Research On Fiber-optic Aircraft Fuel Level Measurement Based On Light Leakage In Optical Fiber

Aircraft fuel gauge is an important part in the aircraft fuel system. In the next genera-tion of aircraft technology, engineers require the aircraft fuel gauge to be safer, more reliable and more accurate. The optical fiber sensor (OFS) is reported to be intrinsically safe, and have the advantages of small size, light weight, more sensitive and resistant to electromag-netic interference. It has been listed as the prior technology in the development of future aircraft fuel gauging technology.This dissertation researched on light leaking type optical fiber liquid level sensor (OFLLS) based on light scattering, aimed to fulfill the requirement of the future aircraft fuel gauging technology. The dissertation focused on the following aspects:The dissertation firstly researched on the attenuation coefficient of SEOF with mode division in geometrical optics, aimed to explain the basic principle of the sensor and pro-vide theoretical foundation for sensor design. The scattering in the fiber was studied and divided into conducting mode, cladding mode, critical mode and radiation mode to explain the change of fiber’s lose in air and liquid. The expressions of the attenuation coefficient in SEOF were then derived based on the calculation of the ratio of each mode in scattering light, and the ways to increase the attenuation coefficient were also discussed.The dissertation proposed a side-coupling OFLLS to avoid the weak output changing of the direct measurement structure. It is based on the modulation of the sensor’s light leaking and coupling by the liquid level. The surface emergent of the emitting fiber and the side coupling efficient of the receiving fiber are the key parameters in the sensor, and were deeply studied in the dissertation. A liquid level sensor and the experimental setup were then fabricated, and the static experiments in the water were taken to demonstrate the sensor’s principle. The experimental results were carefully analyzed and the feasibility of using this sensor for aircraft fuel gauging was then discussed. The results show that the output of the sensor is weak for signal processing.The dissertation proposed a spiral OFLLS to get a larger output. The transmitting light in the fiber was directly detected to increase the sensor’s output power, while fiber curving was used to increase the sensor’s relative output changing ratio. A spiral bending was then used to make the fiber’s submerged length to be proportionate to the liquid level. Several sensors were fabricated, one with a steadily changing screw pitch, three with fixed screw pitches of15mm,30mm,45mm, respectively. Static experiments and dynamic experiments were taken to study the sensors’performances. The results show that the outputs of the spiral OFLLS are much larger than that of the side-coupling OFLLS. The works in this dissertation will be important in the development of a practical OFLLS for aircraft fuel gauging.