Research Of Pressural Fiber Liquid Level Sensors

As a new sensing material, fiber has a lot of advantages.First of all, the recent developments in the area of optical fiber sensors, fiber grating sensors and the measurement of liquid level are reviewed in this thesis. And then, compared all kinds of measurement method, the conclusion is made that the pressural liquid level measuring strategy using optic fiber is a good method. The main contents in this thesis are:(l)In weakly guiding fibers, microbend loss in periodic axial microdeformation is deduced by the coupled mode theory between guide mode and cladding mode. Compared with microbend loss, refractive index distribution of FBG is periodic axial modulated, the reflection spectrum characteristics of FBG is deduced by the coupled mode theory between guide modes. And so, base on fiber microbend loss and FBG reflection spectrum characteristics,microbend fiber liquid level sensor and FBG liquid level sensor, as two pressure sensing method, are analysed in this thesis.(2) On the basic of theoretical analysis, the fiber microbend loss sensing experimental systems are developed and the characteristics of multimode fiber microbend loss are investigated experimentally. And then, the detail of designing process for the optic fiber pressural liquid level sensor is given in the thesis, including calculation of diaphragm and the microdeformation tooth plate.The system configuration and signal process circuits are analysed in.this part also. On the basis of the mechanical theory of elasticity, the central displacement formula of the fixed diaphragm is deduced. According to the result, the diaphragm thickness and interrelated parameters are calculated, and the displacement and the stress distribution are simulated using the finite element method by ANSYS. The emission circuit and photodiode monitoring circuit of microbend optical fiber liquid level sensor system are discussed in this thesis.The selection of light emitting diode (LED) and photodiode is introduced. Several circuits are selected to fulfill the function of this system, including integrated manostat circuit with LM317, preamplifier circuit with ICL7650 and Butterworth fourth-order low pass filter circuit.(3) On the basic of theoretical analysis, the characteristics of fiber Bragg grating strain and temperature sensitivities are investigated experimentally. And then, a FBG liquid level sensor system are designed in the thesis, including the system configuration and discrimination between strain and temperature effects using reference FBG Finally, various wavelenth demodulation technologies are also decribed in this thesis, include wavelenth-division coupler filter method, tunable Fabry-Perot cavity method,unbanlanced Mach-Zehnder interferometer method.