Study On The Key Technology Of All-fiber Low-frequency Vibration Sensor

Low-frequency vibration measurement technology has been playing important roles in monitoring of high buildings, mechanical equipment, aerospace, earth activites etc. The smart structure integrated with main structure, sensors and controllers is the development trend of measurement technology, which is capable of self-sensing, self-diagnosise, self-adaption and self-mending. The optic-fiber sensor is promising in detecting low-frequency vibration duo to the ability of anti-interference, multiplexing, remote measurement etc. However, it has some disadvantages, such as bulky, insensitive to low-frequency vibration and cross-sensitive to the temperature. In order to solve the problems, this thesis analyzes the theory model, damping and resonant frequency of vibration response structure. Also, three different all-fiber compact low-frequency sensors are proposed and demonstrated in design, fabrication method, optimization and environmental response characteristics systematically.The main contents are as follows:① We have studied the vibration model, damping and resonant frequency of the inertia low-frequency vibration sensor, analyzed the reasons and optimization methods of the damping loss. The theoretical analysis shows that a micro fiber is suitable for a small-damping vibration beam, because the ratio of surface to volume is small, the clamp is integrated with the beam, the surface is smooth after annealing and it is non-plastic. Therefore, the micro-fiber low-frequency vibration sensor is promising in academic research and potential applications in practice.② We have proposed and demonstrated an all-fiber support-beam low-frequency vibration sensor based on the hollow core fiber(HCF) and the micro structure fiber(MSF). The advantages are as follows:(1) The sensor is compact for the vibration response structure is integrated in the fiber;(2) The sensor has high sensitivity, because it can measure the deformation of the whole beam, but not the deformation of a part of the beam as that in traditional Fabry-Perot(FP) sensor based on mechanical vibration beam;(3) The temperature cross sensitivity has been decreased by using suitable quartz tube, and the experimental results show that the error is only 2.5% with the temperature fluctuation from 20°C to 120°C.③ We have proposed and demonstrated an all-fiber pendulum low-frequency vibration sensor based on micro structure fiber Bragg grating(MSFBG). The advantages are as follows:(1) The detectable vibration-frequency range of the sensor is low enough by measuring the deformation of the bended MSFBG, and the experimental results show that the sensor can detect random low-frequency vibration within a certain amplitude, even the static inclination.(2) It can realize the demodulation easily. The vibration signal is easy to be gained by demodulating the reflective intensity from the MSFBG.(3) The sensor can solve the problem of temperature cross sensitivity.④ We have proposed and demonstrated a quasi-distributed micro-fiber-based FBG sensor for simultaneous measurement of vibration and temperature. It can solve the multiplexing problem of point vibration sensors effectively. Meanwhile, the sensor has the advanteges of compactness and insensitivity to temperature.