Comparing polymer optical fiber, fiber Bragg grating ... - OSA Publishing

Comparing polymer optical fiber, fiber Bragg grating, and traditional strain gauge for aircraft structural health monitoring Javier Gomez,1,* Joseba Zubia,1 Gerardo Aranguren,1 Jon Arrue,1 Hans Poisel,2 and Idurre Saez3 1

Department of Electronics and Telecommunications, University of the Basque Country, Alda. Urquijo s/n, Bilbao 48013, Spain 2

Polymer Optical Fiber Application Center, University of Applied Sciences, Wassertorstrasse 10, Nuremberg D-90489, Germany

3

Aeronautical Technologies Center, Juan de la Cierva 1, Alava Tecnological Park, Vitoria 01510, Spain *Corresponding author: [email protected] Received 12 January 2009; accepted 30 January 2009; posted 11 February 2009 (Doc. ID 105519); published 3 March 2009

Systems for structural health monitoring in aeronautical structures use methods of measuring the elongation that normally require too heavy setups or difficult assembly jobs, such as those based on traditional strain gauges. Alternative methods based on fiber Bragg gratings tend to be very expensive. We analyze the possibility of improving the existing designs with the aid of low-cost plastic optical fiber sensors. For this purpose we test these sensors in a rudder flap subjected to different types of bending forces. The results show that they offer good stability and repeatability, and the measured values are very similar to those obtained with Bragg sensors. © 2009 Optical Society of America OCIS codes: 060.3735, 130.6010, 160.5470, 280.4991.

1. Introduction

In the past few years, there has been an increase in the use of intelligent sensors employed in critical structures (e.g., buildings, bridges, ships, windmills, and aeronautical structures) for structural health monitoring (SHM), which can be embedded or fixed by means of surface mount technology. This greater use not only relates to the initial test period, but also relates to the rest of the lifetime of the structure. The utility of using these sensors during the whole lifetime can be very great. For example, vibrations during flights can reduce the lifetime of wings. Optical fibers constitute a suitable tool for SHM, and their benefits are enhanced by their small 0003-6935/09/081436-08$15.00/0 © 2009 Optical Society of America 1436

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weight and small size and by their immunity to electromagnetic fields [1]. An example of their utility is the deformation sensor developed recently at the Polymer Optical Fiber Application Center in Nuremberg, Germany, which measures the amount of stress undergone by a polymer optical fiber (POF) by means of the phase shift between two optical signals propagating in the fiber [2]. Among the different types of optical fibers, polymer ones offer the additional advantages of low cost, easy handling, relatively high resistance to fracture [3,4], and high sensitivity to the amount of strain when they are stretched or pulled [5]. Moreover the experimental methodology that can be employed with POFs [2] involves a much easier and cheaper processing than that needed with fiber Bragg gratings (FBGs), for example. SHM based on POFs has already been successfully tested on models of turbine blades, but our objective is to

show the effectiveness of this method in an aircraft structure in comparison with other methods that are commonly adopted, such as the employment of strain gauges or FBGs. To compare the aforementioned methodologies, we make use of a rudder flap that is bent by means of a hydraulic actuator. Various tests are carried out, from single movements to cyclical and step-by-step tests at different velocities and with different durations. These serve to check some performance characteristics that must be met by all SHM systems, such as the stability of the measures and the sensitivity to changes in tension. The tests also serve to analyze the suitability of adopting SHM methods based on POFs in the near future for the aeronautical industry, especially by integrating POFs in aircraft pieces. We begin by introducing the POF-based strain sensor developed by us. Afterward we use it in a rudder flap and compare the results with those obtained with FBGs. Finally we summarize the main conclusions. 2. Description of the Polymer-Optical-Fiber-Based Strain Sensor

The sensor is based on the change in the phase undergone by a signal due to the distance traveled. If a sinusoidally modulated optical signal is launched into two fibers of different length, there will be a phase difference between the respective signals at the output of the fibers due to the different transit distances. The frequency of the launched signal is adjusted by means of a voltage-controlled oscillator (VCO), which allows changing the frequency of the sinusoidal signal. The frequency that is needed for doing the measurements depends on the structural elements under test. In general, the higher the frequency, the better the resolution will be, although there is a maximum value that cannot be exceeded, which is imposed by the finite bandwidth of the fiber, the transmitter, and the receiver. Furthermore the frequency value is preferably chosen in such a way that the phase shift is always smaller than π for the whole range of expected changes in the lengths of the fibers. The transmitter used is an LED that takes the electrical signal from the VCO. Before launching the optical signal into the fibers, a Y coupler divides the output of the LED into two parts in such a way that the signals at the input of the fibers are identical. At the output of each fiber, a receiver converts each optical signal into an electrical one. An analog phase comparator compares the phase of the two electrical signals and provides an output voltage proportional to their phase shift, which is shown and recorded in a personal computer (PC) after being conveniently adapted in an analog-to-digital (A/D) converter. We can see the schema of setup in Fig. 1. As phase comparator, we used an AD8302 integrated circuit that can work from low frequencies up to 2:7 GHz. The slope efficiency of the device is

Fig. 1. Setup used for the measurements. The VCO generates a sinusoidally modulated signal that is fed into the transmitter (T). A Y coupler launches two identical signals into the POFs (POF 1 and POF 2). Two optical receivers (R) generate the respective electrical signals, and with a phase comparator and an A/D converter, we can see the phase shift in a PC.

10 mV (arc degree of the phase shift) [6]. Any difference in length between the two fibers (Δl) can be readily calculated from the phase shift (Δφ) by means of

Δl ¼

c0 1 · Δφ; · nc0 · f m 360

ð1Þ

in which c0 is the speed of light in vacuum, nco is the index of refraction of the fiber core, and f m is the modulation frequency. In our case, nco is 1.49, and f m is 765 MHz. In our case the object under test is a specimen of rudder flap. The influence of environmental factors such as temperature, humidity, or atmospheric pressure is null, because both fibers are to be exposed to the same conditions. Each fiber is fixed on a different side of the flap. During the measurements the flap is held horizontally by one of its edges, and a bending force is applied to the opposite edge (Fig. 2). Depending on the orientation of the force (downward or upward), the upper fiber will be exposed to an elongation or a compression due to the bending, while the bottom fiber will undergo the opposite effect. The analog signal of the phase comparator will be the result of both simultaneous effects. Therefore its

Fig. 2. Flap is held by one of its sides by means of a metal framework and a clamping jaw, and it is bent by a hydraulic actuator attached to another clamping jaw located on the other side. The movements of the flap are monitored using 3D cameras placed on top of the camera column. 10 March 2009 / Vol. 48, No. 8 / APPLIED OPTICS

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amplitude will correspond to twice the real elongation (or compression) of the upper side. 3. Experimental Setups with Different Types of Sensors

Figure 2 shows the setup employed for the measurements. The part of the aircraft under test was progressively bent with the aid of a linear variable differential transformer (LVDT), which is a type of electrical transformer used for measuring linear displacement. The rudder flap was held by one of its sides by means of a metal framework and a clamping jaw, and it was bent by means of an actuator placed on the other side. To protect the flap, panels of neoprene 4 mm thick were utilized. The actuator employed was a hydraulic one, namely, the model 243.17 from MTS Company, which features a stroke of 508 mm for a nominal load of 50 kN. It is controlled by the servo controller MTS 894.50, which is a proportional–integral–derivative one. During the test the necessary applied load was 84 N for bending the flap an amount of 10 cm. The elongation of the flap during the bending process could be visually monitored by using threedimensional (3D) cameras placed on top of the camera column (Fig. 2), which is a 6 m high column placed on one side of the testing structure. This 3D monitoring system served to record the longitudinal movements on the upper side of the flap, with the aid of white labels marked on it and a reference position marked on top of a tripod (Fig. 3). The cameras have a frequency of 12:5 frames=s and a display resolution of 1.3 million pixels, which yields a sensor resolution of 200 μm. As shown schematically in Fig. 1, a set of two POFs can be employed to measure the difference in elongations between the upper and the bottom sides of the bent flap. For this purpose both POFs were placed on different sides of the flap in such a way that they were lying longitudinally in the central part of it in the shape of a loop on each side (Fig. 4). Since the length of each loop was approximately 4 m, the measurements yielded the differences in the elongations between 2 m long sides of the flap. In order to isolate the elongation of the upper part of the bent flap, we had to divide by 4 the measurement corresponding to the total phase shift (since one of the sides expands and the other one contracts, and there are two lengths of fiber on each side). In order to attach the POFs to the surface of the flap, different bonding materials were tried, such as adhesives or a fiberglass filling. Finally we used the mixture M-Bond 200 from Vishay Instruments Company, which was covered with a fiberglass filling to secure the fixing, as can be seen in Fig. 4. The MBond 200 is basically made of cyanoacrylate, and it is usually used to fix strain gauges. The flap was made of carbon fiber reinforced in the inside with a honeycomb pattern. The electronic components were placed on the upper side of the flap and near the fastening vertical 1438


Fig. 3. Assembly of all the parts of the structure, photographed from the narrow side of the flap. In the foreground we can see the actuator and its clamping jaw as well as a tripod that was used to provide a reference position for the 3D camera system. The metal framework is in the background.

framework. In order to reduce the effects of the temperature and to dissipate the heat, they were mounted on a 1 cm thick aluminum plate. Two power supplies were used for feeding all the components, including the VCO that feeds the LED. A set of two photographs showing the sides of the flap with the electronic setup mounted on one of the sides can be seen in Fig. 4. Figure 5 is a photograph of the electronic setup plus several numbers showing the different parts comprising it, which are mainly the VCO (3), the transmitter (4), the two receivers (5), the phase-shift detector (6), and the A/D converter (7). By setting the frequency of the VCO conveniently, the phase shift was adjusted to be close to π=2. The necessary frequency for this phase shift was near 765 MHz, which was measured by means of a spectrum analyzer. The experimental results obtained with this method of the phase shift will be discussed later, together with those corresponding to an alternative method employed to monitor the flap. As for this other method, it consisted of using FBGs. Specifically we used four sensors purchased

Fig. 4. Photographs of the upper side (left) and the bottom side (right) of the flap. The electronic components were placed on the upper side. For the measurements a loop of POF was employed on each side, extending nearly from one end to the opposite one (the shorter loop that also appears in the photographs was used to compare the amount of stress in different parts of the flap).

from Micron Optics Company, corresponding to the models OS110 (two of them) and OS310 (the other two sensors) [7]. The former model can be described as a bare fiber including a Bragg grating, while the latter is a FBG sensor of encapsulated fiber. The Bragg wavelength was chosen to be 1550 nm. We fixed the sensors to the surface using a mixture from Vishay Instruments Company called M-Bond AE10, which is made of an epoxy resin and a curing agent. The OS310 sensors were placed inside the loop used in the POF-based method, whereas the other two sensors (OS110) were attached outside the loop but in close proximity to the former ones in such a way that the respective distances from the end of the flap coincided (Fig. 6). FBG sensors reflect a wavelength that depends on the period of the grating structure. This is called Bragg wavelength (λB ), and it is calculated as λB ¼ 2nΛ;

ð2Þ

Fig. 5. Electronic setup, with the components numbered as follows: 1, wires for power supply; 2, circuit board; 3, VCO; 4, transmitter; 5, receivers; 6, phase-shift detector; 7, A/D converter; and 8, metal plate.

Fig. 6. Schematic diagram of the location of the sensors on the specimen under test (top view). Two pairs of FBG sensors are used (a pair of OS310 and a pair of OS110). Both pairs are placed along directions parallel to the flap, which lie, respectively, inside and outside the fiber loop employed for the POF-based strain sensor. At both sides of each OS110 sensor, there are strain gauges.

where n is the refractive index of the fiber core, and Λ is the period of the grating [8]. Changes in strain (Δε) or temperature (ΔT) produce changes in Λ, which can be noticed by measuring the corresponding changes in the wavelength of the reflected light. The equation that relates all these changes is ΔλB ¼ K ε Δε þ K T ΔT;

ð3Þ

where K ε and K T are constants. In the FBG sensors used in this work, K ε was 1:25 pm=μstrain and K T was 10 pm=°C. As interrogator device the system SM130-200 from Micron Optics Company was employed. It has two channels and an interrogation rate of 100 Hz. This device can be used in the range of wavelengths between 1510 and 1590 nm. It is able to sense up to 250 different gratings in each channel, with a measurement resolution of 0:5 pm. Tests were carried out in several stages, connecting different combinations of sensors to the interrogation device each time. First the two OS310 Bragg sensors were used for our data acquisition. Next the two OS110 sensors were employed for the same purpose. To conclude the process the criterion was to connect the two sensors that were closer to the framework, namely, one OS310 and one OS110, on the basis that the strain tends to become greater toward the framework. Most of the tests were done with bending amplitudes ranging from þ10 to −10 cm, and some tests were repeated with amplitudes between þ5 and −5 cm. Since the temperature affects the values of the parameters of Eq. (3), we measured the temperature of the room during the tests in order to make the necessary corrections. For this purpose, an LM35 analog temperature sensor with an amplifier circuit was employed. Apart from the POF-based and the FBG-based methods, during the tests we also used strain gauges placed at both sides of each OS110 sensor. They were employed to measure the strain cyclically, namely, each time the flap acquired its maximum bending deformation during the process of bending it downward and upward. Specifically data were collected when the displacements of the end of the flap were 0, þ10, and −10 cm. 10 March 2009 / Vol. 48, No. 8 / APPLIED OPTICS

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4. Experimental Results

The stability of the output signal is a key feature to validate the use of any of the aforementioned types of sensors for SHM systems. In the case of the POFbased sensors, it is important to achieve repeatable measurements of the phase shift for the correct calculation of the elongation of the fibers under strain. In this respect Fig. 7 shows satisfactory results since, at the end of each bending cycle, the obtained phaseshift measurements are the same. The results correspond to a test of deflection using POF sensors with bending amplitudes ranging between þ10 and −10 cm. At points 1, 4, 7, and 10 of Fig. 7, which correspond to a null elongation, the errors in the measurement of the elongation are clearly noticeable, as shown in Table 1. Each row of the table shows how much the voltage of the phase comparator changes from the point chosen as reference to the following ones. Although the data shown in Table 1 are voltages, we also obtained the relationship between the phase shift in volts and the deflection of the flap. This relationship could be deduced from several measurements. Since a voltage difference of 14 mV was observed between the bending amplitudes of 10 and 0 cm, we can deduce that an uncertainty of 0:38 mV in the measurement would correspond to an uncertainty of 2:7 mm in the deflection of the flap. There are many reasons that may justify the uncertainties obtained in the measurements. For example, they may be due to the limited resolution of the actuator, to little vibrations, to the limited resolution of the data-acquisition board, or to dilations and contractions in the flap produced by changes in the room temperature. Figure 8 shows the results of a continuous test with cyclic upward and downward movements. It can be seen that there are no noticeable changes in the maximum and minimum voltages of the phase-shift comparator from one cycle to another. Finally we studied the drift of the POF sensor with time. Figure 9 shows measurements carried out during 5 min when the actuator was static at the 0 cm point. The average value was 1:034 V, the standard deviation was 9e − 5 V, and the difference between

Fig. 7. Stability of the deflection measurements. Results of bending the flap upward and downward 10 cm at 40 mm=s are shown. 1440


Table 1.

Voltage Differences between the Points of Null Elongation Marked in Fig. 7

Voltage Difference between Points Reference Point

Point 4

Point 7

Point 10

1 4 7

0:18 mV

0:38 mV 0:20 mV

0:34 mV 0:16 mV −0:04 mV

the maximum and minimum voltages was 5e4 V, which is equivalent to 0:3 mm. This small value can be due to little vibrations in the structure. We also made many different tests including simple, cyclical, and step-by-step ones. For example, in Fig. 10 we show the results of an upward–downward test of three cycles of duration at a rate of 10 mm=s. They were obtained using different types of sensors in the LVDT [Fig. 10(a)], in the strain gauges [Fig. 10 (b)], in the OS110 FBG sensor closest to the framework [Fig. 10(c)], and in the POF-based sensor [Fig. 10(d)]. The rate of the data collected from the LVDT and the strain gauges is lower than that of the data-acquisition system used with the Bragg gratings or the POF. Since the rate was too low, the measurements in Figs. 10(a) and 10(b) were only carried out at the ends of the bending cycles or at the 0 position. On the contrary, with the FBG sensor and the POF-based sensor, the data acquisition was continuous at a rate of 100 Hz. Therefore there are more sample points in the respective graphics [Figs. 10(c) and 10(d)]. Although the best results are those obtained with FBGs, the performance of POF-based sensors is also satisfactory. The strain gauges were placed in two pairs at the sides of the two OS110 FBG sensors, as shown in Fig. 11, which is a photograph of the pair located farthest from the framework that holds the flap. The distance from the framework is the same for the two strain gauges, and the same happens with the other pair, i.e., each pair is placed in the transverse direction. To facilitate the comparison of the four strain values corresponding to the four locations, we number the strain gauges as follows: Gauge

Fig. 8. Continuous deflection. Results of the POF sensor when the flap is continuously bent upward and downward in the range of 10 cm are shown. The horizontal dotted lines serve to check the stability of the output.

Fig. 9. Drift of the POF sensor with time. A 5 min period of measurements carried out when the actuator was static at the point 0 cm is shown.

1 and Gauge 2 are, respectively, the two gages of Fig. 11 starting from the top, the bottom being closer to the longitudinal border of the flap, and Gauge 3 and Gauge 4 are numbered with the same criterion (starting from the top), the only difference being that they are placed at the sides of the other OS110 FBG sensor. For any deflection of the flap different from zero, the strains measured with the four strain gauges were rather different, even between gauges of the same pair. If we compare the results of the OS110 FBG sensors with those of the strain gauges, only in the case of Gauge 4 did the results match those of its closest FBG sensor. Specifically the

Fig. 11. Photograph of the pair of strain gauges located farthest from the framework that holds the flap (Gauges 1 and 2, respectively, starting from the top).

value of Gauge 4 with the maximum deflection was 205 μstrain, whereas the value of Gauge 3 was 168 μstrain. The difference between both values seems to indicate differences in the structure of the flap in the transverse direction. We also carried out a step-by-step test from 0 to 10 cm with steps of 10 mm and a velocity of 10 mm=s. In this test there was a certain similarity between the results of the OS310 FBG sensor closest to the framework and those of Gauges 3 and 4. There also was a certain similarity between the results of

Fig. 10. Deflection measurements when bending the flap upward an downward in the range of 10 cm: (a) positions of the actuator, (b) strains undergone by the strain gauges, (c) strains undergone by the OS110 FBG sensor, and (d) voltage of the POF-based system. 10 March 2009 / Vol. 48, No. 8 / APPLIED OPTICS

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Fig. 12. (a) Deflections, (b), (c) strains, and (d) voltages obtained as a result of bending the flap upward and downward up to 10 cm with steps of 1 cm: (a) positions of the actuator, (b) strains measured by the strain gauges, (c) strains measured by FBG Sensors 1 and 2, and (d) voltages yielded by the POF-based system according to the phase shift.

the OS310 FBG sensor closest to the actuator and those of Gauges 1 and 2. However, the strains close to the actuator were very different from those close to the framework. For example, the first step upward was detected correctly by the sensors close to framework, and it remained unnoticed by the sensors close to actuator. At the end of the bending (þ10 cm), there was also a worse measurement of the step amplitudes in the case of the FBG sensor and the strain gauges located closer to the actuator. While the POF-based sensor is subjected to the strains of the entire structure, the FBG sensor and the strain gauges are only subjected the local strains. Therefore measurements with the POF-based sensor yield an average response. All these results (of the step-bystep test) can be observed in Fig. 12. Although the analysis presented here seems to corroborate the suitability of using a POF-based sensor for SHM of a flap of an aircraft, similar measurements could be carried out to test the behavior of the flap in the case of having random vibrations, quicker movements, or larger loads. The setup could also serve to monitor cracks, bumps, and deformations in the flap in destructive tests. Another option would be to use piezoelectric actuators in order to study the sensitivity of the POF-based sensor when waves are generated by the piezoelectric device. It would also be possible to adapt the hardware for 1442


using it in the interior of an aircraft in flight. In this case it should integrate elements such as phase comparators, VCOs, light sources and photodiodes, and if necessary, a digital signal processor employing protocols of avionics such as Aeronautical Radio, Inc. Moreover it would be possible to use a central computer capable of processing and storing all the data from different sensors installed in various structures. 5.

Conclusions

The POF-based sensor has been proved to constitute a suitable tool to monitor the deformation of a piece of an aircraft (a rudder flap in the experiments) since the results coincide with those obtained with strain gauges or FBGs, but the proposed setup using POFs has added advantages. On the one hand, its cost is lower than that of a FBG. On the other hand, the setup is better in weight, wiring, and complexity than in the case of employing strain gauges. Another feature is that POFs yield information of the entire area in which the POFs are located, whereas FBGs or strain gauges only provide local information. Unlike FBGs, the proposed POF-based sensor, provided it is mounted correctly, does not require any correction for temperature variations since the two fiber branches are affected in the same way. Therefore if the temperature causes a

fiber elongation in one of the branches, the same elongation will occur in the other branch, so the phase difference is not altered. This work was supported by Ministerio de Educación y Ciencia, Universidad del País Vasco/ Euskal Herriko Unibertsitatea, Gobierno Vasco/ Eusko Jaurlaritza, Diputación Foral de Bizkaia/ Bizkaiko Foru Aldundia, and the European Union (EU) 7th Research Framework Programme under projects TEC2006-13273-C03-01, GIU05/03, EJIE07/ 12, SHMSENS, AISHAII, and AIRHEM. The Polymer Optical Fiber Application Center acknowledges funding through the Bavarian Research Foundation within ForPhoton. References 1. J. M. López-Higuera, ed., Handbook of Optical Fiber Sensing Technology (Wiley, 2002). 2. H. Poisel, M. Luber, S. Loquai, Neuner, and A. Bachmann, “POF strain sensor using phase measurement techniques,”

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presented at the 16th POF Conference 2007, Turin, Italy, 10–13 September 2007. S. Kiesel, K. Peters, T. Hassan, and M. Kowalsky, “Large deformation in-fiber polymer optical fiber sensor,” IEEE Photonics Technol. Lett. 20, 416–418 (2008). S. Kiesel, K. Peters, T. Hassan, and M. Kowalsky, “Behaviour of intrinsic polymer optical fibre sensor for large-strain applications,” Meas. Sci. Technol. 18, 3144–3154 (2007). M. Silva-López, A. Fender, W. N. MacPherson, J. S. Barton, and J. D. C. Jones, “Strain and temperature sensitivity of a single-mode polymer optical fiber,” Opt. Lett. 30, 3129– 3131 (2005). Analog Devices, AD8302 Datasheet, www.analog.com/static/ imported‑files/data_sheets/AD8302.pdf. Micron Optics, OS110 and OS310 Fiber Bragg Grating Sensor Datasheet, http://www.micronoptics.com/pdfs/os1100.pdf, http://www.micronoptics.com/pdfs/os3100.pdf. H. Zhao, B. Zhang, R. Wang, Z. Wu, and D. Wang, “Monitoring of composite pressure vessel using two kinds of fiber optic sensors,” presented at 6th International Workshop on Structural Health Monitoring, Stanford University, Stanford, Calif., 11–13 September 2007.

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