Study Of Low-frequency FBG Vibration Sensor System

Fiber Bragg grating (FBG) vibration sensor has become a new research field due to its many advantages, including high sensitivity, strong ability of anti-interference, good performance in low-frequency detection, and easy operation in multiplexing. In this dissertation, to realize high performance detection of low frequency vibrating signal, the FBG vibration sensor as well as its demodulation system are theoretically analyzed and experimentally studied, and based on which, three different structure of low-frequency FBG acceleration vibration sensors are developed.First, by analyzing the basic theory of FBG sensing technology, and the comparison of different design methods, three sensing configurations are proposed: metal-barrel encapsulated single cylinder mandrel structure, polyurethane encapsulated single-cylinder mandrel structure, and double-cylinder symmetrically push-pull mandrel structure. The theoretical analysis and simulation are performed for these types of sensors.To realize the precise demodulation of vibration signals, a FBG demodulation system is designed and constructed based on interferometer PGC technology, and the performance of this system is experimentally studied. The results show stable performance of the system with background noise below -90dB, detection precision of system wavelength deviation of 3.8×10?3pm, and detection precision of dynamic strain of 3.2nε. To increase the stability and low-frequency demodulation ability, and realize the double-FBG demodulation function, a Michelson interferometer double-FBG demodulation system is designed and constructed. By twin-channel signal matching technology, the system's ground noise can be effectively reduced. Experimental result shows that the background noise gets a decrease of 10dB when the operation frequency is below 200Hz.The contrast tests are made for each of the three types of sensors compared to the traditional piezoelectric acceleration sensors. The metal-barrel encapsulated single cylinder mandrel sensor has relatively simple structure and good performance. It is suitable for use below 200Hz with natural frequency of 388Hz and sensitivity of about 80pm/g. The Polyurethane encapsulated single cylinder mandrel sensor has high sensitivity of 450pm/g with natural frequency of 155Hz, which is more applicable in measurement of frequency below 100Hz. The double-cylinder symmetrically push-pull mandrel sensor has many advantages, including temperature self-compensation function, good performance of low-frequency detection, high stability, simple structure, and easy expansion to three-dimension. This type of sensor has natural frequency of 210Hz and sensitivity as high as 450pm/g.