Study On Fiber Microbending Sensing Technology

A novel engine knocking sensor with fiber microbending technology was described in this paper. The conventional knocking sensor uses piezo-electricity or hysteresis telescopic principle to detect the knocking signal. Firstly, the microbending fiber sensor was introduced to detect knocking phenomenon. After analyzing mode-coupling theory, a procedure was programmed using MATLAB. The software OptiBPM was also used to analyze microbend waveguide. According to theory analysis, a lot of key parts were designed, and a number of corresponding experiments were done. Finally, optical fiber microbending sensor was used on detecting engine knocking phenomenon.The main work of this paper was described as following:1. The mode-coupling theory was used to analysis the relation among kinds of parameters for single mode step-index microbending fiber. When a fiber is subjected to axial deformation, the mode coupling occurs between the guided modes and the cladding modes. The attenuated stage of optical power coupled to the cladding modes is heavily due to lots of parameters of fiber structure and deformer (i.e. core radius, cladding radius, refractive index of core, refractive index of cladding, microbend spatial period, number of microbends, the length of the microbend deformation region, amplitude of deformation and wavelength of light source etc.). The comparison between the calculation results based on mode coupling theory and G. Murtaza's experimental measurements had been demonstrated over a range of microbend periods with a maximum percentage difference of less than 2.36%. The calculated mode was demonstrated to be correct. By analyzing the relation among lots of parameters, it is helpful to design the structure of fiber microbending sensor in the future.2. The 2D-FDBPM was used to analyze the microbend phenomenon of single mode step-index fiber. Comparing with mode coupling theory, advantages of FDBPM are electronic field profile detected in axial direction of the fiber, magnetic field profile, and the actual level of attenuation at maximum determined. The effective refractive method was utilized to transfer 3D light waveguide to 2D light waveguide in 2D-FDBPM. Using a center-based finite difference scheme to substitute partial differential, Crank-Nicholson scheme with implicit finite difference format of being unconditionally stable was deduced. The transparent boundary condition was introduced, and the boundary problem of waveguide was solved. Linear system of equations was solved using numerical method. The software OptiBPM was used to simulate microbend fiber waveguide. The comparison between the simulation results with OptiBPM and G. Murtaza's experimental measurements had been demonstrated over a range of microbend periods with a maximum percentage difference of less than 3.66%.3. The experimental design of periodic microbending fiber sensor. A halogen light source was designed and manufactured to detect serial infrared spectrum, and it could be inserted into fiber. A circuit was designed to supply the electric energy to infrared emitting diode and receive detecting signal. Impedance conversion for signal was done, to convert high impedance input to low impedance output. Then, the signal was amplified, using operational amplifier to make the current signal to voltage signal. A filtering circuit was used to resist self-oscillation. The structure of connector between infrared emitting diode or infrared photodiode and FC ring flange was designed. The parameters about deformed structure were designed based on mode coupling theory. The AutoLISP language was used to generate the sine curve in AutoCAD and the deformation tooth was manufactured, using linear cutting machine. The scale plate was designed and manufactured with numerical control machine. The corresponding experiments were done about periodic microbending structured. The error in rake angle between theory calculated result and experimental result was about 0.3195%.4. The experimental design of non-periodic microbending fiber sensor with the shape of a figure of eight. The design of this type sensor is smart, and the geometry allows microbending radii to change freely without any other mechanical deformer. Its geometry allows the natural stiffness of the fiber to act as a restoring force which holds the shape of the sensor during extension and compression. Cause the performance of sensor is similar either in extension or in compression. The linearity between gage length and output voltage of this type sensor was about 0.87121%.5. To design knocking sensor of microbending fiber sensor. By detecting engine oscillation, to adjust the igniting time is the main principle of the knocking sensor at present. A knocking sensor with cantilever beam was designed; a knocking sensor with resonance was designed; a knocking sensor with off-resonance was designed. The modal of vibration for the knocking sensor with off-resonance was built, and time domain analysis and frequency-domain analysis were done. Combining the natural performance of knocking phenomenon, the limit to the natural frequency and the ratio of damping was introduced for the knocking sensor with off-resonance. The electromagnetic damping was used to achieve the function of damping. The method about how to detect the natural frequency and the ratio of damping was also illuminated.