Communicating Pipes Deflection Monitoring System Using Intensity Based Plastic Optical Fiber Sensor

The deformation of the bridge under load is a direct reflection of the structure stiffness.The effect of load and environment and variation of structural materials can be expressed through deflection.Deflection is the important control index in the design,detection and monitoring of bridges,therefore it is of great significance to monitor the deflection of bridges.For the limitations of existing long-term bridge deflection monitoring technology and the great challenge of long-term online real-time monitoring,it has certain academic significance and practical value to break the technical bottleneck and develop a new deflection monitoring technique for long-term monitoring of the bridge.This paper firstly analyzes the research background and significance of long-term bridge deflection monitoring,then introduces the existing bridge deflection monitoring technology theory,its advantages and disadvantages,and summarizes the existing problems,the challenges in the long-term monitoring of deflection.On the basis of introducing optical fiber sensor and plastic optical fiber systematically,we try to use plastic optical fibers with excellent toughness,durability and stability as sensor elements to develop the long-term monitoring system based on the intensity change of plastic optical fiber.The system can perceive the height change of the liquid level through the change of light intensity in the plastic optical fiber and convert it into electrical signal by using photoelectric conversion device.So the automatic monitoring of the communicating pipes deflection is realized by measuring the electrical signal.The system not only provides a new solution for the long-term monitoring of bridge deflection,but also can be used as an important part of bridge health monitoring system by optical fiber sensing.The main research work and conclusions of the paper are as follows:1.The cause of loss of plastic fiber is analyzed.This paper introduces the optical fiber loss theory and its application in the field of sensing.Then derives a relationship between longitudinal fiber spacing and intensity loss of POF sensor and a plastic optical fiber liquid level sensor based on light intensity variation is proposed.2.A plastic optical fiber liquid level sensor was designed and manufactured.The production process adopts the hot creep cut method to ensure the flatness of the fiber cutting face.Adopting a U-shaped glass tube to permanently set the plastic optical fiber,and it was separated from the liquid to be tested,which improved the stability and adaptability of the sensor.The sensing area is placed inside the plastic tube with smooth inner wall,ensuring that the upper and lower movement of U-shaped optical fiber can accurately reflect the fluctuation of the liquid surface.3.The liquid level measurement experiment was carried out on the designed plastic optical fiber sensor.The results show that the sensor has a good linearity within the measurement range of 20 mm,and its correlation coefficient is above 0.98 and the sensitivity is 0.44 d B/mm.According to the parameter analysis of the sensor structure,it can be seen that the smaller the core diameter D of the plastic fiber,the more the number n of gaps between the fibers,and the higher the sensitivity of the sensor.Finally,the experimental results under different temperature conditions show that the sensor has good temperature adaptability and stability.4.The plastic optical fiber liquid level sensing element is embedded into the communicating pipes device.Setting up the deflection monitoring system for the simply supported beam with the three measuring points,and a comparison experiment is made with the existing LVDT sensor.The results of the experiment show that the two results are basically the same,and the relative error is within 3%,which can meet the engineering surveying requirements.