MINIATURE OPTICAL SEISMIC SENSORS FOR MONITORING … · 2011. 6. 28. · Figure 7. Noise...
Transcript of MINIATURE OPTICAL SEISMIC SENSORS FOR MONITORING … · 2011. 6. 28. · Figure 7. Noise...
-
si-audio
Figure 1. Schematic of a photonics based motion detection principle. Light from a semiconductor laser such
as a vertical cavity surface emitting laser (VCSEL) illuminates a diffraction grating. A portion of the incident
light reflects directly off of the grating fingers, while the remaining light travels in between the grating fingers
and to the proof mass and back to accrue additional phase. A diffracted field results consisting of a zero and
higher orders whose angles remain fixed, but whose intensities are modulated by the relative distance
between the proof mass and grating with the sensitivity of a Michelson type interferometer.
MINIATURE OPTICAL SEISMIC SENSORS FOR MONITORING APPLICATIONS
Caesar T. Garcia, Guclu Onaran, Brad Avenson, Alex Liu, Matt Christensen, and Neal A. Hall
Silicon Audio Labs
Sponsored by the National Nuclear Security Administration
Contract No DE-FG02-08ER85106
TECHNOLOGY OVERVIEW:
Figure 2. (left) Theoretically predicted relationship between the diffracted beams labeled in Figure 1 vs.
gap distance “d” labeled in Figure 1. (right) The difference signal is then used to detect the proof mass
motion within a single interference fringe.
The Department of Energy (DOE) and the National Nuclear Security Administration (NNSA) seek revolutionary innovations with
respect to miniature seismic sensors for the monitoring of nuclear detonations. Specifically, the performance specifications are to be
consistent with those obtainable by only an elite few products available today, but with orders of magnitude reduction in size, weight,
power, and cost. This next-generation sensor technology calls upon several advanced fabrication methods and read-out
technologies being pioneered by Silicon Audio, including the combination of silicon microfabrication, advanced meso-scale
fabrication and assembly, and the use of advanced photonics-based displacement / motion detection methods. Prior development
has demonstrated 1) verified and repeatable sub 2ng/√Hz noise floor from 5 to 100Hz, 2) compact integration of 3-axis prototypes
and 3) robust deployment exercises. Ongoing developments are focusing on low frequency challenges, low power consumption,
ultra-miniature size, and low cross axis sensitivity. Successful implementation will result in a demonstration unit roughly the size of a
9-volt battery and with the ability to address the advanced needs of the monitoring community. Additional applications envisioned
include military/defense, scientific instrumentation, oil and gas exploration, inertial navigation, and civil infrastructure monitoring.
ABSTRACT:
Figure 3. Sensor block diagram including feedback control. Control circuitry conditions the dynamic
response of the output, Vout, and also provides logic instructions to ensure sensor stability.
Additionally, the control circuitry is equipped with an option for self calibration based upon the known
optical wavelength.
Microfabricated
grating region
Micro optoelectronic
components
2g tungsten proof mass
and non-magnetic low
thermal expansion springs
Figure 4. Various components used in the assembly of prototype units. Further miniaturization is
underway using Silicon Audio’s micro-optoelectronic packaging capabilities.
Figure 5. CAD image along with actual photograph of Silicon Audio’s 3-axis GeoLight prototype.
0.01 0.1 1 10 100
-10
-8
-6
-4
-2
0
2
Frequency(Hz)
No
rma
lize
d R
esp
on
se
(d
B r
ef 7
60
V/(
m/s
2))
10-1
100
101
-200
-180
-160
-140
-120
-100
-80
-60
Frequency (Hz)
PS
D (
10*l
og
10(m
2/s
4/H
z))
STS-2 EW
STS-2 NS
STS-2 Z
Si Audio X
Si Audio Y
Si Audio Z
Q330HR Digitizer (Si Audio)
Q330HR Digitizer (STS-2)
NLNM
10 20 30 40 50 60 70
-5
0
5
x 10-6
Velo
city (
m/s
)
STS-2
10 20 30 40 50 60 70
-5
0
5
x 10-6
Velo
city (
m/s
)
Si Audio
Time (seconds)
EWNSZUSGS Predicted Arrival Time
EWNSZUSGS Predicted Arrival Time
GENERAL
Topology Three Axis
Feedback Force balance with interferometric transducer
Mass centering Automatic centering
Leveling Integrated bubble level, adjusted locking leveling feet
PERFORMANCE
Noise 1ng/√Hz
Passband 100 seconds to 100 Hz
Clip level 4.2mg pk-pk
Sensitivity 760 V/(m/s^2)
Linearity (3%THD) 3.2mg pk-pk
ANALOG INTERFACE
Acceleration output ±10 volts
UVW coordinate system (Galperin orientation)
DIGITAL INTERFACE
Type Available upon request
POWER
Supply voltage 12 V
Power Consumption 20mW/channel
HANDLING
Transport No mass lock required for transport
ACKNOWLEDGEMENTS: The authors graciously thank the NNSA and the DOE SBIR program for support. We also thank the University of Texas, Institute of
Geophysics for assistance with Phase I and Phase II field test demonstrations. Finally, we would like to thank Bob Hutt and his team at
the USGS Albuquerque Seismology Lab for their assistance with prototype field testing.
A sensing structure similar to that described in Figure 1 is in development. In this embodiment, an optical detection scheme is
responsible for detecting a proof mass displacement which is proportional to ground acceleration. The sensitivity of the resulting
output signal is flat to acceleration and is therefore expressed in units of V/(m/s^2).
Figure 6. (left) Single sensors packaged as geophones for low power remote monitoring applications. (right)
Frequency response measured optically while using magnetic actuator as a self calibration signal.
Figure 7. Noise measurement conducted during visit to USGS Albuquerque Seismology Laboratory (ASL)
demonstrating Silicon Audio’s miniature 3-axis GeoLight prototype detecting down to site noise at 3 Hz
and above and capturing portions of the 0.2 Hz microseism. For comparison, the NLNM is included as are
the digitizer noise levels. Sources of 1/f noise at lower frequencies are presently being addressed.
Figure 8: P-wave arrival of Tarapaca, Chile event recorded by STS-2
and by Silicon Audio’s GeoLight prototype during testing visit to
USGS ASL, May 3-6th 2010.
Table 1: Preliminary specifications for 3-axis GeoLight
prototype.
FUTURE WORK • Improve low frequency noise (1ng /√Hz at 100 mHz) • Electrostatically actuated MEMS grating for ultra low power and miniaturization down to 1cm^3 / axis. • All MEMS embodiment for 10ng/√Hz noise floor strong motion applications.
Figure 9: a) Profilometer image of electrostatically actuated MEMS grating. b) CAD image of MEMS strong motion
embodiment with (c) top and (d) bottom photograph views of fabricated devices.
1mm
a b c d
100µm