Study On An Optical Fiber Fabry-Perot Cavity Liquid Level Sensor By Interference Fringe Counting

Optical fibre liquid level sensor has been extensively investigated and applied for its intrinsic safety and high precision. An Optical fibre interference liquid level sensor with the ability of reducing the effects of light fluctuations and outside disturbances has been described. After thoroughly analysing and carefully comparing the most of liquid level measurement methods, the fringe counting method is introduced for treating the output of the fibre F-P interferometer sensor. Flexible metal bellows, which can afford a large strain displacement, is suggested to be one surface of the F-P cavity. Two methods were developed to determine the fringe move direction in order to give the up and down of the liquid level, and the experimental equipment is assembled.Firstly, the extensively survey of the literature about liquid level sensors was conducted, and their characteristics, advantages and disadvantages were summarised. Secondly, the basic principle, the function structure and mechanism of the sensor system by using fringe counting were analyzed and concluded. The features, the strain model, the composed materials, and the large displacement formula of the bellows were thoroughly analyzed and discussed, and some useful suggestions for choosing the bellows were given. Thirdly, both the hardware electronics design scheme and the software treatment methods were developed to count the fringe and determine the up and down of the liquid level. Fourthly, the F-P liquid level sensor was fabricated, the practical measurements were carried out both for the air pressure and the water level. Finally, the factors affecting the sensor were discussed and some suggestions about improving the sensor were given.