Design Of Sagnac Vibration Detection System Based On FPGA

With the continuous improvement of urbanization and modernization level in our country,large infrastructure such as subways,bridges and pipelines are being built at a rapid pace and have played more and more important roles in the aspects of urban development of economy,transportation,industry and so on.Therefore,the real-time operation status and fault monitoring of the abovementioned infrastructure becomes particularly important.Vibration parameter detection is often used for fault monitoring.Most of these infrastructures have the characteristics of large scale,long distance,complex and changeable working environment,etc.Thus traditional point-separated vibration sensors can hardly meet the monitoring requirements in these scenarios.Distributed optical fiber vibration detection technology based on Sagnac interference takes optical fiber as the carrier of sensing and transmission,could overcome the shortcomings of traditional sensors,and has broad application prospects.In engineering applications,the real-time detection of the vibration signals along the long distance optical fiber puts forward higher requirements for data acquisition and transmission processing speed.Therefore,FPGA technology is introduced into the Sagnac vibration detection system to realize real-time acquisition and highspeed transmission of vibration signals.In order to realize the real-time detection and positioning demodulation of the vibration signal,the Sagnac vibration detection system designed in this paper is divided into three parts: Sagnac interferometric optical system,high-speed data acquisition and transmission system and the host computer data demodulation software.The phase difference between optical signals is caused by the modulating effect of vibration on optical signals in the fiber by the Sagnac interference optical system,and then the interference optical signals are generated,and finally the interference optical signals with vibration information are converted into electrical signals.The high-speed data acquisition and transmission system converts the analog electrical signals output by the optical system into digital signals and transmits the data to the host computer.The host computer data demodulation software is mainly responsible for the control,data processing,display and other functions of the whole system.It performs algorithm processing on the collected data and finally restores and locates the vibration signal.The research content mainly includes the following aspects:Firstly,the mechanism of Sagnac vibration sensor is studied,mainly including the phase modulation mechanism of the vibration to the optical signal,the vibration demodulation mechanism of ring,linear and doule Sagnac interference structure.The overall design scheme of the Sagnac vibration detection system is introduced,and the device is selected according to the system requirements.Secondly,a high-speed data acquisition and transmission system is designed based on AD9218 chip,CYUSB3014 chip and Altera's FPGA chip.The whole acquisition scheme and differential drive circuit are introduced in detail.Besides,this paper expounds the basic principle of asynchronous FIFO and realizes each logic module of the system.After analyzing and debugging the program,it is verified that the system has the function of real-time acquisition and transmission.Thirdly,the host computer software based on LabVIEW is designed by analyzing the function requirements of the system.The main functional modules and corresponding program block diagrams are specifically explained,including calibration and alarm module,data reading module and data processing module.Finally,the host computer software interface is designed and optimized.Fourthly,a Sagnac vibration detection system based on FPGA is established.the data of a single channel is analyzed at first,to explore the positioning advantages and disadvantages of the null-frequency point method.Then the signals of the two channels are analyzed,and different types of vibration signals such as single frequency and impact vibrations could be detected and located.The advantages of the location algorithm are elaborated.Finally,the performance of this system is tested.The results show that the system can achieve the positioning error within 100 m among the sensing distance of 5km.In addition,the real knocking and trampling vibration signals are tested.These results prove that the scheme has a broad prospect in practical application.