Research On The Key Technology Of Fiber Bragg Grating Vibration Sensing

Vibration measurement which is widely used in structural health monitoring of large buildings, large modern equipment, transportation, aerospace and some other fields is an important research topic in the field of engineering and technology currently. With the improving of fiber Bragg grating(FBG) sensing technology and the advantages of AntiEMI, good electric isolation, capability of multiplexing, etc. FBG vibration sensing technology has become one of the most widely used sensing technology in the field of health monitoring.This paper studies the key technology of FBG sensing. Firstly, the development status of FBG sensing technology at home and abroad is reviewed. The coupled-mode theory of FBG is introduced. The strain, temperature and vibration sensing principles of FBG are analyzed.Secondly, a cantilever FBG vibration sensor is designed. Theoretical analysis of sensor’s mechanical model is given. Using Workbench finite element software, a simulation model of the sensor is constructed and the sensor’s related features are analyzed. The result of stiffness analysis verifies the reliability of the simulation modeling; The result of modal analysis gives the natural frequency of the sensor; The best fixed position of FBG on beam is obtained through the analysis of the static strain on beam; The analysis of simulation frequency response curve shows that the operating frequency of the sensor ranges from 0 to 38 Hz.Thirdly, three intensity demodulation FBG vibration sensing methods are introduced.The key factors which may affect the performance of FBG demodulation system based on these methods are analyzed. The demodulation sensitivity and range of three methods are compared by Matlab software. Then, the trends of the demodulation sensitivity and range of three methods are given when 3 dB bandwidth of sensing FBG is changed.Finally, a fiber Bragg grating vibration demodulation system with temperature compensation is developed. Using two transmission mode matching FBGs with center wavelengths symmetrically located at both sides of that of sensing FBG, two symmetrical and complementary sensing channels are established in the system and then the vibration demodulation is realized based on difference principle.The system performance is simulated by Matlab software and the results show that compared with traditional method, the proposed method can effectively improve the demodulationsensitivity and range of the system. The matching FBGs are packaged together with a TEC and a judging channel of temperature compensation is established. When the output of judging channel exceeds the threshold set previously for temperature regulation judgment, the center wavelengths of matching FBGs are re-matched with the sensing FBG by changing the package temperature of matching FBGs with TEC controller,thereby realizing the temperature compensation of the system. The experimental results show that the normalized system demodulation sensitivity is 5.168/nm, the demodulation range is 1.2 nm, and the normalized threshold voltage of judging channel is 0.8905.The feasibility of system temperature compensation and judgment mechanism is demonstrated by measurement of the same vibration signal under four different environment temperatures using the developed demodulation system.