Distributed strain measurement based on long-gauge FBG and delayed transmission/reflection ratiometric reflectometry for dynamic structural deformation monitoring Michiko Nishiyama,1,* Hirotaka Igawa,2 Tokio Kasai,2 and Naoyuki Watanabe3 1 2
Faculty of Engineering, Soka University, 1-236 Tangi, Hachioji, Tokyo 192-8577, Japan
Institute of Aeronautical Technology, Japan Aerospace Exploration Agency, 6-13-1 Ohsawa, Mitaka, Tokyo 181-0015, Japan 3
Department of Aerospace Engineering, Tokyo Metropolitan University, 6-6 Asahigaoka, Hino, Tokyo 191-0065, Japan *Corresponding author: [email protected] Received 18 November 2014; revised 19 November 2014; accepted 9 January 2015; posted 12 January 2015 (Doc. ID 216974); published 9 February 2015
In this paper, we propose a delayed transmission/reflection ratiometric reflectometry (DTR3 ) scheme using a long-gauge fiber Bragg grating (FBG), which can be used for dynamic structural deformation monitoring of structures of between a few to tens of meters in length, such as airplane wings and helicopter blades. FBG sensors used for multipoint sensing generally employ wavelength division multiplexing techniques utilizing several Bragg central wavelengths; by contrast, the DTR3 interrogator uses a continuous pulse array based on a pseudorandom number code and a long-gauge FBG utilizing a single Bragg wavelength and composed of simple hardware devices. The DTR3 scheme can detect distributed strain at a 50 cm spatial resolution using a long-gauge FBG with a 100 Hz sampling rate. We evaluated the strain sensing characteristics of the long-gauge FBG when attached to a 2.5 m aluminum bar and a 5.5 m helicopter blade model, determining these structure natural frequencies in free vibration tests and their distributed strain characteristics in static tests. © 2015 Optical Society of America OCIS codes: (140.3490) Lasers, distributed-feedback; (060.3735) Fiber Bragg gratings; (060.2370) Fiber optics sensors. http://dx.doi.org/10.1364/AO.54.001191
1. Introduction
It is widely known that structural health monitoring (SHM) methods can be applied for use in aerospace systems. By embedding sensors of various types into a structure, loading, stress, and damage can be detected in order to prevent fatal structural disorders. A particularly important challenge in aerospace vehicle monitoring and control is created by the time-varying mass and inertia of a vehicle in flight and the consequent deformation and changes in the modal frequencies [1] of airplane wing deflection or blade shape in windmills or helicopters. Standard 1559-128X/15/051191-07$15.00/0 © 2015 Optical Society of America
sensing approaches for such structures include the use of noncontact systems such as laser Doppler vibrometers or in situ electric devices such as accelerometers and strain gauges. Although the use of additional on-board instrumentation can provide more accurate estimation of modal properties and enable better real-time control, higher-quality estimation requires the use of increasing numbers of sensors, the additional weight and complexity of which can pose problems. In addition, the extensive wiring needed for sensors increases system weight and susceptibility to electromagnetic interference (EMI). To overcome these problems, excellent multiplexing techniques involving data interrogation at high sampling rates have been sought. 10 February 2015 / Vol. 54, No. 5 / APPLIED OPTICS
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Fiber optic sensors represent an attractive SHM technology owing to their simple wiring, high sensitivity, and resistance to harsh environments such as EMI noise. In particular, fiber Bragg grating (FBG) sensors, which have been proposed and developed for strain measurement applications in several studies [2–6], open up the possibility of measuring strain and estimating the shape and modal properties of aerospace structures in real time. Previous FBG solutions have included using a nine FBG sensor array to estimate the effects on modal shape of mass step changes caused by exhausted propellant in real time [3], the use of four wavelength multiplexing FBG sensors for estimating dynamic structural displacement in aluminum and acryl beam specimens [4], and modal analysis using single and multiple embedded FBG sensors [5,6]. In previous work, the FBG sensors used for dynamic displacement and modal estimation were single or multipoint systems based on WDM, a technique requiring that each sensor has a unique central Bragg wavelength. However, multiple Bragg wavelength FBG systems require more costly fabrication processes than single wavelength systems and must use wavelength tunable lasers or wavelength spectrum measurement instruments to interrogate Bragg wavelength change, which, in turn, requires complex measurement systems. FBG interrogating systems based on ratiometric techniques that utilize optical filtering combined with a superluminescent diode (SLD) continuous broadband light source have also been proposed, although such systems require special optical filters for each Bragg wavelength, which again results in large, complex system configurations. The combination of a delayed transmission/ reflection ratiometric reflectometry (DTR3 ) interrogator that uses time-division multiplexing techniques [7] with continuous pulse arrays based on a pseudorandom number (PN) code and a long-gauge FBG with a single Bragg wavelength has been proposed as a means of simplifying the interrogation of multiplexing and dynamic FBG responses in applications such as dynamic deformation and modal parameter estimation in aerospace vehicles. A DTR3 consists of simple hardware devices, including a broadband SLD light source, a single wavelength slope cutting filter, a detector, and correlation computing hardware. Because time arrival differences from a longgauge FBG can be detected in real time through the correlation computing of a continuous pulse array modulated by a PN code, the long-gauge FBG can have a single Bragg wavelength, which reduces its fabrication cost. Techniques using this combination of equipment can compensate light source fluctuation and bending loss from transmission fiber lines by differentiating between reflected and transmitted spectra through a wavelength slope-cutting filter. In this paper, we present and evaluate the dynamic distributed strain measurement characteristics of a long-gauge FBG and a DTR3 interrogator that can achieve dynamic measurements with a 50 cm spatial 1192
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resolution at a 100 Hz sampling rate. We employed a 2.5 m aluminum bar test piece with a single longgauge FBG to conduct static strain calibration testing and a 5.5 m helicopter blade model with four longgauge FBGs to conduct dynamic strain detection using free vibration testing and distributed strain measurement. 2. DTR 3 with Long-Gauge FBG Interrogation System A. System Configuration
Figure 1 shows the configuration of the proposed DTR3 interrogator with a long-gauge FBG. A DTR3 consists of the broadband SLD light source, the wavelength slope-cutting filter, the detector, and correlation computing hardware. The reflection and transmission (RT) power ratio is detected by means of a continuous pulse modulated by a PN code. It is well suited for DTR3 interrogation [7] through a process of fiber distributed reflection measurement of the DTR3 in which a continuous pulse PN digital modulated signal, rather than an impulse, is utilized as an interrogating light. Because the interrogating light is continuous-wave, it does not require a high power source. Additionally, interrogation speed can be increased because time arrival differences from a long-gauge FBG are computed by correlation of a continuous PN code modulated pulse. The wavelength slope-cutting filter as shown in Fig. 1 has complementary reflectivity and transmissivity with the wavelength. Since the FBG sensor has a steep reflected spectrum, the reflection and transmission (RT) power ratio of the sensor changes with the Bragg wavelength shift. The Bragg wavelength also changes with strain and temperature; therefore, the RT ratio changes with physical properties such as strain and temperature. B. Calibration Test for DTR3 Interrogator
Figure 2 shows an experimental setup used for calibration tests utilizing a 2.5 m aluminum bar with a single long-gauge FBG as a test piece. A 2.5 m longgauge FBG was pasted width-wise to the center of a hollow aluminum prismatic bar with a 30 mm × 15 mm cross section. Measurements were taken in alternate trials using the DTR3 and an optical frequency domain reflectometer (OFDR) [8], which can
Fig. 1. Dynamic disctributed strain measurement system based on long-gauge FBG with DTR3 interrogation.
Fig. 2. Measurement setup using 2.5 m aluminum bar as a test peace and DTR3 and OFDR as reference equipment.
detect Bragg wavelength shifts with a high spatial resolution of 1 mm at a 1 Hz reference sampling rate for static measurement. In order to align the DTR3 coordinates with the location of the applied load and identify its center at a scale resolution of 50 cm, the calibration characteristics of the RT ratio distribution were evaluated by applying the load locally along the long-gauge FBG. The OFDR played the role of a reference instrument that detected the position of the applied load and strain using a Bragg wavelength shift, and the applied load was monitored by the referenced electrical strain gauges caused by maintaining the load at various loading positions. To evaluate the RT ratio characterizing the DTR3 at a given position, the ratio was normalized for a given amount of applied stress along the fiber. It was assumed that the normalized RT ratio has a Gaussian profile for a given spatial position, with the center of the Gaussian profile indicating the central position of DTR3 scale resolution. The normalized RT ratio, ζNorm , is given by ζ Norm R
Δζ ; input ΔλB dx
(1)
where Δζ and ΔλB are the RT ratio shift and the Bragg wavelength shift corresponding to the applied load. The loading position was assumed to be the center of the applied stress distribution. Figure 3 shows the normalized RT ratio characteristics for
Fig. 3. Normalized RT ratio profile produced by locally weighting RT ratio measured by DTR3.
Fig. 4. Experimental results measuring distributed strain detected by (a) Bragg wavelength shift of the OFDR and (b) RT ratio of the DTR3 employing long-gauge FBG for four-point bending tests using a 2.5 m aluminum bar.
each loading position and their respective Gaussian fittings. A loading instrument was used to locally weight the test piece over 100 mm intervals, and the position coordinates in Fig. 3 indicate the locations of the OFDR. It is seen that the normalized RT ratios demonstrate approximately Gaussian profiles against the loading positions at a scale resolution, indicating that the center position of the scale resolution in DTR3 interrogation can be accurately estimated using the peak of the Gaussian profile of the RT ratio characteristics. Figures 4(a) and 4(b) show experimental results produced by the OFDR and DTR3 , respectively, obtained by calibrating the RT ratio to the Bragg wavelength shift in elongation strain measurements taken from four-point bending tests of the 2.5 m aluminum bar test piece. In these assessments, the
Fig. 5. Experimental result measuring characteristics of ratio of DTR3 produced RT to Bragg wavelength detected by OFDR under four-point bending tests. 10 February 2015 / Vol. 54, No. 5 / APPLIED OPTICS
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Fig. 6. Free vibration test results using 1 kg weight at the center of two supports. (a) Strain and FFT spectrum measured by DTR3 with 100 Hz sampling. (b) Strain and FFT spectrum measured by electrical gauge using 1 kHz sampling.
Fig. 7. Free vibration test results from test piece with two weights between two supports. (a) Strain measured and FFT spectrum by DTR3 using 100 Hz sampling. (b) Strain and FFT spectrum measured by electrical gauge using 1 kHz sampling.
test piece was placed on a four-point bending setup consisting of two simply supporting and two applied loading positions at distances of 2 and 0.8 m, respectively. Calibration data for the DTR3 were taken from strain measurement of the test piece at a coordinate of approximately 1.4 m. Because reflections in the fiber line seemed to affect measurement of the load distribution between 1.21 and 1.3 m, the Bragg wavelength shift does not appear in this range [Fig. 4(a)]. In addition, as the long-gauge FBG had unwritten sections of 20 mm length between every 500 mm long section of written grating, the distributed Bragg wavelength shown in Fig. 4(a) includes undetectable areas. It is seen from the figure that distributed strain was uniformly induced along the longitudinal direction of the test piece between the positions of the two weights, while, from Fig. 4(b), it is seen that the RT ratio monotonically increased with strain. Figure 5 shows the characteristics of the RT ratio as determined by the DTR3 at each Bragg wavelength detected by the OFDR produced in the 2.5 m aluminum bar test piece undergoing the four-point bending 1194
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test described above. The compressive and elongation strains could have been induced in the long-gauge FBG on the bar when the loading direction of the test piece was changed as the FBG attachment side was altered. Because it is known that the FBG wavelength shifts linearly with strain, the results in Fig. 5
Fig. 8. 5.5 m helicopter blade model structure with strain electrical gauges and four FBGs for deformation measurements.
indicate that the RT ratio of the test piece changed monotonically upon application of strain; this nonlinearity can be attributed to the fact that the wavelength slope-cutting filter had nonlinearities in terms of wavelength in its reflective and transmitted characteristics. In order to estimate the measurement stability, the strain characteristics of the RT ratio were linearly fitted, as shown in Fig. 5. The rate of Bragg wavelength change to an RT ratio was 0.3989. The DTR3 interrogating system using long-gauge
FBG had a deviation of 0.029 3σ of an RT ratio, which was estimated by means of 1 min measurement. Therefore, the deviation of the long-gauge FBG using the DTR3 was approximately 0.016 nm wavelength, which corresponds to 9.6 με strain [7]. In order to estimate a calibration curve for the RT ratio that includes the measured values, we employed a Spline interpolation calibration (Fig. 5), which enabled the characteristics of the RT ratio to be smoothed by including the measured values. Using the Spline
Fig. 9. Photos of 5.5 m helicopter blade model structure. (a) FBG sensors arranged on the blade surface. (b) Cantilever setup for the static loading test.
Fig. 10. Strain distribution for helicopter blade model detected by OFDR, nearby electrical strain gauge, and DTR3 from FBGs: (a) L1, (b) L2, and (c) L3 attached to a simply supported helicopter blade model, and from (d) FBG-L1 attached to a cantilever under uniform loading. 10 February 2015 / Vol. 54, No. 5 / APPLIED OPTICS
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interpolation characteristics, the RT ratio was used to calculate the strain, as shown, again, in Fig. 5. 3. Experimental Results A.
Dynamic Sensing Characteristics Using Aluminum Bar
Figure 6 shows the free vibration test results and fast Fourier transform (FFT) spectra produced by 100 Hz DTR3 sampling and 1 kHz electrical strain gauge sampling measurements of a system consisting of a 1 kg weight at the center of two supports. As indicated by the figure, the RT ratio response from the DTR3 was similar to that of the strain gauge; the natural frequencies detected by the respective instruments were both about 4.5 Hz, and the FFT spectra had similar shapes, as shown in Figs. 6(a) and 6(b), respectively. On the other hand, whereas the strain gauge measured symmetric strain change, the real-time strain change measured by the DTR3 was asymmetric. As shown in Fig. 5, this is because the RT ratio is easily affected by nonlinear sensitivity changes. Figure 7 shows free vibration test results produced by the DTR3 and the strain gauge from measurements of two weights at the center of two supports. It is seen that the DTR3 could detect the dumped strain waveform and natural frequency, which confirms that
the DTR3 can accurately measure dynamic strain and suggests its potential for use in the estimation of structural dynamic deformation. B. Distributed Sensing Characteristics Using a Helicopter Blade Model
Figure 8 shows a model of a 5.5 m helicopter blade structure with strain electrical gauges and four long-gauge FBGs attached for the evaluation of static distributed strain. FBG pairs −L1, −L2 and −L3, −L4 are attached near the leading and trailing edges, respectively, while FBG pairs −L1, −L3 and −L2, −L4 are attached on the upper and lower sides, respectively. 10 mm referenced strain gauges are placed at intervals of 300 mm along the side of trailing edge from the FBG lines. Figures 9(a) and 9(b) show photos of the 5.5 m helicopter blade model structure with FBG sensors arranged on the blade surface and as configured for static cantilever loading testing, respectively. As shown in Fig. 8, the long-gauge FBGs were located at between 250 and 5000 mm from the blade tip. As the DTR3 interrogator can measure from a maximum distance of 20 m, two long-gauge FBG and transmission line assemblies of approximately 9 m each in length were connected in tandem to enable simultaneous measurements with a single
Fig. 11. Comparison of detected strain from DTR3 and OFDR measurement equipment FBGs: (a) L1, (b) L2, and (c) L3 from a simply supported helicopter blade model, and from (d) FBG-L1 attached to a cantilever under uniform loading. 1196
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fiber line. FBG pairs L1–L2 and L3–L4 were each connected, and loading positions were established at 1.3, 2.2, 3.1, and 4.0 m in the blade coordinate system, as shown in Fig. 8. Figures 10(a)–10(d) show the static distributed strain test results produced by long-gauge FBGs L1, L2, and L3 in the simply supported helicopter blade model and by FBG-L1 in the cantilever under uniform loading, respectively. It is seen from the figure that, although the DTR3 measured strains separately at intervals of 50 cm, the patterns of strain distribution of elongation and compression produced by the DTR3 was the same as that produced by the OFDR and strain gauges, confirming that the proposed long-gauge FBG and DTR3 interrogator system could accurately detect the deformation shape of the helicopter blade model. The OFDR measurement results in Fig. 11 show fluctuations of strain distribution that increase with loading, a phenomenon that was confirmed in the other loading test cases. The long-gauge FBG could detect the uninformed surface strain distribution caused by carbon fiber cloth material because the OFDR has the advantage of high spatial resolution for distributed strain measurement; on the other hand, the DTR3 was unaffected by the uninformed surface strain distribution owing to the fact that it detects strain averaged over detectors spaced at 50 cm intervals. Figure 11 shows a comparison among the averaged strain measured by the OFDR over the range −25 to 25 cm at the above-referenced DTR3 sensing positions; the strain measured by the DTR3 interrogator with long-gauge FBGs-L1, L2, and L3 in the simply supported helicopter blade model; and the strain measured by FBG-L1 in the cantilever under the uniformly loading test conditions indicated in Fig. 10. It is seen that strain measured by the DTR3 was as low or slightly lower than that measured by the OFDR because the calibration characteristics of the RT ratio had a nonlinear relationship to the Bragg wavelength, as shown in Fig. 5. This caused fluctuations in RT ratio to easily affect the Bragg wavelength shift. In cases of relatively small strain (less than 300 με), the strain measured by the DTR3 is similar to that measured by the OFDR, as the wavelength slope-cutting filter has a stable slope in the range of a few hundreds of με.
4. Conclusion
In this paper, a dynamic distributed strain measurement technique using a long-gauge FBG and a DTR3 interrogator was proposed. A 2.5 m aluminum bar test piece and a 5.5 m helicopter blade model were used to evaluate, via loading and free vibration tests, the ability of the proposed technique to conduct dynamic strain and static distributed strain measurements. Using dynamic strain measurement at a 100 Hz sampling rate, the DTR3 can detect the natural frequency of a structure; using distributed strain measurement, it can successfully measure cantilever deformation in a 5.5 m helicopter blade model. These results demonstrate that the proposed DTR3 interrogator with a long-gauge FBG has the potential to conduct distributed dynamic strain measurement at a 50 cm spatial resolution in order to detect vibration deformation modes in structures with dimensions ranging from a few to tens of meters. The authors would like to thank to Watanabe, Co., Ltd. for development of DTR3 instrument. References 1. W. Staszewski, C. Boller, and G. Tomlinson, Health Monitoring of Aerospace Structures (Wiley, 2003), pp. 1–12. 2. A. D. Kersey, M. I. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putman, and E. J. Friebele, “Fiber grating sensors,” J. Lightwave Technol. 15, 1442–1463 (1997). 3. H. Jiang, B. Veek, D. Kirk, and H. Guierrez, “Real-time estimation of time-varying bending modes using fiber Bragg grating sensor arrays,” AIAA J. 51, 178–185 (2013). 4. L. H. Kang, D. K. Kim, and J. H. Han, “Estimation of dynamic structural displacements using fiber Bragg grating strain sensors,” J. Sound Vibr. 305, 534–542 (2007). 5. A. Cusano, P. Capoluongo, S. Campopiano, A. Cutolo, M. Giordano, F. Felli, A. Paolozzi, and M. Caponero, “Experimental modal analysis of an aircraft model wing by embedded fiber Bragg grating sensors,” IEEE Sens. J. 6, 67–77 (2006). 6. H. I. Kim, L. H. Kang, and J. H. Han, “Shape estimation with distributed fiber Bragg grating sensors for rotating structures,” Smart Mater. Struct. 20, 035011 (2011). 7. S. Onoda, K. Inoue, K. Aita, T. Nakada, M. Nakano, and Y. Komatsu, “Delayed transmission/reflection ratiomatic reflectometry,” Proc. SPIE 7503, 750332 (2009). 8. H. Igawa, K. Ohta, T. Kasai, I. Yamaguchi, H. Murayama, and K. Kageyama, “Distributed measurements with a long gauge FBG sensor using optical frequency domain reflectometry (1st report, system investigation using optical simulation model),” J. Sol. Mech. Mat. Eng. 2, 1242–1252 (2008).
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Faculty of Engineering, Soka University, 1-236 Tangi, Hachioji, Tokyo 192-8577, Japan
Institute of Aeronautical Technology, Japan Aerospace Exploration Agency, 6-13-1 Ohsawa, Mitaka, Tokyo 181-0015, Japan 3
Department of Aerospace Engineering, Tokyo Metropolitan University, 6-6 Asahigaoka, Hino, Tokyo 191-0065, Japan *Corresponding author: [email protected] Received 18 November 2014; revised 19 November 2014; accepted 9 January 2015; posted 12 January 2015 (Doc. ID 216974); published 9 February 2015
In this paper, we propose a delayed transmission/reflection ratiometric reflectometry (DTR3 ) scheme using a long-gauge fiber Bragg grating (FBG), which can be used for dynamic structural deformation monitoring of structures of between a few to tens of meters in length, such as airplane wings and helicopter blades. FBG sensors used for multipoint sensing generally employ wavelength division multiplexing techniques utilizing several Bragg central wavelengths; by contrast, the DTR3 interrogator uses a continuous pulse array based on a pseudorandom number code and a long-gauge FBG utilizing a single Bragg wavelength and composed of simple hardware devices. The DTR3 scheme can detect distributed strain at a 50 cm spatial resolution using a long-gauge FBG with a 100 Hz sampling rate. We evaluated the strain sensing characteristics of the long-gauge FBG when attached to a 2.5 m aluminum bar and a 5.5 m helicopter blade model, determining these structure natural frequencies in free vibration tests and their distributed strain characteristics in static tests. © 2015 Optical Society of America OCIS codes: (140.3490) Lasers, distributed-feedback; (060.3735) Fiber Bragg gratings; (060.2370) Fiber optics sensors. http://dx.doi.org/10.1364/AO.54.001191
1. Introduction
It is widely known that structural health monitoring (SHM) methods can be applied for use in aerospace systems. By embedding sensors of various types into a structure, loading, stress, and damage can be detected in order to prevent fatal structural disorders. A particularly important challenge in aerospace vehicle monitoring and control is created by the time-varying mass and inertia of a vehicle in flight and the consequent deformation and changes in the modal frequencies [1] of airplane wing deflection or blade shape in windmills or helicopters. Standard 1559-128X/15/051191-07$15.00/0 © 2015 Optical Society of America
sensing approaches for such structures include the use of noncontact systems such as laser Doppler vibrometers or in situ electric devices such as accelerometers and strain gauges. Although the use of additional on-board instrumentation can provide more accurate estimation of modal properties and enable better real-time control, higher-quality estimation requires the use of increasing numbers of sensors, the additional weight and complexity of which can pose problems. In addition, the extensive wiring needed for sensors increases system weight and susceptibility to electromagnetic interference (EMI). To overcome these problems, excellent multiplexing techniques involving data interrogation at high sampling rates have been sought. 10 February 2015 / Vol. 54, No. 5 / APPLIED OPTICS
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Fiber optic sensors represent an attractive SHM technology owing to their simple wiring, high sensitivity, and resistance to harsh environments such as EMI noise. In particular, fiber Bragg grating (FBG) sensors, which have been proposed and developed for strain measurement applications in several studies [2–6], open up the possibility of measuring strain and estimating the shape and modal properties of aerospace structures in real time. Previous FBG solutions have included using a nine FBG sensor array to estimate the effects on modal shape of mass step changes caused by exhausted propellant in real time [3], the use of four wavelength multiplexing FBG sensors for estimating dynamic structural displacement in aluminum and acryl beam specimens [4], and modal analysis using single and multiple embedded FBG sensors [5,6]. In previous work, the FBG sensors used for dynamic displacement and modal estimation were single or multipoint systems based on WDM, a technique requiring that each sensor has a unique central Bragg wavelength. However, multiple Bragg wavelength FBG systems require more costly fabrication processes than single wavelength systems and must use wavelength tunable lasers or wavelength spectrum measurement instruments to interrogate Bragg wavelength change, which, in turn, requires complex measurement systems. FBG interrogating systems based on ratiometric techniques that utilize optical filtering combined with a superluminescent diode (SLD) continuous broadband light source have also been proposed, although such systems require special optical filters for each Bragg wavelength, which again results in large, complex system configurations. The combination of a delayed transmission/ reflection ratiometric reflectometry (DTR3 ) interrogator that uses time-division multiplexing techniques [7] with continuous pulse arrays based on a pseudorandom number (PN) code and a long-gauge FBG with a single Bragg wavelength has been proposed as a means of simplifying the interrogation of multiplexing and dynamic FBG responses in applications such as dynamic deformation and modal parameter estimation in aerospace vehicles. A DTR3 consists of simple hardware devices, including a broadband SLD light source, a single wavelength slope cutting filter, a detector, and correlation computing hardware. Because time arrival differences from a longgauge FBG can be detected in real time through the correlation computing of a continuous pulse array modulated by a PN code, the long-gauge FBG can have a single Bragg wavelength, which reduces its fabrication cost. Techniques using this combination of equipment can compensate light source fluctuation and bending loss from transmission fiber lines by differentiating between reflected and transmitted spectra through a wavelength slope-cutting filter. In this paper, we present and evaluate the dynamic distributed strain measurement characteristics of a long-gauge FBG and a DTR3 interrogator that can achieve dynamic measurements with a 50 cm spatial 1192
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resolution at a 100 Hz sampling rate. We employed a 2.5 m aluminum bar test piece with a single longgauge FBG to conduct static strain calibration testing and a 5.5 m helicopter blade model with four longgauge FBGs to conduct dynamic strain detection using free vibration testing and distributed strain measurement. 2. DTR 3 with Long-Gauge FBG Interrogation System A. System Configuration
Figure 1 shows the configuration of the proposed DTR3 interrogator with a long-gauge FBG. A DTR3 consists of the broadband SLD light source, the wavelength slope-cutting filter, the detector, and correlation computing hardware. The reflection and transmission (RT) power ratio is detected by means of a continuous pulse modulated by a PN code. It is well suited for DTR3 interrogation [7] through a process of fiber distributed reflection measurement of the DTR3 in which a continuous pulse PN digital modulated signal, rather than an impulse, is utilized as an interrogating light. Because the interrogating light is continuous-wave, it does not require a high power source. Additionally, interrogation speed can be increased because time arrival differences from a long-gauge FBG are computed by correlation of a continuous PN code modulated pulse. The wavelength slope-cutting filter as shown in Fig. 1 has complementary reflectivity and transmissivity with the wavelength. Since the FBG sensor has a steep reflected spectrum, the reflection and transmission (RT) power ratio of the sensor changes with the Bragg wavelength shift. The Bragg wavelength also changes with strain and temperature; therefore, the RT ratio changes with physical properties such as strain and temperature. B. Calibration Test for DTR3 Interrogator
Figure 2 shows an experimental setup used for calibration tests utilizing a 2.5 m aluminum bar with a single long-gauge FBG as a test piece. A 2.5 m longgauge FBG was pasted width-wise to the center of a hollow aluminum prismatic bar with a 30 mm × 15 mm cross section. Measurements were taken in alternate trials using the DTR3 and an optical frequency domain reflectometer (OFDR) [8], which can
Fig. 1. Dynamic disctributed strain measurement system based on long-gauge FBG with DTR3 interrogation.
Fig. 2. Measurement setup using 2.5 m aluminum bar as a test peace and DTR3 and OFDR as reference equipment.
detect Bragg wavelength shifts with a high spatial resolution of 1 mm at a 1 Hz reference sampling rate for static measurement. In order to align the DTR3 coordinates with the location of the applied load and identify its center at a scale resolution of 50 cm, the calibration characteristics of the RT ratio distribution were evaluated by applying the load locally along the long-gauge FBG. The OFDR played the role of a reference instrument that detected the position of the applied load and strain using a Bragg wavelength shift, and the applied load was monitored by the referenced electrical strain gauges caused by maintaining the load at various loading positions. To evaluate the RT ratio characterizing the DTR3 at a given position, the ratio was normalized for a given amount of applied stress along the fiber. It was assumed that the normalized RT ratio has a Gaussian profile for a given spatial position, with the center of the Gaussian profile indicating the central position of DTR3 scale resolution. The normalized RT ratio, ζNorm , is given by ζ Norm R
Δζ ; input ΔλB dx
(1)
where Δζ and ΔλB are the RT ratio shift and the Bragg wavelength shift corresponding to the applied load. The loading position was assumed to be the center of the applied stress distribution. Figure 3 shows the normalized RT ratio characteristics for
Fig. 3. Normalized RT ratio profile produced by locally weighting RT ratio measured by DTR3.
Fig. 4. Experimental results measuring distributed strain detected by (a) Bragg wavelength shift of the OFDR and (b) RT ratio of the DTR3 employing long-gauge FBG for four-point bending tests using a 2.5 m aluminum bar.
each loading position and their respective Gaussian fittings. A loading instrument was used to locally weight the test piece over 100 mm intervals, and the position coordinates in Fig. 3 indicate the locations of the OFDR. It is seen that the normalized RT ratios demonstrate approximately Gaussian profiles against the loading positions at a scale resolution, indicating that the center position of the scale resolution in DTR3 interrogation can be accurately estimated using the peak of the Gaussian profile of the RT ratio characteristics. Figures 4(a) and 4(b) show experimental results produced by the OFDR and DTR3 , respectively, obtained by calibrating the RT ratio to the Bragg wavelength shift in elongation strain measurements taken from four-point bending tests of the 2.5 m aluminum bar test piece. In these assessments, the
Fig. 5. Experimental result measuring characteristics of ratio of DTR3 produced RT to Bragg wavelength detected by OFDR under four-point bending tests. 10 February 2015 / Vol. 54, No. 5 / APPLIED OPTICS
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Fig. 6. Free vibration test results using 1 kg weight at the center of two supports. (a) Strain and FFT spectrum measured by DTR3 with 100 Hz sampling. (b) Strain and FFT spectrum measured by electrical gauge using 1 kHz sampling.
Fig. 7. Free vibration test results from test piece with two weights between two supports. (a) Strain measured and FFT spectrum by DTR3 using 100 Hz sampling. (b) Strain and FFT spectrum measured by electrical gauge using 1 kHz sampling.
test piece was placed on a four-point bending setup consisting of two simply supporting and two applied loading positions at distances of 2 and 0.8 m, respectively. Calibration data for the DTR3 were taken from strain measurement of the test piece at a coordinate of approximately 1.4 m. Because reflections in the fiber line seemed to affect measurement of the load distribution between 1.21 and 1.3 m, the Bragg wavelength shift does not appear in this range [Fig. 4(a)]. In addition, as the long-gauge FBG had unwritten sections of 20 mm length between every 500 mm long section of written grating, the distributed Bragg wavelength shown in Fig. 4(a) includes undetectable areas. It is seen from the figure that distributed strain was uniformly induced along the longitudinal direction of the test piece between the positions of the two weights, while, from Fig. 4(b), it is seen that the RT ratio monotonically increased with strain. Figure 5 shows the characteristics of the RT ratio as determined by the DTR3 at each Bragg wavelength detected by the OFDR produced in the 2.5 m aluminum bar test piece undergoing the four-point bending 1194
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test described above. The compressive and elongation strains could have been induced in the long-gauge FBG on the bar when the loading direction of the test piece was changed as the FBG attachment side was altered. Because it is known that the FBG wavelength shifts linearly with strain, the results in Fig. 5
Fig. 8. 5.5 m helicopter blade model structure with strain electrical gauges and four FBGs for deformation measurements.
indicate that the RT ratio of the test piece changed monotonically upon application of strain; this nonlinearity can be attributed to the fact that the wavelength slope-cutting filter had nonlinearities in terms of wavelength in its reflective and transmitted characteristics. In order to estimate the measurement stability, the strain characteristics of the RT ratio were linearly fitted, as shown in Fig. 5. The rate of Bragg wavelength change to an RT ratio was 0.3989. The DTR3 interrogating system using long-gauge
FBG had a deviation of 0.029 3σ of an RT ratio, which was estimated by means of 1 min measurement. Therefore, the deviation of the long-gauge FBG using the DTR3 was approximately 0.016 nm wavelength, which corresponds to 9.6 με strain [7]. In order to estimate a calibration curve for the RT ratio that includes the measured values, we employed a Spline interpolation calibration (Fig. 5), which enabled the characteristics of the RT ratio to be smoothed by including the measured values. Using the Spline
Fig. 9. Photos of 5.5 m helicopter blade model structure. (a) FBG sensors arranged on the blade surface. (b) Cantilever setup for the static loading test.
Fig. 10. Strain distribution for helicopter blade model detected by OFDR, nearby electrical strain gauge, and DTR3 from FBGs: (a) L1, (b) L2, and (c) L3 attached to a simply supported helicopter blade model, and from (d) FBG-L1 attached to a cantilever under uniform loading. 10 February 2015 / Vol. 54, No. 5 / APPLIED OPTICS
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interpolation characteristics, the RT ratio was used to calculate the strain, as shown, again, in Fig. 5. 3. Experimental Results A.
Dynamic Sensing Characteristics Using Aluminum Bar
Figure 6 shows the free vibration test results and fast Fourier transform (FFT) spectra produced by 100 Hz DTR3 sampling and 1 kHz electrical strain gauge sampling measurements of a system consisting of a 1 kg weight at the center of two supports. As indicated by the figure, the RT ratio response from the DTR3 was similar to that of the strain gauge; the natural frequencies detected by the respective instruments were both about 4.5 Hz, and the FFT spectra had similar shapes, as shown in Figs. 6(a) and 6(b), respectively. On the other hand, whereas the strain gauge measured symmetric strain change, the real-time strain change measured by the DTR3 was asymmetric. As shown in Fig. 5, this is because the RT ratio is easily affected by nonlinear sensitivity changes. Figure 7 shows free vibration test results produced by the DTR3 and the strain gauge from measurements of two weights at the center of two supports. It is seen that the DTR3 could detect the dumped strain waveform and natural frequency, which confirms that
the DTR3 can accurately measure dynamic strain and suggests its potential for use in the estimation of structural dynamic deformation. B. Distributed Sensing Characteristics Using a Helicopter Blade Model
Figure 8 shows a model of a 5.5 m helicopter blade structure with strain electrical gauges and four long-gauge FBGs attached for the evaluation of static distributed strain. FBG pairs −L1, −L2 and −L3, −L4 are attached near the leading and trailing edges, respectively, while FBG pairs −L1, −L3 and −L2, −L4 are attached on the upper and lower sides, respectively. 10 mm referenced strain gauges are placed at intervals of 300 mm along the side of trailing edge from the FBG lines. Figures 9(a) and 9(b) show photos of the 5.5 m helicopter blade model structure with FBG sensors arranged on the blade surface and as configured for static cantilever loading testing, respectively. As shown in Fig. 8, the long-gauge FBGs were located at between 250 and 5000 mm from the blade tip. As the DTR3 interrogator can measure from a maximum distance of 20 m, two long-gauge FBG and transmission line assemblies of approximately 9 m each in length were connected in tandem to enable simultaneous measurements with a single
Fig. 11. Comparison of detected strain from DTR3 and OFDR measurement equipment FBGs: (a) L1, (b) L2, and (c) L3 from a simply supported helicopter blade model, and from (d) FBG-L1 attached to a cantilever under uniform loading. 1196
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fiber line. FBG pairs L1–L2 and L3–L4 were each connected, and loading positions were established at 1.3, 2.2, 3.1, and 4.0 m in the blade coordinate system, as shown in Fig. 8. Figures 10(a)–10(d) show the static distributed strain test results produced by long-gauge FBGs L1, L2, and L3 in the simply supported helicopter blade model and by FBG-L1 in the cantilever under uniform loading, respectively. It is seen from the figure that, although the DTR3 measured strains separately at intervals of 50 cm, the patterns of strain distribution of elongation and compression produced by the DTR3 was the same as that produced by the OFDR and strain gauges, confirming that the proposed long-gauge FBG and DTR3 interrogator system could accurately detect the deformation shape of the helicopter blade model. The OFDR measurement results in Fig. 11 show fluctuations of strain distribution that increase with loading, a phenomenon that was confirmed in the other loading test cases. The long-gauge FBG could detect the uninformed surface strain distribution caused by carbon fiber cloth material because the OFDR has the advantage of high spatial resolution for distributed strain measurement; on the other hand, the DTR3 was unaffected by the uninformed surface strain distribution owing to the fact that it detects strain averaged over detectors spaced at 50 cm intervals. Figure 11 shows a comparison among the averaged strain measured by the OFDR over the range −25 to 25 cm at the above-referenced DTR3 sensing positions; the strain measured by the DTR3 interrogator with long-gauge FBGs-L1, L2, and L3 in the simply supported helicopter blade model; and the strain measured by FBG-L1 in the cantilever under the uniformly loading test conditions indicated in Fig. 10. It is seen that strain measured by the DTR3 was as low or slightly lower than that measured by the OFDR because the calibration characteristics of the RT ratio had a nonlinear relationship to the Bragg wavelength, as shown in Fig. 5. This caused fluctuations in RT ratio to easily affect the Bragg wavelength shift. In cases of relatively small strain (less than 300 με), the strain measured by the DTR3 is similar to that measured by the OFDR, as the wavelength slope-cutting filter has a stable slope in the range of a few hundreds of με.
4. Conclusion
In this paper, a dynamic distributed strain measurement technique using a long-gauge FBG and a DTR3 interrogator was proposed. A 2.5 m aluminum bar test piece and a 5.5 m helicopter blade model were used to evaluate, via loading and free vibration tests, the ability of the proposed technique to conduct dynamic strain and static distributed strain measurements. Using dynamic strain measurement at a 100 Hz sampling rate, the DTR3 can detect the natural frequency of a structure; using distributed strain measurement, it can successfully measure cantilever deformation in a 5.5 m helicopter blade model. These results demonstrate that the proposed DTR3 interrogator with a long-gauge FBG has the potential to conduct distributed dynamic strain measurement at a 50 cm spatial resolution in order to detect vibration deformation modes in structures with dimensions ranging from a few to tens of meters. The authors would like to thank to Watanabe, Co., Ltd. for development of DTR3 instrument. References 1. W. Staszewski, C. Boller, and G. Tomlinson, Health Monitoring of Aerospace Structures (Wiley, 2003), pp. 1–12. 2. A. D. Kersey, M. I. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putman, and E. J. Friebele, “Fiber grating sensors,” J. Lightwave Technol. 15, 1442–1463 (1997). 3. H. Jiang, B. Veek, D. Kirk, and H. Guierrez, “Real-time estimation of time-varying bending modes using fiber Bragg grating sensor arrays,” AIAA J. 51, 178–185 (2013). 4. L. H. Kang, D. K. Kim, and J. H. Han, “Estimation of dynamic structural displacements using fiber Bragg grating strain sensors,” J. Sound Vibr. 305, 534–542 (2007). 5. A. Cusano, P. Capoluongo, S. Campopiano, A. Cutolo, M. Giordano, F. Felli, A. Paolozzi, and M. Caponero, “Experimental modal analysis of an aircraft model wing by embedded fiber Bragg grating sensors,” IEEE Sens. J. 6, 67–77 (2006). 6. H. I. Kim, L. H. Kang, and J. H. Han, “Shape estimation with distributed fiber Bragg grating sensors for rotating structures,” Smart Mater. Struct. 20, 035011 (2011). 7. S. Onoda, K. Inoue, K. Aita, T. Nakada, M. Nakano, and Y. Komatsu, “Delayed transmission/reflection ratiomatic reflectometry,” Proc. SPIE 7503, 750332 (2009). 8. H. Igawa, K. Ohta, T. Kasai, I. Yamaguchi, H. Murayama, and K. Kageyama, “Distributed measurements with a long gauge FBG sensor using optical frequency domain reflectometry (1st report, system investigation using optical simulation model),” J. Sol. Mech. Mat. Eng. 2, 1242–1252 (2008).
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