Advanced Materials Development and Performance (AMDP2011) International Journal of Modern Physics: Conference Series Vol. 6 (2012) 203-208 World Scientific Publishing Company DOI: 10.1142/S2010194512003182 LOAD CELL DESIGN USING FIBER BRAGG GRATING SENSORS Dong Jin Park, Min Kyu Kang, Jong Hyun Lee and Seok Soon Lee * Department of Mechanical Engineering, Gyeongsang National University 501,Jinju-daero, Jinju, Gyeonnam, 660-701, Republic of Korea * leess@gsnu.ac.kr Hyo Seok Jung Technical Research Center, MIRAE Industries Co.,Ltd, 162, Daechiri, Chilseomyeun, Hamangun Gyeongnam,637-941, Republic of Korea jhs@miraewinch.com A load cell is the representative converter that changes load to the quantity of electricity. The load cell is used to a large mechanical structure and offshore structures to measure the force. Currently, the load cell using electrical strain gauges are commonly used. Basic measuring principle of electrical strain gauge is the electrical method. A load cell with electrical strain gauges is not available in the electromagnetic and corrosion environment. A Fiber Bragg Grating (FBG) sensor is not affected by the EMI (Electro Magnetic Interference)/EMC (Electro Magnetic Compatibility) and is strong in corrosion under the sea water. In this paper, we use the FBG sensors to make a load cell under the sea water condition and the electromagnetic environment and show FBG sensors availability. Keywords: Load Cell; Fiber Bragg Grating Sensor; Health Monitoring. 1. Introduction Marine plants are exposed to a harsh sea water and high level electro-magnetic field. A load cell using electrical strain gauges is affected by electrical resistance and corrosion. To complement these problems, Fiber Bragg Grating sensors has been gradually expanding in the areas of mechanical engineering, aviation, civil engineering. But in the marine area, a load cell design of the FBG sensors is incomplete. In this paper, we design a load cell with electrical strain gauges and FBG sensors which is used in the fairleader of marine structures and compare experimental results. 203
204 D. J. Park et al. Fig. 1. Fairleader 2. Fiber Bragg Grating sensor A Fiber Bragg Grating sensor is a passive optical element that reflects a specific wave of incident light. A FBG sensor have characteristic that change reflected wavelength of light according to external environment change like strain or temperature. In the case of light source with a broad band spectrum is incidence to optical fiber, wavelength component is determined by Bragg condition like Eq. (1). A remainder wavelength is passed. 2n (1) B In Eq. (1), is a bragg grating spacing, is distance of wavelength, n e is validity refractive index of FBG. In case of irradiated grain with X-ray, X-ray is diffracted by a grain grid and the strong diffracted X-ray is created. Because these are reflected X-ray by a grain grid plane, this phenomenon is the bragg reflection of X-ray. A grating plane is a group of equally interval parallel atoms. The distance between the grating plane is the grating plane spacing, the angle between grating plane and X-ray is, wavelength of X-ray is, n is random integer, then the condition of bragg reflection is 2sin. This condition is bragg condition. e Fig. 2. FBG sensor s principle
Load Cell Design using Fiber Bragg Grating Sensors 205 We can know strain or temperature of FBG by the valid index of refraction and the grating plane spacing. We can quantitatively deduct strain or temperature applied to the FBG from Eq. (2), (3) [( ) T (1 p ) ] (2) B B e 2 n pe ( )[ p12 ( p11 p12 )] (3) 2 In Eq. (2), (3), p e is a photo elasticity coefficient, is strain, is Poisson s ratio of the optical fiber, T is temperature, is a thermal expansion coefficient. FBG sensor can measure the change of strain and temperature, and many FBG sensors can be arranged to a single optical fiber by the wavelength division multiplexing method. It can monitor the change of strain and temperature of various points by a single optical fiber. So the distance sensor network deployment is easy. 3. Experimental verification To verify accuracy of a FBG sensor performed experiment at cantilever beam. In this experiment, we compared the experimental results using electrical strain gauges and FBG sensors. 3.1. Experiment using strain gauge and FBG sensor In this experiment, we used 7mm strain gauges and an aluminum cantilever like Fig. 3. Fig. 3. Strain gauges attached to the cantilever Near a fixed area, two strain gauges are attached on the both plane, and add a downward 2mm displacement at the end of cantilever. An average strain is about 85 10-6 from the experiment using electrical strain gauges. In order to compare the results, FBG sensors are attached in the same area as in the previous experiment. We used FBG sensors and IS-7000 Interrogation system shown in Fig. 4 and Fig. 5. Fig. 4. IS-7000 Interrogation system
206 D. J. Park et al. Fig. 5. Measurement of strain using FBG sensor Experiment results are shown in Table. 1. An average strain is about 80 10-6 from the experiment using FBG sensors and shows the almost same value. Based on these verified results, we designed a load cell. Table 1. Comparing the results of experiment Average strain Using a FBG sensors 80.241 10-6 Using an electrical strain gauges 85.360 10-6 4. Load cell experiment A load cell in the fairleader is fixed on the both ends and the load is applied at the center. Considering these conditions, a load cell was designed. In order to determine an optimal location of each sensor, we performed a finite element analysis. A FEA result is shown in Fig. 6. 4 electrical strain gauges are attached on the maximum tension and compression areas in the specimen based on the FEA results. Each attached points of the sensors and the experimental setting are shown in the Fig. 7. In this experimental case, a compensating circuit must be considered. By constructing a bridge circuit with electrical strain gauges, the measured results using the indicator are shown in Table 2. Fig. 6. Finite element analysis results (a) (b) (c) Fig. 7. (a) Load cell (b) Experimental setting (c) Digital indicator
Load Cell Design using Fiber Bragg Grating Sensors 207 Table 2. Comparison between the applied load and indicator load Applied value (kn) 0 1.0 2.0 3.0 4.0 5.0 Indicator value (kn) 0 0.979 1.970 3.001 3.985 4.987 Error (%) 0 2.1 1.5 0 0.4 0.3 As shown in the Table. 2, applied value and indicator value are almost the same. This specimen is suitable for producing a load cell. And then, we attach FBG sensors to a load cell and compare experiment results with electrical strain gauges and with FBG sensors in Fig. 8. The correlation coefficient is close to 1 as shown in the Table 3 which means the results with FBG sensors are linear. Fig. 8. Comparison of strain gauge, FBG sensor and wet condition experiment Table 3. The correlation coefficient for each sensor Correlation coefficient Strain gauge (Compression) 0.9987 Strain gauge (Tension) 0.9999 FBG sensor (Compression) 0.9997 FBG sensor (Tension) 0.9996 As mentioned in the introduction, the load cell in the marine structure endures in the sea water environment. In order to verify corrosiveness of a FBG sensor, as shown in Fig. 9, we immerse the load cell in the water about 10minutes. Fig. 9. Immersed load cell in the water
208 D. J. Park et al. Fig. 10. Comparison of strain gauge, FBG sensor and wet condition experiment As shown in Fig. 10, there is no significant difference between the normal condition experiment and the wet condition experiment. Therefore we know that water does not affect FBG sensor. 5. Conclusion In this study, we verify a load cell manufacture possibility by measuring strains with electrical strain gauges and FBG sensors. From experimental results, there are some gaps due to the difference characteristic between an electrical strain gauge and a FBG sensor. A FBG sensor is narrow and long, but electrical strain gauge is relatively wide and short, therefore sensing areas are different. We verify the availability of FBG sensor in a wet condition. A load cell used in the seawater environment is exposed to a harsh environment physically and electromagnetically. So, a corrosion and electromagnetic environment must be considered. Therefore, research activity using FBG sensor in the seawater environment is required. References 1. D. H. Kim, Development of load Cell Using Fiber Bragg Grating Sensors and Differential Method for Structural Health Monitoring, Journal of the Korean society for nondestructive testing., Vol. 29, No. 4, (1999), p.299. 2. N. S. Kim and N. S. Cho, Estimation of Bridge Deflection Using Fiber Optic Bragg-grating Sensors, Journal of Civil Engineering., Vol. 22, No. 6A, (2002), p.1357. 3. J. H. Yi and C. G. Kang, Signal Processing and Performance of a Six-Axis Force-Torque Sensor Using Strain Gauges, Journal of control, automation and systems engineering., Vol. 7, No. 2, (2001), p.146. 4. R. H. Kang, D. K. Kim and J. H. Han, Estimation of Structural Deformation Using FBG sensors, KSAS, KSAS05-2426, (2005), p.553. 5. C. Y. Chang, J. Y. Lee and D. J. Yoon, Development of fiber Bragg grating corrosion monitoring sensor, KSAS, KSAS09-1119, (2009), p.203. 6. J. S. Heo and J. J. Lee, Development of Uniaxial Force Sensor Array for Tactile Sensation Using Fiber Bragg Gratings, KSME., Vol. 30, No. 9, (2006), p.1160.