Transmissive grating-reflective mirror- based fiber optic accelerometer for stable signal acquisition in industrial applications

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Transmissive grating-reflective mirror- based fiber optic accelerometer for stable signal acquisition in industrial

applications

Yeon-Gwan Lee

Dae-Hyun Kim

Chun-Gon Kim

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Transmissive grating-reflective mirror-based fiber optic accelerometer for stable signal acquisition in industrial applications

Yeon-Gwan Lee KAIST

Department of Aerospace Engineering 291 Daehak-ro, Yuseong-gu

Daejeon 305-701, Republic of Korea

Dae-Hyun Kim

Seoul National University of Science and Technology

Department of Mechanical and Automotive Engineering

172 Gongneung 2-dong, Nowon-gu Seoul 139-743, Republic of Korea

Chun-Gon Kim KAIST

Department of Aerospace Engineering 291 Daehak-ro, Yuseong-gu

Daejeon 305-701, Republic of Korea E-mail:cgkim@kaist.ac.kr

Abstract. This paper discusses an applicable fiber-optic accelerometer composed of a transmissive grating panel, a reflection mirror, and two optical fibers with a separation of quarter grating pitch as transceivers that monitor the low-frequency accelerations of civil engineering structures. This sensor structure brings together the advantages of both a simple sensor structure, which leads to simplified cable design by 50%

in comparison with the conventional transmission-type fiber optic accelerometer, and a stable reflected signals acquisition with repeatability in comparison to the researched grating-reflection type fiber optic accelerometer.

The vibrating displacement and sinusoidal acceleration measured from the proposed fiber optic sensor demonstrated good agreement with those of a commercial laser displacement sensor and a MEMS accelerometer without electromagnetic interference. The developed fiber optic accelerometer can be used in frequency ranges below 4.0 Hz with a margin of error that is less than 5% and a high sensitivity of5.06 rad∕ðm∕sÞ².

Subject terms: reflective mirror; transmissive grating panel; vibrating displacement;

low frequency acceleration; fiber optic accelerometer.

Paper 111608 received Dec. 20, 2011; revised manuscript received Mar. 4, 2012;

accepted for publication Mar. 26, 2012; published online May 4, 2012.

1 Introduction

Fiber optic sensors are promising candidates for application in large-size structures due to their easy cabling, light weight, and long service life.¹Furthermore, an accelerometer using an optical fiber (OF) allows ambient vibration monitoring within high electromagnetic environments. Over the past three decades, many types of fiber optic accelerometers (FOAs) have been developed.^2,3 Fiber Bragg grating (FBG) sensors have been the most widely studied due to their multiplexing functionality.^3,4 However, while most intensity-based sensor systems are relatively low cost, FBG sensors require rather higher cost wavelength shift interrogation systems with high-frequency sampling rate.

Alternatively, for wider application where conventional sensors are commonly used and with the development of more cost-efficient fiber optic sensors (FOSs), intensity modulation techniques have been conducted on hydrophone⁵and accelerometers.⁶^–¹⁰ These schemes use transmitted light between separated OFs through two fringe panels^5–8 in order to measure the low-frequency acceleration of the infra- structure’s structural health monitoring (SHM).^7,8 Most reported transmission mechanisms obtained relatively stable signals, which allow signal processing to obtain the parameter to be measured with a small amount of error under unstable real industrial environments because the emitters and receivers are fixed to the sensor case. However, in the case of the mechanism in which the optical emitter or optical receiver was directly used as a transmitted signal modulator as well as a proof mass,^9,10it does not guarantee stable signal

acquisition in the generally unstable field application.

Furthermore, the fabricated prototypes of such sensors have demonstrated disadvantages resulting from the complicated structure of the four OF lines: each side of the prototype has two OF lines with those on one side being light-emitting OF lines and those on the other side being light receiving OF lines. Thus most transmission type FOAs also have complex structures due to the layout of the OF lines on both sides.^7–10 This leads to limitations in installation of the FOAs due to the geometric conditions. Furthermore, the problem of complex cabling is intensified when the FOAs are used to monitor large size structures through multipoint sensing in hazardous locations.

Therefore, in an effort to simplify the sensor structure, reflection-based FOSs^11–13and FOAs^14–16 have been developed. These sensors are hypersensitive to the target distance and the parallel state between the end face of the OF and the reflective surface during operation^17,18due to the small acceptance angle of the OF.¹⁹ In real industrial fields, light reflected on the movable reflection material undergoes unexpected modulations and misalignments between the optical receiver and reflective materials due to the effect of unwanted parameters such as microscopic non-axial vibrations, tilting, and rotation of the reflection target.

These unwanted effects are induced by non-axial exciting forces and varied gravity forces applied to a proof mass.

The distortions of these received and reflected signals make signal processing to obtain the parameter to be measured difficult because these signal distortions lead to large errors. If these errors are severe, then the signal processing becomes almost impossible.¹⁷Consequently, in the conventional reflection-based mechanisms, the proof mass with

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reflection material is too sensitive to unwanted variations in physical parameters (gravitational accelerations induced by tilt of the sensor) and unexpected non-axial excitations, limiting the scope of real industrial field applications. The difficulties surrounding stable signal acquisition, which only varies with changes of the parameter to be measured, in the reflection mechanism FOSs and complex cabling in the transmission mechanism FOSs should be resolved.

In this study, a transmissive grating-reflective mirror- based applicable FOA is presented in order to measure low-frequency accelerations of large civil engineering structures. The working principles of the sensor are described, and signals to be acquired were verified. The verification tests were carried out for the relative vibrating displacement and acceleration measurements. In this manner, the technique proposed in this paper is verified as a sensor that can measure the vibrating displacement and acceleration with both advantages of transmission and reflection-type FOAs.

2 Working Principles

The proposed low-frequency FOA structure, in which a single degree of freedom structure is embedded, is designed as shown in Fig.1. The sensor head consists of a single transmissive grating panel (TGP), a reflective mirror, and two transceiver OFs. The distance a as shown in Fig. 1 is the distance between the end of the OF and the TGP, and b is the distance between the TGP and the aluminum reflective mirror. The TGP consists of multiple opaque gratings with evenly spaced transparent spacings: the opaque grating width (l₁) and transparent spacing width (l₂) are smaller than the optical beam width. The grating period of l is the sum of l₁ and l₂. In order to produce stable reflectivity, the OFs and reflective mirror, placed on the back of the TGP and perfectly aligned to the end surface of the OFs, are fixed to the sensor case. Therefore, severe distortions can be prevented

because no misalignment and the variation of target distance between the OFs and reflective mirror occur. Consequently, this sensor structure prevents the acquired signals of the proposed FOA from distortions, which cause signal processing to become difficult or impossible, even though the TGP was slightly affected by unwanted parameter variation.

The sensor function is as follows: The emitted light from the two OFs arrives on the TGP, and while the opaque grating blocks the optical incident light, the light that is partially transmitted via the transparent spacing arrives on the mirror.

Then the reflected light on the mirror is transmitted again into the OFs that were used as the light emitter via the TGP. When the TGP shifts by one grating pitch l in a perpendicular direction to the optical beam emitted, the reflected signal, of which the light intensity varies sinusoidally, will also shift by l. Furthermore, the directional information of the displacement can be obtained by using two transceiver OF lines that are separated by a distance ofðn þ 1∕4Þ · l, where n is a positive integer, to have a phase difference of 90 deg between the two received signals.

The variable u₁ refers to the displacement of the seismic mass, and u₀refers to the displacement of the sensor case. If the periodic sine wave signals in quadrature are obtained, the relative displacement u, which is defined as the difference between u₁ and u₀, can be obtained through the dynamic motion equation with a mass (m), spring stiffness (k), and damper (c), as shown in Eq. (1):

m¨uþ c ˙u þ ku ¼ −m¨u0; (1)

where ¨x₀ is the acceleration imparted on the seismic mass.

Assuming that the excited acceleration is defined as

¨u₀¼ Aexc· eⁱ^ωt, whereω is the excited frequency, then the steady-state response should be the relative displacement of the mass u¼ Bres· eⁱ^ωt. A_exc refers to the acceleration measured via the reference accelerometer, and B_res refers to the measured displacement of the seismic mass in the FOA. Therefore, the acceleration can be obtained theoreti- cally from the deformation response factor R, which is expressed using the relationship between the relative displacement and excited acceleration, as represented by the following equation:

R¼

B_res A_exc

· ωⁿ²¼ 1

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1 − r²Þ²þ ð2ζrÞ²

p ; (2)

where r¼ ω∕ωn, the damping ratio isζ, and ωnis the natural frequency of the integrated mass-spring system. Using this mechanism, the quantification of the parameters such as the relative displacement and acceleration were accomplished.

3 Measurement of the Relative Vibrating Displacement

The FOA proposed in this paper functions through tracking the relative displacement of the seismic mass.

The quantification of the displacement, calculated using the proposed technique, is compared stage by stage with that of a commercial laser displacement sensor based on a sinusoidal vibration.

Fig. 1 Schematic diagram of the transmissive grating-reflective mirror-based FOA.

Lee, Kim, and Kim: Transmissive grating-reflective mirror-based fiber optic accelerometer: : :

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3.1 Verification of the Reflected Signal

In a prototype, separations among the TGP, reflective mirror, and OFs with any value of a and b are necessary for independent motion of the seismic mass. The light received by the OF undergoes diffraction twice by the many gratings on the TGP during two passages via the TGP.²⁰Diffracted signal mixing with high-order diffraction (subsidiary maxima) should be avoided because the proposed scheme is only modulated by the reflected light of 0’th order diffraction (principal maxima). In order to demonstrate the sinusoidal signal acquisition experimentally without the diffracted signals of subsidiary maxima, the appropriate values of a and b, which are described in Fig.1, had to be determined in order to fabricate the prototype. The TGP (thickness: 700 μm), which has an l of 240 μm, l1 (Cr material) of 120 μm, and l₂ of 120μm, was attached to the motor-controlled linear-motion jig, as shown in Fig. 2. An Erbium doped fiber amplifier (EDFA; 1530∼ 1565 nm) was used as the light source, and a single mode OF combined with the collimator (LPC-05-1535/1565-9/125-S, OZ Optics, Co., Canada), was used as the transceiver. As the TGP moved to the OF fixing jig, the received light was stored in the data acquisition (DAQ) system at sampling rates of 1 kHz through the one circulator and photo detector (ν2011, New Focus, Inc., USA). The reflected signals in the three cases were identified via one OF line as shown in the bottom of the Fig.2. The

reflective mirror (PF05-03-G01, Thorlabs, Inc., USA) was not employed in the first case, while the reflective mirror was used in the other cases. The black line with representative square symbols described as “Align to reflective grating (l¼ 240 μm, a ¼ 1.0 mm)” in Fig.2indicates the result at l¼ 240 μm and a ¼ 1.0 mm without the reflective mirror when the end of the OF was perfectly perpendicular to the reflective grating panel that consists of the reflective grating instead of the opaque grating in the TGP. This result was compared with those of“Align to reflective mirror (l ¼ 240 μm, a¼ 1.0 mm, b ¼ 3.5 mm)” and “Align to reflective mirror (l¼ 240 μm, a ¼ 5.0 mm, b ¼ 5.0 mm),” which were measured when the end of the OF was perfectly perpendicular to the reflective mirror.

In order to verify the sinusoidal signal, a sine curve fitting analysis was implemented. The R-square values and half periods of the three acquired signals in each case are described in Table1. Within a and b, which were less than 5.0 mm, and the specifications of experimental setup (the OF and TGP with l¼ 240 μm) used in the study, the proposed structure demonstrated sinusoidal signal acquisitions with a half period of 119.7 μm and high R-square value of 0.99 without receiving the diffracted light. This analysis demonstrated that the acquired signals of “Align to reflective mirror (l¼ 240 μm, a ¼ 1.0 mm, b ¼ 3.5 mm)” and “Align to reflective mirror (l¼ 240 μm, a ¼ 5.0 mm, b ¼ 5.0 mm)”

based on the sensing structure proposed in this paper are

Fig. 2 Experimental setup and result of the verification of the reflected signal.

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in good agreement with the reflected signal of “Align to reflective grating (l¼ 240 μm, a ¼ 1.0 mm).” Although it is difficult to directly compare between reflected signals of the reflective grating and reflective mirror, the magnitudes of the sine wave signals via reflective mirror were increased.

Furthermore, increases of a and b only result in the decrease of the magnitudes while maintaining the reflected sinusoidal form. Therefore, the signal of the proposed sensor principles can be used for the displacement measurement technique.

Consequently, a and b of 5.0 and 5.0 mm, respectively, can be admitted for sinusoidal signal acquisition when this scheme is realized by the prototype.

3.2 Experimental Setup

Figure3shows a schematic diagram of the sinusoidal shaking test setup for the measurement of the relative moving displacement of the measurand. The TGP (l of 240 μm) was attached to the measurand. The EDFA was used as the light source, and two single mode OFs were used as the transceiver. In order to minimize the reception of high-order diffracted reflection light from a neighboring optical fiber’s emitted light, the center of the other optical fiber end surface was separated by a distance of 3.66 mm from the neighboring optical fiber. As the distance between the OF lines is defined by ½ðn þ 1∕4Þ · l, n is equal to 15.

The measurand was placed on the shaker (VTS100, VTS, Inc., USA), whose motion was controlled using a fixed sine function generator. In a prototype, the separation distances of a and b are necessary for the independent motion of the seismic mass. Furthermore, in order to prevent the TGP from damage induced by contact and friction between the OF and TGP, a was maintained at a minimum separation distance of 2 mm; b was maintained at 5 mm during

operation in order to guarantee the independent motion of the measurand. Consequently, as the TGP on the measurand vibrates with respect to the OF fixing jig, the received lights were stored in the DAQ system at sampling rates of 2 kHz through the two circulators and photo detectors. A commercial laser displacement sensor (LDS; LK031, Keyence, Co., Japan), which features a high resolution of 1μm and a sensitivity of 1μm∕mV, was used as the reference sensor.

As the measurand was vibrated by the shaker, the reflected signals at the two observation points were periodic sine wave functions of the displacement with a phase difference of 90 deg with respect to each other. The raw reflected signals at these two points can be expressed as a DC com- ponent. After normalization to a sinusoidal function, which has an amplitude of one and a mean magnitude of zero, the two normalized reflected signals were expressed as follows using the grating period of l:

¯S1 ¼ sin

2π · uðtÞ l

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and

¯S2 ¼ cos

2π · uðtÞ l

: (4)

The relative displacement, uðtÞ, can be inferred by unwrap- ping the two signals as follows:

uðtÞ ¼ 1

2π· unwrap

tan⁻¹

¯S1

¯S₂

: (5)

3.3 Vibrating Displacement

In order to evaluate the performance of the proposed fiber optic sensing principle, the measurand was vibrated with 5.0-Hz sine excitation frequencies. The vibration movement of the measurand was measured using the proposed scheme and then compared with the LDS measurements. From the normalized reflected signals that feature a range greater than the half periods with maximum and minimum values of the DC reflected sinusoidal signals, the signal processing procedure can be understood easily, as shown in Fig.4(a).

This figure describes the normalized reflected optical signals of optical fiber 1 (OF1) and optical fiber 2 (OF2) at the 5.0-Hz excitation. Figure4(b)describes the comparison of the displacements measured by the FOS and LDS. Although the measurand was not vibrated at the specified 5.0-Hz sinusoidal frequency, the moving displacement measured using the proposed scheme agrees very well with that measured by

Table 1 R-square values and half period of the acquisition signals based on the sine curve fitting analysis.

R-square value Half period (μm)

Align to reflective grating (l ¼ 240 μm, a ¼ 1.0 mm) 0.9979 119.718

Align to reflective mirror (l ¼ 240 μm, a ¼ 1.0 mm, b ¼ 3.5 mm) 0.9964 119.722

Align to reflective mirror (l ¼ 240 μm, a ¼ 5.0 mm, b ¼ 5.0 mm) 0.9932 119.723

Fig. 3 Configuration of the experimental setup for measurement of the vibrating displacement.

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LDS. The noise level of the Erbium doped light source is 0.003 mV. The peak-to-peak resolution of the sensor depends on the noise level of the light source. Using the lowest amplifier level of the photo detector, the presented sensor system can provide a minimum measurable displacement of 0.3μm.

4 Fiber Optic Accelerometer 4.1 Experimental Setup

The foundational quantification principle of acceleration states that the change in the relative displacement of a seismic mass is proportional to its change in acceleration.

Furthermore, the measured relative displacement of the seismic mass can be converted into its acceleration using Eq. (6).

The natural frequency and damping ratio of the FOA should be identified in order to convert the relative displacement into acceleration as follows:

ü₀¼ uðtÞ · ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1 − r²Þ²þ ð2ζrÞ² q

·ωn2: (6)

In order to evaluate the scheme proposed in this paper, a cylindrical prototype accelerometer was fabricated. The prototype had a size of 26× 36 mm²(outer diameter× height), and a total weight of 50 g. For the resilience of this prototype, a leaf plate was employed as a spring. The leaf springs are composed of phosphor bronze (PB) with a thickness of 60 μm for high sensitivity of seismic motion. These are placed on both the top and bottom with horizontal sym- metry in order to minimize the eccentricity error effect and unbalancing weight error during vibrating operation, as shown in Fig. 1. The experimentally measured spring stiffness was 0.017 N∕mm. Furthermore, in the prototype, in order to avoid plastic deformation of the leaf springs, the movable range of the seismic mass was limited to 2.9 mm, considering the yield strength of the leaf spring. The FOA prototype and the microelectromechanical system (MEMS) reference sensor (3711B112G, PCB Piezotronics, Inc., USA), which has a high sensitivity of

101.9 mV∕ðm∕s²Þ at low-frequency accelerations, were fixed to the shaker as shown in Fig.5.

4.2 Performance of the Fiber Optic Accelerometer The natural frequency of 14.49 Hz (at 24.6°C) and the damping ratio of 0.00064 were experimentally obtained from the free vibration test of the FOA. In Fig. 6, the measured low-frequency accelerations were plotted before lowpass filtering was applied. The measured root mean square (RMS) values of the vibrated acceleration were approximately 0.85 m∕s² and 1.06 m∕s² at 1.0- and 4.0-Hz excitation, respectively. For the reference accelerometer (Ref_Acc), while the electrical noise was clearly included, the vibrated accelerations that were measured using the proposed accelerometer of the FOS (FOS_Acc) verified its immunity to electrical interference. At a 1.0-Hz excitation, although the shaker plate was not vibrated at the fixed frequency, the acceleration measured by the proposed FOS_Acc agrees very well with that of the Ref_Acc.

The frequency response curve of the FOA was experimentally obtained using the shaker. The shaker was excited by

Fig. 4 Measurement of the vibration displacement at a 5.0-Hz excitation: (a) The two normalized received optical signals in the quadrature;

(b) comparison of the displacements measured using the FOS and LDS.

Fig. 5 Experimental setup for the performance test of the FOA.

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the sine signals at selected frequencies covering a large range and amplitude of the steady state acceleration. The frequencies of the excitation signals were selected as 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 8.0, 10.0, 11.0, 12.0, 12.5, 13.0, 13.3, 13.5, 13.8, 14.0, 14.2, 14.5, 15.0, 18.0, and 20.0 Hz. The frequency response curve in the form of a deformation response factor (R) was plotted as a function of the excited frequencies using three repetitions, as shown in Fig.7. The experimental points, which were calculated using displacements and accelerations measured respectively by the FOA and MEMS accelerometer, were compared with the theoretical result, which was plotted using Eq. (2). Using a chi-square curve fitting, the exact theoretical dynamic characteristics determined that the natural frequency was 14.34 Hz and the damping ratio was 0.00682 with a reduced chi-square value of 0.068. These results agree very well with the experimental results using the logarithmic decrement theory. Furthermore, the deformation response factors of the proposed FOA

demonstrated good agreement with the theoretical curve with an R-square value of 0.9995.

The small graph in the inset of Fig. 7 shows both accelerations versus time at the same fixed 4.0-Hz excitation measured using the FOS_Acc and Ref_Acc after a lowpass filtering of 40.0 Hz. The measured relative displacement of the seismic mass was 415μm (peak-to-peak magnitude). The acquired voltage from the normalization signal processing was 10.87 rad because the acquired period of the sinusoidal signal was 1.73 periods. The average sensitivity was 5.06 rad∕ðm∕s²Þ based on the RMS acceleration value criterion because the magnitude of the measured RMS acceleration value was 2.15 m∕s² (peak-to-peak magnitude) after lowpass filtering. This indicates that the proposed FOA has a higher sensitivity of 49.60 rad∕½G (gravitational acceleration G). The minimum and maximum measurable acceleration of the fabricated prototype were 0.0016 m∕s² and 15.0241 m∕s² based on the RMS acceleration value criterion because of a minimum measurable displacement of 0.3μm and limited movable range (2.9 mm) of the proof mass. The deformation response factor of the developed FOA was almost one in the frequency ranges below 4.0 Hz.

This implies that the designed and fabricated FOA can be used in the frequency ranges below 4.0 Hz within a 5% error margin in order to measure the low-frequency accelerations of civil engineering structures. The sensor data observed during the shaking tests were replicable because the average standard deviation was 0.013 up to 4.0 Hz, which was obtained from more than four repetitions.

In order to verify the sensor performance regarding the temperature effect, the natural frequency variations of the sensor were investigated. When the sensor is used at the equilibrium state of the elevated temperature, the measurable frequency range is affected by a temperature variation. In this equilibrium state, the experimentally obtained natural frequencies of the sensor are plotted according to the corre- sponding equilibrium temperatures. Therefore, the measurable frequency range of the FOA was studied according to the temperature variations, as shown in Fig.8.

The measurable frequency range of the FOA depends on the natural frequency of the embedded single degree of

Fig. 6 Measured low-frequency accelerations before lowpass filtering: (a) comparison of the accelerations measured using the proposed accelerometer of the FOS and a commercial reference accelerometer at a 1.0-Hz excitation; (b) comparison of the accelerations measured using the proposed FOS and a commercial reference accelerometer at a 4.0-Hz excitation.

Fig. 7 Comparison between the measured and theoretical deformation response factor (R).

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freedom system. The initial natural frequency was 14.52 Hz with a standard deviation of 0.02 at 17.6°C (room temp.).

Temperatures of the sensor were varied from 17.6°C to 131.8°C. Below the temperature of 131.8°C, the R-square value of the linear regression analysis between the temperature and natural frequency was 0.969. This indicates that the natural frequency of the sensor linearly decreased with an increase in the sensor temperature. The decrease ratio of the natural frequency is approximately 0.003 Hz∕°C. Most accelerometers can be used below one-third of the natural frequency. 2.6% of the initial natural frequency was decreased until an elevated equilibrium temperature of 131.8°C was reached. However, in the prototype, the bond- ing material (KFR-730F with KHF-730F, Kukdo Chemical, Co., Korea), which was used to attach the TGP to the seismic mass and the optical fiber to the sensor case, can maintain dimensional stability up to 65°C. Consequently, the fabricated sensor can measure accelerations at frequencies below 4.0 Hz and at temperatures up to 65°C due to a small decrease (1.1% reduction) in natural frequency.

5 Conclusion

The developed fiber-optic accelerometer measured low- frequency accelerations below 4.0 Hz within a 5% error margin under a surrounding temperature of 65°C. Further- more, the proposed fiber optic accelerometer has a very high sensitivity of 5.06 rad∕ðm∕s²Þ, compared with most commercial low-frequency accelerometers, including microelectromechanical system and piezoelectric sensors.

This technique has the advantages of simpler and more effective design than conventional transmission-type fiber optic accelerometers; furthermore, a more reliable and stable reflected signal than conventional reflection-type fiber optic accelerometers can be achieved through improvement of working principles. Therefore, an applicable fiber optic accelerometer is developed for application in real industrial fields, and the proposed fiber optic accelerometer combines the advantages of both transmission and reflection type fiber optic accelerometers. If a leaf spring with high yield strength is used, the maximum measurable accelerations can be extended, and if a light source with stable and minimized noise is used, the minimum measurable accelerations can be improved.

Acknowledgments

This research was supported by the Leading Foreign Research Institute Recruitment Program (Grant No. 2011- 0030065) and Nuclear Research and Development Program (Grant No. 2011-0019198) through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology. Co-author, Dr. Dae-Hyun Kim was partially supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (Grant No. 2010-0004312).

References

1. B. H. Lee,“Review of the present status of optical fiber sensors,”Opt.

Fiber Tech.9(2), 57–79 (2003).

2. T. T.-Y. Lam et al.,“Optical fiber three-axis accelerometer based on lasers locked toπ phase-shifted Bragg gratings,”Meas. Sci. Technol.

21(9), 094010 (2010).

3. G. Gagliardi et al., “Design and test of a laser-based optical-fiber Bragg-grating accelerometer for seismic applications,” Meas. Sci.

Technol.19(8), 085306 (2008).

4. P. Antunes, H. Varum, and P. Andre,“Uniaxial fiber Bragg grating accelerometer system with temperature and cross axis insensitivity,”

J. Inter. Meas. Conf.44(1), 55–59 (2011).

5. W. B. Spillman, Jr.,“Multimode fiber-optic hydrophone based on a schlieren technique,”Appl. Opt.20(3), 465–470 (1981).

6. E. Udd and P. A. Turek,“Single-mode fiber-optic vibration sensor,” in Fiber Optic and Laser Sensors, Vol. III, E. L. Moore and O. G. Ramer, Proc. Soc. Photo-Opt. Instrum. Eng., 566, 135–140 (1985).

7. D. H. Kim,“A fiber-optic tiltmeter system based on the moiré-fringe effect,”Meas. Sci. Technol.20(2), 025203 (2009).

8. M. Q. Feng and D. H. Kim,“Novel fiber optic accelerometer system using geometric moiré fringe,” Sens. Actuators A, 128(1), 37–42 (2006).

9. M. Morante et al.,“New approach using a bare fiber optic cantilever beam as a low-frequency acceleration measuring element,”Opt. Eng.

35(6), 1700–1706 (1996).

10. J. M. López-Higuera, M. A. Morante, and A. Cobo, “Simple low-frequency optical fiber accelerometer with large rotating machine monitoring applications,”J. Light. Tech.15(7), 1120–1130 (1997).

11. S. W. Harun, M. Yasin, and H. Ahmad,“Micro-displacement sensor with multimode fused coupler and concave mirror,” Laser Phys.

21(4), 729–732 (2011).

12. S. Zhao et al., “Optical tilt sensor with direct intensity-modulated scheme,”Opt. Eng.50(11), 114405 (2011).

13. H. Z. Yang et al., “Environment-independent liquid level sensing based on fiber-optic displacement sensors,”Microw. Opt. Tech. Lett.

53(11), 2451–2453 (2011).

14. A. S. Gerges et al.,“High-sensitivity fiber-optic accelerometer,”Opt.

Lett.14(4), 251–253 (1989).

15. Y. G. Lee, D. H. Kim, and C. G. Kim,“Performance of single reflective grating based fiber optic accelerometer,”Meas. Sci. Technol. 23(4), 045101 (2012).

16. Q. Lin et al.,“A high-resolution fiber optic accelerometer based on intracavity phase-generated carrier (PGC) modulation,” Meas. Sci.

Technol.22(1), 015303 (2011).

17. Y. G. Lee et al.,“Test of single reflective grating based fiber optic sensor design for measurement of tilt angle,” Proc. ICCM18 (2011).

18. W. Shen et al.,“Long distance fiber-optic displacement sensor based on fiber collimator,”Rev. Sci. Instrum.81, 123104 (2010).

19. Y. G. Lee et al.,“Measurement method and system of optical fiber- based beam width using a reflective grating panel,” Int. J. Aero.

Space Sci.12(2), 175–178 (2011).

20. E. Hecht, Optics, 4th ed, Addison Wesley, New York, pp. 460–464 (2002).

Yeon-Gwan Lee received his BS degree from Chonnam National University in 2007 and his MS degree from the School of Mechanical, Aerospace and System Engineering in Korea Advanced Institute of Science and Technology. He has been a PhD candidate in KAIST since 2009. His current research areas include structural analysis, structural health monitoring, and optical fiber sensors for multiparameter measurements.

Fig. 8 First natural frequency variations of the FOA according to the surrounding temperatures.

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Dae-Hyun Kim has been working in the Department of Mechanical and Automotive Engineering at Seoul National University of Science and Technology as an assistant pro- fessor since September 2006. He is currently developing several optical fiber sensor systems for monitoring of civil infra-structures and mechanical systems. His current research interests are optical sensor systems, NDE/NDT, structural health monitoring (SHM), composite materials, and structural analysis, etc. He worked at University of California Irvine as a post-doctoral researcher from October 2004 to August 2006. He received BS and MS degrees from the Department of Aerospace Engineering, Korea Advanced Institute of Science and Technology (KAIST), in 1997 and 1999, respectively; PhD degree from department of mechanical engineering, KAIST, in 2004.

Chun-Gon Kim received a PhD degree in the Department of Aerospace Engineering from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 1987. From 1987 to 1991, he was engaged in Composite Materials Laboratory with Korea Institute of Machinery & Materials (KIMM) as a senior researcher. He was a visiting researcher of Wright-Patterson Air force Laboratory in 1988. He has been a faculty member in Department of Aerospace Engi- neering at KAIST since 1991. He spent his sabbatical year in UCLA in 1998. His primary research interest is in the area of smart structure design using optical fiber sensors, material properties in various environments, and multifunctional composite materials and structures. Pro- fessor Kim is a vice-president of the Korean Society for Composite Materials and editor-in-chief of the Advanced Composite Materials.