Research On FBG Accelerometer With Low Frequency

Fiber Bragg grating(FBG)accelerometet play an important role in the field of low frequency vibration measurement due to its many advantages,including high sensitivity,high accuracy and high SNR.In this dissertation,in-depth studies are made on structure design and signal demodulation technology of low-frequency FBG accelerometer.The mainly results achieved in this dissertation are listed as below.(1)From the basic theory of FBG sensing technology and mechanical model of accelerometer to start,the paper analyze the basic design methods of FBG accelerometer and propose a FBG accelerometer based on L-shaped rigid beam and elastic diaphragm.The structure of this accelerometer is analyzed theoretically,and the influence of each parameter on the sensitivity and resonant frequency is discussed by simulation analysis with Matlab.Optimization design is made on the basis of simulation.To enhance the immunity to interference,this structure is improved to a two Bragg gratings symmetry push-pull structure,and the sensing characteristics are analyzed.(2)To realize the precise demodulation of the accelerometer with one FBG,a FBG demodulation system is designed and constructed based on Michelson interferometer PGC technology.The factors which affect the degree of coherence of this system are theoretically analyzed.The paper effectively improve the quality of output signal from system by controlling the arm difference of interferometer and eliminate random phase shift and polarization decline respectively by PGC and FRM technology.To realize the precise demodulation of the accelerometer with two FBGs,another FBG signal demodulation program based on grating matching method is proposed.The theoretical simulation indicate that this demodulation system have simple structure and convenient signal processing method.Also this system can compensate the wavelength shift of FBG which are from noise because two FBGs are packaged in this accelerometer together and compose this symmetry push-pull structure.(3)To enhance the performance in low-frequency detection of these FBG low-frequency vibration sensors,the manufacturing methods of sensing FBG are studied.The factors which affect the reflectivity and the reflective spectrum width of FBG are theoretically analyzed and a fiber hydrogen-loading system which can control temperature is developed.This system can realize the fast photoactivation of fiber and shorten the period of hydrogen-loading to 24 hours.By this method,a FBG which length is only 5mm with 99.93% reflectivity and 0.13 nm reflective spectrum width is made and be used for the sensor with one FBG.Relying on new FBG production system and backup bar with especial structure,a pair of matching apodized FBGs are made by controlling the exposure time of different position,these two FBGs have beautiful reflective spectrum which correspond to the simulated results.(4)The two different FBG accelerometers are made.The sensing performances of low-frequency signals are tested experimentally.Experimental results show that the resonant frequency of the accelerometer with one FBG is 115 Hz.It has flat response from 20 Hz to 70 Hz and the mean sensitivity is 200 pm/g with the linear coefficient of 99.98% and the interference degree can be less than-32.73 dB which correspond to the theoretical analyses.The accelerometer with two FBGs symmetry push-pull structure mainly measures the vibration signals which frequency are below 20 Hz.The experimental results show that the accelerometer possesses high sensitivity which can reach as high as 188pm/g between 2Hz and 5Hz and high linear response degree which is more than 99%.Especially,this FBG accelerometer also gets good output when the frequency of input acceleration signals at 1Hz and 0.5Hz.The experimental testing proves that this low-frequency acceleration sensor system has high SNR at very low frequency range and can be used for high performance detection of very low frequency vibration.