A Novel Multimode Interference Based Optical Fiber Ultrasonic Sensing Technique For Bridge Deflection Monitoring

Following with the development of national economy and modern science, more and more attention has been paid to the security issues of transport infrastructure. As an important part of the structure monitoring and health assessment, the bridge deflection monitoring is referred to a new height. Since the limitation of the existing monitoring technologies and the challenge of long-term monitoring for bridge deflection, it is very important to break the limitations of existing technologies and develop a new type of monitoring technology for bridge monitoring.This paper firstly introduces the significance and the existing technologies for bridge deflection monitoring. By the analysis of the principle, the advantages and disadvantages of existing technologies, the demanding and challenge for bridge deflection monitoring are analyzed and summarized. Based on this, a novel multimode interference based optical fiber ultrasonic sensor system is proposed and studied for the first time for bridge deflection on-line monitoring. This paper make an intensive study on the principle of the sensing system, designing and establishment of non-contact displacement sensor system. By designing and optimizing of displacement sensing scheme, the non-contact displacement sensor experiment and the simulation experiment of bridge deflection monitoring have also been conducted. The main research contents include:1. This paper has investigated and introduced the characteristics of ultrasonic, the principle as well as advantages and limitations of existing ultrasonic non-contact ranging technologies.2. The advantages and limitations of optical fiber sensor in the field of bridge deflection monitoring are investigated. By giving full play to complementary advantages of ultrasonic ranging technologies and optic fiber sensor, this paper proposed a novel multimode interference based optical fiber ultrasonic sensing technique for bridge deflection monitoring.3. The sensing principle of multimode interference based optic fiber sensor are investigated. It have been confirmed that the multimode interference based optic fiber sensor can detect ultrasonic pulse signal with a frequency of 25 kHz. At the same time, with an aim to eliminate the measurement error caused by ultrasonic wave distortion and environmental condition changes, the scheme of double sensor for displacement reference is proposed. By analyzing the pulse time delay between the reference sensor and the displacement sensor, the stability of the system have been greatly improved4. The principle of ultrasonic optical fiber sensing system based on multimode interference for non-contact displacement measuring has been investigated, and the non-contact displacement sensor system has been designed and established. Through the displacement sensing experiment, the measurement accuracy of the pulse detection method and phase detection method of quasi continuous wave have been tested. Based on the results of the displacement sensing experiment, a novel displacement sensing scheme based on pulse-phase detection method is proposed, and its feasibility has been confirmed by expreiment. The results show that the displacement resolution can reach 0.3mm above 5 meters contactless distance and the maximum error is 0.37mm. It demonstrate that the proposed displacement sensing scheme can satisfy the demand for bridge monitoring.5. A bridge deflection test system has been built in laboratory to simulate the actual bridge deflection development. Through the simulated deflection monitoring experiment, the feasibility of proposed sensing system for non-contact deflection monitoring is verified. The results show that the displacement resolution can keep in 0.3 mm. It provides a theoretical basis and technical support for the engineering application of proposed scheme in bridge deflection monitoring.