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IEEE SENSORS JOURNAL, VOL. 12, NO. 4, APRIL
A Robust and Compact Fiber Bragg Grating
Vibration Sensor for Seismic Measurement
Yinyan Weng, Xueguang Qiao, Tuan Guo, Member, IEEE, Manli Hu, Zhongyao Feng, Ruohui Wang, and
Jing Zhang
Abstract—A compact fiber Bragg grating (FBG) vibration
sensor consisting a flat diaphragm and two L-shaped rigid cantilever beams for seismic measurement has been proposed and
experimentally demonstrated. The specially designed sensing
configuration contributes many desirable features such as a wide
frequency response range (10–120 Hz), an extremely high sensitivity coefficient ( 100 pm g) together with a robust metal
package for improving the mechanical strength and a decreased
transverse sensitivity ( 5%), making it a good candidate for
in-field seismic wave measurement in oil and gas exploration.
Index Terms—Cantilever beam, fiber Bragg grating (FBG), vibration sensor.
I. INTRODUCTION
N THE OIL AND GAS exploration industry, the increasing demand for seismic wave measurement excites
the research on vibration measurement techniques. Various
kinds of vibration sensors have been proposed, including the
piezoelectric, piezoresistive, capacitive, fiber-optic-based techniques [1]–[3]. Among these techniques, FBG (FBG) shows
a great potentials in vibration measurement because of its
distinguished advantages such as immunity to EMI and RFI,
multiplexing ability, small size and lightweight, and much high
sensitivity [4], [5]. Based on the in-field experiences [2], for the
seismic measurement of oil and gas exploration, the required
vibration frequency for detection is usually range from 10 to
100 Hz. Metal-cantilever-beam-based FBG vibration sensor,
with special configuration design, can meet above frequency
I
Manuscript received July 18, 2011; accepted August 17, 2011. Date of publication August 30, 2011; date of current version February 08, 2012. This work
was supported in part by the National Natural Science Foundation under Grant
60727004 and Grant 61077060, in part by the National High Technology Research and Development Program 863 Foundation under Grant 2007AA03Z413
and Grant 06Z203, in part by the Ministry of Education Project of Major Science and Technology Innovation Foundation under Grant Z08119, in part by
the Ministry of Science and Technology Project of International Cooperation
Foundation under Grant 2008CR1063, in part by the Shaanxi Province Project
of Science and Technology Innovation Foundation under Grant 2009ZKC01-19
and Grant 2008ZDGC-14, in part by the Guangdong Natural Science Foundation of China under Grant S2011010001631, and in part by the Fundamental
Research Funds for the Central Universities of China under Grant 11611601.
The associate editor coordinating the review of this paper and approving it for
publication was Prof. Ozanyan Krikor.
Y. Weng, X. Qiao, M. Hu, Z. Feng, R. Wang, and J. Zhang are with
the Department of Physics, Northwest University, Xi’an 710069, China
(e-mail: [email protected]; [email protected]; [email protected];
[email protected]; [email protected]; [email protected]).
T. Guo is with the Institute of Photonics Technology, Jinan University,
Guangzhou 510632, China (e-mail: [email protected]).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JSEN.2011.2166258
range detection requirement. For example, a two-parallel- rectangular-beam-based FBG accelerometer was reported in 1998
[6], which provided a wide bandwidth about 100 Hz and a good
cross-sensitivity of less than 1%. However, its acceleration
). A FBG vibration sensor
sensitivity is much low (31.5
using a double-cantilever beam structure was proposed in 2009
[7]. It showed a narrow frequency resonance range from 0.1
to 15 Hz and a high sensitivity 189 mv/g, with the maximal
cross-axis anti-interference about 33 dB. Designing sensors
with better quality and lower cost encourages further research
in this field.
In this paper, we report the design and performance of a FBG
accelerometer composing of a flat diaphragm and a U-shaped
rigid cantilever beam. The U-shaped rigid cantilever beam is
used to enhance the vibration effect and the flat diaphragm is
to minimize the cross-axis sensitivity. Experimental results
demonstrate that this FBG accelerometer provides a high senand an extended frequency response
sitivity up to
ranging from 10 to 120 Hz, covering the key frequencies to
be measured for civil engineering structural monitoring and
seismic measurements. Meanwhile, the cross-sensitivity is
for an excellent immunity to cross-axis
depressed to
signals.
II. PRINCIPLE OF SENSORS DESIGN
The schematic structure of the U-shaped rigid cantilever
beam based FBG vibration sensor is shown in Fig. 1. Two
L-shaped rigid cantilever beams were made as two L-shaped
levers which were supported by two bars and can be flexibly rotated around the precision bearing. The U-shaped lever is made
up of two L-shaped levers through the hinge. The diaphragm
was clamped by two locking pillars either side. A FBG was
glued between points A and B on left and right levers, respectively. It is worth to note that the FBG was two-point-fixed
(instead of entirely pasting along the whole grating section to
avoid the potential grating chirp) and it is prestrained in order
to make sure a real-time response to external vibration signal.
The brass mass consists of three parts function as a connection
between level and diaphragm. The above and bottom masses
were made for bolt and lock nut, respectively, and the middle
mass was used as counter weight.
According to elastics [8], the deflection of the diaphragm
(center point) caused by the inertial force of mass is given
by
1530-437X/$26.00 © 2011 IEEE
(1)
WENG et al.: A ROBUST AND COMPACT FIBER BRAGG GRATING VIBRATION SENSOR FOR SEISMIC MEASUREMENT
801
Fig. 1. Structural schematic of the vibration sensor.
and the deflection of the diaphragm (center point) caused by the
reaction force resulting from the force of FBG is given by
TABLE I
THE PARAMETER OF THIS ACCELEROMETER
(2)
So, the actual deflection of the center of diaphragm can be expressed as
(3)
According to geometric knowledge, the relationship between
strain and deflection can be expressed as
(4)
So, strain
can be described as
(5)
Therefore, sensitivity coefficient
expression
can be represented by the
(6)
According to the equation of motion of the structure, natural
frequency can be expressed as
Fig. 2. Experimental setup of the accelerometer.
III. EXPERIMENT AND DISCUSSION
(7)
In the above formulas, is the total mass of brass,
and
are the elastic modulus and cross-section area of fiber, and
is the distance between points A and B, respectively. and
is Young’s modulus and Poisson’s ratio of diaphragm, and
is radius and thickness of diaphragm,
and
is the length of
is the
short arm and long arm of L-shaped lever, respectively.
numerical coefficient depending on radius and contact radius
of the diaphragm,
is the bending rigidity of diaphragm,
and
is the spring
stiffness of FBG and diaphragm, respectively.
The parameters of the FBG vibration sensor are listed in
Table I. We can figure out that this sensor provides a natand sensitivity coefficient
ural frequency
in theory.
A series of experiments were carried out to verify the sensing
properties of accelerometer. Fig. 2 shows the experimental
setup of the accelerometer. A standard piezoelectric acceleration sensor (type BK8305) is attached above the accelerometer
for calibration. In the experiment, a small precision shaking
table (type JZ-40 produced by Beijing Spectrum) provides a
serious of sine excitations with tunable frequency as the input
signal and the output of fiber grating sensor is monitored by
wavelength interrogator (type SM-130 produced by American
Micron Optics) with the resolution of 1 pm and sampling frequency of 1000 Hz. We ignored the influence of temperature to
FBG because it is really a slow change fluctuation ( minutes)
comparing to that of vibration change ( microseconds). To
minimize the potential thermal-induced geometrical structure
nonuniformity between the two beams either side, the whole
802
Fig. 3. Frequency response characteristic of the accelerometer (Y axis is the
ratio between the measured amplitude response and the imposed acceleration).
IEEE SENSORS JOURNAL, VOL. 12, NO. 4, APRIL
Fig. 4. Linear response of peak-to-peak wavelength shift versus acceleration.
metal beam (
left and
right) is fabricated by one
moulding (not a configuration combined with several separated
sections) and the supporting point is accurately positioned in
the beam center to avoid any unwanted nonaxial alignment.
Experimental results showed that the output error caused by
the temperature fluctuations (between 20 C and 50 C) is less
than 5%, which identified the proposed sensing configuration
is stable enough for the most in-field applications. However,
for the high temperature
C under-well vibration
measurement or seismic monitor, further investigation and tests
should be carried out to improve its feasibility.
A. Frequency Response
Frequency response range of the accelerometer is one of
the key characters as it dominates the possible applications the
sensor to be employed. In this experiment, the input signal frequency was increased from 10 to 400 Hz with the same exciting
amplitude at 0.5 g, which is calibrated by a commercial piezoelectric acceleration sensor. We used the dynamic demodulator
to obtain the accelerometer output of each frequency (with a
frequency step of 10 Hz) and detect the maximum wavelength
shift correspondingly. The amplitude-frequency characteristic
of the accelerometer is shown in Fig. 3. The figure identifies a
flat response range (effective frequency measurement range) is
from 10 to 120 Hz (with a resonant frequency about 170 Hz),
covering the key frequencies to be measured in seismic wave
exploration. The experimental value of resonant frequency
is lower than theoretical prediction is mainly because of the
nonvertical assembly between U-shaped lever and mass.
B. Sensitivity Coefficient
In this experiment, the acceleration amplitude of sine excitation varied from 1 to 20
with a constant frequency at 30
and 80 Hz, respectively. The peak-to-peak wavelength shift was
recorded by the dynamic demodulator. Fig. 4 shows a linear response between peak-to-peak wavelength shift and acceleration
amplitude for the vibration frequency of 30 and 80 Hz, with the
sensitivity coefficient of 98.06 pm/g (30 Hz) and 102.24 pm/g
(80 Hz), respectively. Although the expected sensitivity curve
under the beam’s natural resonance frequency (170 Hz) should
be flat. However, the measured sensitivity coefficients are slight
Fig. 5. Input waveform of PE accelerometer.
different (e.g., the 30 and 80 Hz, as shown in the Fig. 4) with
a slight increasing trend (but less than 5% totally). Meanwhile,
the detected sensitivity coefficient is higher than that of theoretical calculation in the above section, this is due to we simplify
the L-shaped beam configuration in theoretical model and the
elastic modulus referenced may not be exactly correct.
C. Contrast to PE Accelerometer
During the experiment, the standard piezoelectric (PE) accelerometer was used to record the input signal of actuator and
make a contrast to FBG sensor output. Figs. 5 and 6 show the
input waveform of standard PE accelerometer and the output
waveform of FBG accelerometer, respectively, under a sinusoidal driving signal at 50 Hz and acceleration of 1 g. Figs. 7
and 8 are frequency spectra of standard PE accelerometer and
FBG accelerometer which are Fast Fourier Transform (FFT)
of Figs. 5 and 6, respectively. The discrepancy of frequency
response between them is less than 1%. Moreover, the output
waveform is almost a sinusoidal curve, demonstrating that FBG
is without chirp.
D. Cross-Axis Sensitivity
To ensure one-dimension vibration measurement, a low
cross-axis sensitivity is essentially important. Here, in our
experiment, the sine vibration input with 0.5 g acceleration
is induced to the FBG sensor in main-axial direction and
WENG et al.: A ROBUST AND COMPACT FIBER BRAGG GRATING VIBRATION SENSOR FOR SEISMIC MEASUREMENT
803
Fig. 6. Output waveform of FBG accelerometer.
Fig. 9. Cross-axis anti-interference characteristic.
3.33% of the main-axis). It identifies that the proposed sensing
configuration performs good vibration direction discrimination
ability.
IV. CONCLUSION
Fig. 7. Frequency spectrum of PE accelerometer.
The feasibility of U-shaped rigid cantilever beam based
FBG vibration sensor for seismic measurement has been
demonstrated. The U-shaped rigid cantilever beam is used
to enhance the vibration effect and the flat diaphragm is to
minimize cross-axis sensitivity. Experimental tests, calibrated
with commercial PE accelerometer, show that the proposed
sensing device provides the excellent performance, including a
wide frequency response range (0–120 Hz), a high sensitivity
, together with the good cross-axis
coefficient
anti-interference degree
and robust metal package,
making it a good candidate for in-field seismic wave monitoring system in oil and gas exploration.
REFERENCES
Fig. 8. Frequency spectrum of FBG accelerometer.
cross-axial direction, respectively, over the frequency range
from 20 to 160 Hz. The experimental results are shown in
Fig. 9. The output wavelength shift of main-axis is of 30 pm,
whereas it is only about 1 pm in that of cross-axis (equal to
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IEEE SENSORS JOURNAL, VOL. 12, NO. 4, APRIL
Yinyan Weng received the B.Sc. degree in physics
from Jiangsu University, Zhenjiang, China, in 2009.
Currently, she is working towards the M.Sc. degree
in optics at Northwest University, Xi’an, China.
Her current research interests are centered on the
field of geophone, fiber-optic sensors.
Xueguang Qiao received the B.Sc. degree in physics
from Xi’an Jiaotong University, Xi’an, China, in
1982, and the Ph.D. degree in optics from Xi’an
Institute of Optics and Precision Mechanics of CAS,
Xi’an, China, in 1998.
From 1999 to 2000, he was at Massachusetts Institute of Technology, Cambridge, as a Visiting Scholar.
Currently, he is a Professor of Physics at Northwest
University, Xi’an, China. He is coauthor of over 160
publications in journals and conference proceedings
and is co-inventor on seven invention patents. His research work mainly focus on photonics technology, fiber communication and
sensing, fiber logging in oil and gas fields, geophysical prospecting, and oil and
gas pipeline inspection.
Tuan Guo (M’09) received the B.Sc. and M.Sc. degrees in electronics engineering from Xi’an Shiyou
University, Xi’an, China, in 2001 and 2004, respectively, and the Ph.D. degree in optics from Nankai
University, Tianjin, China, in 2007.
From 2007 to 2008, he was with the Department
of Electronics, Carleton University, Ottawa, ON,
Canada, as a Postdoctoral Fellow working on tilted
fiber grating sensors and surface plasmon fiber
sensors. Since August 2008, he joined the Photonics
Research Centre at The Hong Kong Polytechnic
University, China, as a Postdoctoral Research Fellow working on speciality
fiber sensors and fiber laser sensors. Currently, he is an Associate Professor at
the Institute of Photonics Technology, Jinan University, Guangzhou, China. His
research work mainly focuses on photonic components and devices, fiber-optic
sensors, fiber lasers, and bio-photonics. He is coauthor of over 80 publications
in journals and conference proceedings and is co-inventor on three patents.
Dr. Guo is a member of the Optical Society of America (OSA). He is the
Associate Editor of the IEEE SENSORS JOURNAL.
Manli Hu received the B.Sc. and M.Sc. degrees in
optics from Northwest University, Xi’an, China, in
1983 and 1993, respectively, and the Ph.D. degree in
optics from Xi’an Institute of Optics Precision Mechanics of CAS, Xi’an, China, in 1999.
From 2003 to 2004, he was at Kobe University,
Japan, as a Visiting Scholar. Currently, he is a
Professor of Physics at Northwest University, Xi’an,
China. He is coauthor of over 90 publications in
journals and conference proceedings. His research
work mainly focuses on information photonics, fiber
sensor, and photoelectric inspection technology.
Zhongyao Feng received the B.Sc. degree and Ph.D.
degree in optics from Northwest University, Xi’an,
China, in 1997 and 2008, respectively.
Currently, he is an Associate Professor of Physics
at Northwest University. He is coauthor of over 20
publications in journals and conference proceedings.
His research work mainly focuses on information
photonics and optic fiber sensing technology.
Ruohui Wang received the B.S. degree in optics
from Northwest University, Xi’an, China, in 2008.
Currently, he is working towards the Ph.D. degree in
optics at Northwest University.
His research interests are mainly in optical fiber
sensing technology.
Jing Zhang received the B.S. degree in physics
from Northwest University, Xi’an, China, in 2009.
Currently, he is working towards the M.Sc. degree
in optics at Northwest University.
His research interests focus on fiber sensors and
fiber lasers.