18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS TEMPERATURE COMPENSATED FBG STRAIN SENSORS FOR MONITORING OF WIND TURBINE BLADE K. Choi 1 , G. Kim 1 , C. Kim 1 , I. Kwon 1* , D. Yoon 1 1 Center for Safety Measurement, Korea Research Institute of Standards and Science, 1 Doryong-dong, Yuseong-gu, Daejeon, South Korea * ibkwon@kriss.re.kr Keywords : FBG strain sensor, temperature compensation, wind turbine blade, dual FBG 1 Introduction of fiber core material, and K and K are strain and T The size of wind turbine blades has gradually been temperature coefficients respectively. increased to improve the efficiency of wind power (3) T K K 1 generation in recent years. These big wind turbine 1 T 1 b 1 K K ( K K K K ) blades may experience harsh environment such as 2 T 2 b 2 1 T 2 2 T 1 gust wind, bird strike, and lightning etc that make For two Bragg wavelength shifts to be observed, the some damages in the blades [1]. Therefore, the resulting two equations can be expressed in a matrix monitoring of strain and temperature of the blades is form. The changes of temperature and strain can be very important to operate them safely, and can calculated in the equation (3) by measuring reduce their maintenance costs. Fiber optic sensors experimentally both of the Bragg wavelength shifts. have been deeply considered in blade damage detection techniques such as acoustic emission and ultrasonic detection [2-6]. However, fiber optic 3 Analysis of dual FBG sensor probe sensors need to compensate temperature effects in 3.1 Fabrication strain sensing signals [7]. In this study, a dual fiber A dual FBG sensor probe is fabricated with epoxy Bragg grating (FBG) sensor probe molded with material that is appropriate to use to bond on the epoxy is proposed to measure the strain with temperature compensation of wind turbine blades. surface of wind turbine blade composites, and the probe size is 70 3 3, shown in Fig. 1. One FBG Both temperature and strain dependencies of the epoxy molded FBG probe are investigated and (FBG2) is surrounded by a fiber zirconia stub and analyzed. Also, some FBG sensor signals are two ends of this stub are thermally cured by high acquired from a blade bending test and analyzed to temperature epoxy (Tg-90degree). Another (FBG1) get the deformation behavior of the blade. is just a normal FBG. 2 Sensing principle of a dual FBG sensor The Bragg wavelength ( B ) of fiber Bragg grating is affected by strain and temperature changes. The relative change in the B due to strain and Fig.1. Structure of the dual FBG sensor probe temperature change is expressed as molded by epoxy. [( ) T (1 p ) ] (1) B B e 3.2 Strain and temperature sensitivities (2) K T K A composite specimen of wind turbine blade is B T attached with three dual FBG sensor probes, an where is thermal expansion and is thermo-optic electric strain gauge, and a thermo-couple as shown coefficient, P is the effective photo-elastic constant in figure 2 (a). The temperature sensitivities of these e three probes are investigated by a commercialized
FBG interrogator during heating up process using a forced convection oven. In the graph of Fig. 2 (b), the temperature sensitivities of normal FBGs are fitted to about 40 pm/°C, and that of covered FBGs are about 20 pm/°C. The strain sensitivities of normal and covered FBGs are about 1.0 pm/micro- strain and 0.20 pm/micro-strain, respectively. (a) Measurement spectral peaks 0.12 probe 1 probe 2 0.10 probe 3 probe7 Wavelength shift (nm) probe 8 0.08 probe 9 0.06 (a) 0.04 2.5 P1-1 P1-2 0.02 P2-1 P2-2 2.0 P3-1 P3-2 Normal FBG Wavelength shift (nm) 0.00 1.5 0 50 100 150 200 250 300 1.0 Time (min) Covered FBG 0.5 (b) Single FBG probe 0.0 0.10 20 30 40 50 60 70 80 90 probe4(1) Temperature (degree) probe4(2) (b) 0.08 probe5(1) probe5(2) Wavelength shift (nm) Fig.2. (a) Composite blade specimen with dual FBG probe6(1) probe6(2) 0.06 probes, and (b) temperature sensitivities of dual FBG probes. 0.04 0.02 0.00 0 50 100 150 200 250 Time (min) (c) Dual FBG probes (a) Sensor location on blade web Fig.4. Bragg wavelength monitoring during installation of probes. 4 Application of FBGs on a blade 4.1 Installation of FBG sensor probes An FBG sensor array of six single FBG probes for strain monitoring, a temperature FBG probe, and three dual FBG probes are installed on the surface of a web in a 100 kW blade. As shown in Figure 3, the (b) Sensor attachment on blade web FBG sensor probes and the conventional electrical strain gauges are located on the upper and lower part Fig.3. Location of the FBG sensor probes and of the web at 1 m each through the whole length of electrical strain gauge attached on the web surface of the web. Figure 4(a) shows the spectral peaks of 100 kW wind turbine blades. FBG sensor probes measured by a commercialized
PAPER TITLE FBG interrogator. Bragg wavelength shifts are Two array of FBG sensor probes, 18 strain gauges monitored during four hours after bonding on the are used to monitor strain of the blade. Load and web surface as shown in Fig. 4 (b) and (c). deflection were monitored by two load cells and four LVDTs located as shown in table 1 respectively. 4.2 Blade test setup A 100 kW FRP blade with 20 FBG sensor probes and 18 electrical strain gauges installed along the FBG #1 FBG #2_1 FBG #3 400 FBG #4_1 FBG #5 FBG #6_1 length direction of a web surface are tested under FBG #7 FBG #8 FBG #9 Wavelength shift (pm) static load condition at center for safety 0 measurement of KRISS (Korea Research Institute of Standards and Sciences). This blade is loaded at two -400 points which are connected one load saddle and another spreader bar as shown in Fig. 5. The blade -2000 deflections are monitored at four locations by using -4000 four LVDTs (Linear Variation Displacement Transducer), and applied loads are also measured by 20 % 40 % 60 % 70 % Applied load two load cells mounted on the center of the load spreader bar and load saddle (see Fig. 5 and Table 1). (a) FBG sensors The maximum test loads are designed from 1000 N to 1070 N at the three load saddles. The blade is loaded by the increment, 20 % of the maximum load. SG on 1 SG on 2 SG on 3 400 SG on 4 SG on 5 SG on 6 The step load is applied up to 70 % at the load step SG on 7 SG on 8 SG on 9 of 20 %. 0 Strain ( ) -400 -2000 -4000 20 % 40 % 60 % 70 % Applied load (b) Electrical strain gages 1500 1200 Fig.5. Blade test setup with three point whiffle-tree Load (N) for static load test. 900 600 Table 1. Location of load saddle, load cell and 300 LVDT. 0 # Load cell Load saddles LVDT 0 100 200 300 400 500 600 700 Deflection (mm) (m) (m) (m) 1 5.8 4.8 4 (c) Load – deflection curve 2 9.6 6.8 7 3 9.6 9 Fig.6. Measurement data: (a) wavelength shifts of 4 11, tip FBG sensor probes, (b) electrical strain gages, and (c) tip deflection according to applied load. 3
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