Fibre Bragg Gratings (FBGs) offer several advantages including their immunity to electromagnetic fields making them excellent in situ sensors for feature extraction in electrical machines condition monitoring. However, the pre-requisite of bonding FBGs circumferentially on either the machine cast frame or stator windings can introduce undesired sensing characteristics. This is because the FBG relies on adhesives as the transfer medium for any sensed parameter between the machine and sensor. Whilst FBG sensors rely mainly on wavelength shift, an intolerably low signal-to-noise ratio will result in difficulty in measuring such shifts. As a complementary signature, differential optical power can be combined with wavelength shift to broaden the feature extraction capability of FBG sensors. This makes power level (dBm) an important sensing parameter for FBG sensors. The effect of varying number of bonding points on transmitted optical power is investigated using unstripped and stripped bare fibres as well as an actual FBG sensor. Increasing the number of bonding points beyond an optimum number has been observed to significantly attenuate the optical signal power level and quality for a given dynamic range. Hence, as the number of bonding points is increased, the level of attenuation should be closely monitored to ensure that the optimum number is not exceeded if excellent and accurate FBG sensing characteristics are to be realised.
2. ABB, FOCS applications and benefits, , 2014, [online], available: http://new.abb.com/power-electronics/focs/applications-and-benefits, [accessed: 23-Nov.-2017].
3. Regina, M., et al., "A guide to fiber bragg grating sensors," Current Trends in Short- and Long-period Fiber Gratings, InTech, 2013.
4. Marignetti, F., et al., "Fiber Bragg grating sensor for electric field measurement in the end windings of high-voltage electric machines," IEEE Transactions on Industrial Electronics, Vol. 63, No. 5, 2796-2802, May 2016.
5. Mohammed, A., N. Sarma, and S. Djurovic, "Fibre optic monitoring of induction machine frame strain as a diagnostic tool," 2017 IEEE International Electric Machines and Drives Conference (IEMDC), 1-7, Miami, FL, 2017.
6. Konforty, S., et al., "Bearing health monitoring using optical fiber sensors," European Conference of the Prognostics and Health Management Society, 1-7, Spain, 2016.
7. Jones, K., C. Staveley, and J. F. Vialla, "Condition monitoring of a subsea pump using fibre optic sensing," Proc. SPIE 9157, 23rd International Conference on Optical Fibre Sensors, 2014.
8. Sousa, K. D. M., A. A. Hafner, H. J. Kalinowski, and J. C. C. da Silva, "Determination of temperature dynamics and mechanical and stator losses relationships in a three-phase induction motor using fiber bragg grating sensors," IEEE Sensors Journal, Vol. 12, No. 10, 3054-3061, Oct. 2012.
9. Sousa, K. M., I. Brutkowski Vieira da Costa, E. S. Maciel, J. E. Rocha, C. Martelli, and J. C. Cardozo da Silva, "Broken bar fault detection in induction motor by using optical fiber strain sensors," IEEE Sensors Journal, Vol. 17, No. 12, 3669-367, Jun. 15, 2017.
10. Vilchis-Rodriguez, D. S., S. Djurovic, P. Kung, M. I. Comanici, and A. C. Smith, "Investigation of induction generator wide band vibration monitoring using fibre Bragg grating accelerometers," 2014 International Conference on Electrical Machines (ICEM), 1772-1778, 2014.
11. Mohammed, A. and S. Djurovic, "Stator winding internal thermal stress monitoring and analysis using in-situ FBG sensing technology," IEEE Transactions in Energy Conversion, Vol. 33, No. 3, 1508-1518, 2018.
12. Hudon, C., M. Levesque, M. Essalihi, and C. Millet, "Investigation of rotor hotspot temperature using fiber bragg gratings," 2017 IEEE Electrical Insulation Conference (EIC), 313-316, 2017.
13. Liu, H., W. Chen, P. Zhang, J. Wun, and L. Liu, "Optimization for metal bonding technology of optical fiber sensor," 2011 International Conference on Optical Instruments and Technology: Optical Sensors and Applications , Vol. 8199, 819910, 2011.
14. Picazo-Ródenas, M. J., J. Antonino-Daviu, V. Climente-Alarcon, R. Royo-Pastor, and A. Mota-Villar, "Combination of noninvasive approaches for general assessment of induction motors," IEEE Trans. Ind. Appl., Vol. 51, No. 3, 2172-2180, 2015.
15. Her, S. and C. Huang, "Effect of coating on the strain transfer of optical fiber sensors," Sensors (Basel), Vol. 11, No. 7, 6926-6941, 2011.
16. Zhang, W., W. Chen, Y. Shu, X. Lei, and X. Liu, "Effects of bonding layer on the available strain measuring range of fiber Bragg gratings," Applied Optics, Vol. 53, No. 5, 885, Feb. 2014.
17. Helminger, D., A. Daitche, and J. Roths, "Glue-induced birefringence in surface-attached FBG strain sensors," 23rd International Conference on Optical Fibre Sensors, Vol. 9157, 91577B, 2014.
18. Zhang, W., W. Chen, Y. Shu, J. Wu, and X. Lei, "Degradation of sensing properties of fiber Bragg grating strain sensors in fatigue process of bonding layers," Optical Engineering, Vol. 53, No. 4, 46102, Apr. 2014.
19. Li, W. Y., C. C. Cheng, and Y. L. Lo, "Investigation of strain transmission of surface-bonded FBGs used as strain sensors," Sensors Actuators A Physical, Vol. 149, No. 2, 201-207, Feb. 2009.
20. Wan, K., C. Leung, and N. Olson, "Investigation of the strain transfer for surface-attached optical fiber strain sensors," Smart Materialsand Structures, Vol. 17, No. 3, 35037, Jun. 2008.
21. Wang, Q., et al., "Analysis of strain transfer of six-layer surface-bonded fiber Bragg gratings," Applied Optics, Vol. 51, No. 18, 4129, 2012.
22. Li, J., Z. Zhou, and J. Ou, "Interface strain transfer mechanism and error modification for adhered FBG strain sensor," Proceedings of Fundamental Problems of Optoelectronics and Microelectronics II, Vol. 5851, 278-287, 2005.
23. Zhou, J., Z. Zhou, and D. Zhang, "Study on strain transfer characteristics of fiber Bragg grating sensors," Journal of Intelligent Material Systems and Structures, Vol. 21, No. 11, 1117-1122, Jul. 2010.
24. Kim, S., M. Jeong, I. Lee, I. Kwon, and T. Hwang, "Effects of mechanical and geometric properties of adhesive layer on performance of metal-coated optical fiber sensors," International Journal of Adhesion and Adhesives, Vol. 47, 1-12, Dec. 2013.
25. Cho, S., et al., "Effects of bonding layer characteristics on strain transmission and bond fatigue performance," Journal of Adhesion Science and Technology, Vol. 26, No. 10-11, 1325-1339, 2012.
26. Kwon, H., Y. Park, P. Shrestha, and C. Kim, "Signal characteristics of the surface bonded fiber Bragg grating sensors by bonding length under different load types," 2017 25th Optical Fiber Sensors Conference (OFS), 1-4, Jeju, 2017.
27. Zhang, Y., et al., "Comparison of metal-packaged and adhesive-packaged fiber Bragg grating sensors," IEEE Sensors Journal, Vol. 16, No. 15, 5958-5963, Aug. 1, 2016.
28. Cheng, C., Y. Lo, B. S. Pun, Y. M. Chang, and W. Y. Li, "An investigation of bonding-layer characteristics of substrate-bonded fiber Bragg grating," Journal of Light. Technology, Vol. 23, No. 11, 3907-3915, 2005.