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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Advanced operation & maintenance' taking place on Thursday, 13 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Jae-Kyung Pan Chonbu National University, Korea, Republic of
Co-authors:
Jae-Kyung Pan (1) F Sang-Jin Choi (1) P
(1) Chonbuk National University, Jeonju, Korea, Republic of

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Prof. Pan received B.S., M.S., and Ph.D. degrees in electronic engineering from Yonsei University, South Korea, in 1980, 1982 and 1987, respectively. He joined the Department of Electrical Engineering at Chonbuk National University, South Korea, in 1987 and is now a professor of the Department of Electrical Engineering at Chonbuk National University. Also, since 2011 he is currently a director of Smart Grid Research Center at Chonbuk National University. His research interests are fiber optic sensors and vibration sensor for structure health monitoring.

Abstract

Dynamic strain measurement using self-referencing intensity-based fiber optic sensor for wind turbine blade monitoring

Introduction

In this paper, a novel self-referencing intensity-based fiber optic senor (FOS) and its head are proposed and demonstrated to measure the fiber reinforced plastic (FRP) dynamic strain, which can be applied to implement the strain monitoring patch for wind turbine (WT) blade monitoring. The proposed FOS with the self-referencing and the cascade and/or parallel multipoint sensing characteristics offers the better performance comparing with the conventional fiber Bragg grating (FBG) sensor.

Approach

A novel self-referencing intensity-based FOS shown in {F1}, which consists of a broadband light source (BLS), optical circulator (OC), fiber Bragg gratings (FBG), optical coupler, tunable Fabry-Perot filter (F-P filter), photodiode (PD) module, and LabVIEW program, is proposed and demonstrated for WT blade dynamic strain monitoring. The theoretical analysis of the proposed scheme is provided and the validity of the theoretical analysis is confirmed by experiments. The self-referencing characteristic of the proposed system is validated by showing that the measurement parameter is invariant for BLS optical power attenuations of 0 dB, 3 dB, and 6 dB. Also, the proposed system shows that the measured value is irrelevant to the FBGs with different characteristics. By connecting eight FBGs and six sensor heads (which can be in a remote sensing position in cascade and parallel forms), we demonstrate that the proposed FOS has a multipoint sensing characteristic. To apply the proposed FOS with the dynamic strain measurement, we implement an intensity-based FOS head which consists of a rectangular pulse train shaped steel wire and an optical fiber mounted on FRP surface. The measured data for FRP coupon strain are compared with those of a commercial FBG sensor and a strain gauge. The measured results show that the self-referencing intensity-based FOS with the proposed FOS head has good performance to measure the FRP coupon strain. The proposed FOS has several benefits, including the self-referencing characteristic, the multipoint sensing characteristic, the flexibility to determine the FBGs, and a simple structure in terms of number of devices and measuring procedure. The proposed intensity-based FOS head structure can be applied to implement the strain monitoring patch for WT blade.

Main body of abstract

The proposed intensity-based FOS shown in {F1}, which has self-referencing and cascade and/or parallel multipoint sensing characteristics, can be applied to the measurement of WT blade dynamic strain. The light from the BLS enters through eight FBGs and the intensity sensor head S_(m,n), which are located at the remote sensing points via the OC ports ① and ②. The reflected light from FBG_(m,n) and FBG_(m,n+1) return to the PD module via an optical coupler, OC, and tunable F-P filter. The reflected light from FBG_(m,n+1) includes the power modulation of the travel through the intensity sensor head S_(m,n), which enhances sensitivity.
To evaluate the performance of the proposed intensity-based FOS, we need a reference sensor head with a bidirectional optical power attenuation function. So, we designed and implemented one using a male screw with a diameter of 20 mm and an optical fiber, as shown in {F2}. To achieve the attenuation characteristics for the bidirectional variable optical attenuator, we measured the optical intensity at the output for the input optical power of -16.63 dBm with wavelength of 0.1 nm bandwidth at 1550 nm as a winding optical fiber on the male screw. {F2} shows the measured output power in dBm for input power of -16.63 dBm and the calculated H^2 according to the number of winding turns. {F2} also shows the power attenuation of 0.746 dB per turn and the squared transfer function H^2 with exponential curve, which is used as the reference curve in the experiment.
{F3} shows the experimental results of H^2 for the input optical power with 0 dB, 3 dB, and 6 dB attenuation. The results of this experiment are close to the reference curve in {F2}. The above result shows that the proposed FOS has the self-referencing characteristic, minimizing the influences of long-term aging of source characteristics and of short-term fluctuations in the optical power loss.
As shown in {F4}, the experimental results of H^2 for FBGs with different characteristics in {F5} are close to the reference curve in {F2}. These results show that an FBG with arbitrary characteristics can be used to implement the proposed fiber optic intensity sensor.
To test the multipoint sensing characteristic for the proposed FOS, we implemented the proposed FOS with eight FBGs and six sensor heads with loss in {F6-8}. {F6-8} show that the measured H_(m,n)^2 versus the number of winding turns for six sensor heads (S_2,1, S_2,2, and S_2,3 have losses of 1.5 dB, 2.0 dB, and 2.5 dB, respectively): {F6} S_1,1: varying loss, S_1,2: 0.5 dB loss, S_1,3: 1.0 dB loss; {F7} S_1,1: 0 dB loss, S_1,2: varying loss, S_1,3: 1.0 dB loss; {F8} S_1,1: 0 dB loss, S_1,2: 0.5 dB loss, S_1,3: varying loss, which are in good agreement with the reference curve in {F2}. The other measured H_(m,n)^2 for fixed sensor heads loss are in good agreement with the applied optical power loss in the experiment. However, the smaller reflected optical power from FBGs shows greater measurement error as shown in {F6-8}.
An intensity-based FOS head S shown in {F9} is made of rectangular pulse train shaped steel wire with height of 7.5 mm, periodic length of 15 mm, and total length of 190 mm. The optical fiber and wire are set to cross each other 25 times. The wire is bonded on FRP coupon with length of 220 mm by epoxy resin. The FOS head insertion loss is about 1 dB and the operation range up to 25 dB as applying the force. We measured FRP coupon strain with the proposed FOS head, the FBG sensor (I-MON 512E, Ibsen), and the strain gauge sensor (D4 Data Acquisition, Micro-Measurement) which are shown in {F9 (b)}. The measured data of FRP coupon strain according to the displacement (d) in {F9 (a)} with three types of sensors are shown in {F10}. The normalized RMSE and the maximum relative error of the intensity-based FOS, the FBG sensor, and the strain gauge sensor are 1.84 and 4.06 %, 2.61 and 6.07 %, and 2.37 and 4.86 %, respectively. These results show that the self-referencing intensity-based FOS with the proposed FOS head has good performance to measure the FRP coupon strain.


Conclusion

An intensity-based FOS with self-referencing and multipoint sensing characteristics has been proposed and demonstrated. The self-referencing characteristic for the proposed system has been validated by showing that the measurement parameter is invariant for BLS optical power attenuations of 0 dB, 3 dB, and 6 dB. The proposed FOS has the self-referencing characteristic, minimizing the influences of long-term aging of source characteristics and of short-term fluctuations in the optical power loss. Also, the measured H_(m,n)^2 is irrelevant to the FBGs with different characteristics. This means that the proposed FOS offers the flexibility for determining the FBGs needed for implementation. By connecting eight FBGs and six sensor heads, we have demonstrated that the proposed FOS has a multipoint characteristic for wind turbine blade monitoring. And an intensity-based FOS head consisting of a rectangular pulse train shaped steel wire and an optical fiber has been proposed and applied to the measurement of FRP coupon strain. The measured FRP strain data was compared with those of the FBG sensor and the strain gauge sensor. The normalized RMSE and the maximum relative error of the intensity-based FOS, the FBG sensor, and the strain gauge sensor are 1.84 and 4.06 %, 2.61 and 6.07 %, and 2.37 and 4.86 %, respectively. These results show that the self-referencing intensity-based FOS with the proposed FOS head has good performance to measure the FRP coupon strain. The proposed FOS has several benefits: the self-referencing and the multipoint sensing characteristics, the flexibility to determine the FBGs, and the relatively simple structure. The proposed self-referencing intensity-based FOS structure can be applied to implement the dynamic strain monitoring for WT blade.


Learning objectives
First objective is the proposal self-referencing intensity-based FOS and second objective is the FRP dynamic strain measurement using the proposed an intensity-based FOS head mounted on FRP coupon surface for WT blade dynamic strain measurement.


References
[1] J. M. López-Higuera, L. Rodriguez Cobo, A. Quintela Incera, and A. Cobo. “Fiber optic sensors in structural health monitoring”, J. Lightw. Technol., vol. 29, no. 4, pp. 587-608, Feb. 2011.
[2] K. T. Lau, L. Yuan, L. M. Zhou, J. Wu, and C. H. Woo, “Strain monitoring in FRP laminates and concrete beams using FBG sensors”, Compo. Struc., vol. 51, no. 1, pp. 9-20, Jan. 2001.
[3] D. S. Montero and C. Vázquez, “Remote interrogation of WDM fiber-optic intensity sensors deploying delay lines in the virtual domain”, Sensors, vol. 13, no. 5, pp. 5870-5880, May 2013.
[4] S. J. Choi, H. H. Kim, and J. K. Pan, “A multiplexed structure of intensity based fiber optic sensor”, OECC 2012, pp. 178-179, July 2012.