 Hz
Defined by Controller
and Integrator
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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 Diagram
Ý
ÝIN
X
XÝÝ
V
X
1
6
1.0

10
s2  23s  645
Low Frequency
XÝÝ
F
1
.026
I
28
1
374
V
1
300s  1
High Frequency
XÝÝ 1
.026
F
28
I
1
374
V 1 5.0  10 3 (4.7  10 3 s  1)

2 47  10 3 (5.0  10 4 s  1)
High Pass
Integrate
33s
33s  1
10
(10s  1)
VVELOCITY
VACCELERATION
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