Theoretical And Experimental Research On Dual FBGs Sensing Of Mechanical Vibrations

For the linear vibration,the vibration of torsion angle of rotating shaft,the vibration of gear tooth in various kinds of mechanical equipments,based on the research status and the existing problems of fiber optic grating sensing technique,using the merits of dual fiber optic grating,this paper carried out the following works:(1)Using the numerical integration method for FBG coupling equation,the effects of non-uniform temperature variation and strain on the FBG spectra have been discussed.Calculation results show that,for linear temperature change or strain(gradient terms),the FBG reflection spectrum is symmetrical about its central wavelength,and the central wavelength drift is decided by the strain and temperature change at the midpoint of FBG.The peak intensity decreases when the gradient increases.Progressively,when the fiber Bragg grating is subjected to second order temperature change or strain,the reflection spectrum of FBG is asymmetric;the positive and negative,and the magnitude of the second order temperature change or strain coefficient determine the strong or weak of the side lobe of reflection spectrum.(2)A monitoring method for the linear vibration based on light intensity of dual FBGs has been proposed.The simulations show that the main lobe area of dual FBGs has a good linear relationship with the central wavelength difference of two reflection spectra within certain range,by which,the working region of the center wavelength difference of matching FBGs has been puts forward.The monitoring experiments for linear vibration show that,the magnitude of voltage signal is linear with the magnitude of vibration acceleration,and also is linear with the cosine function of inclination angle of sensor.When the inclination angle is less than 15°,the change of voltage amplitude less than 5% of the voltage amplitude as the sensor was positioned horizontally.(3)Fixing dual FBGs on the shaft surface along certain angles respectively,the quantitative relation between the center wavelength difference of dual FBGs versus the torque and torque angle has been build.The time relationship of vibration center wavelength difference of dual FBGs can be divided as the sum of ‘DC' component and ‘AC' component,the former is related to the torque and torsion angle,and the latter depicted the vibration of torsion angle.Fixing dual CFBGs on the shaft along certain angles,a monitoring method for the angle vibration based on light intensity of dual CFBGs has been proposed.The experimental results show that,the time relationship of voltage signal can be divided as the sum of ‘DC' component and ‘AC' component,the ‘DC' voltage is related to the torque and torsion angle,and the ‘AC' voltage depicted the vibration of torsion angle.(4)Pasting the dual FBGs on two edges of gear tooth,an online monitoring experiment for the extension and compressive deformation and angle vibration of gear tooth have been carried out by wavelength demodulation method.The experimental results show that:(1)the characteristic wavelengths of the dual FBGs exists different overall wavelength drifts;it shows the increasing in temperature during the gear meshing process.(2)The peak magnitude of the vibration curve of characteristic wavelength versus time is linear to the load torque within a small torque range;also depict the extension and compressive deformation and angle vibration of gear tooth.