| With the development of high-speed railway, continuously welded rail is widely used and promoted in the world. The seam between rail joints is eliminated in continuously welded rail, improve the operation speed and comfort. Due to the heat bilges cold shrink, with the change of the Rail's temperature, great force would be created in the rails, when the stress is out of security scope, the rail would break easily, bring hidden danger to the train operation safety, so the broken rail detection system is significant to guarantee the railway transportation's security.In this thesis, a real-time online broken rail monitoring system is developed based on the ultrasonic testing technology. The number of the weld seams fracture accounts for about 90% of the total number of fracture, so this paper lays emphasis on rail welding seam detection system. The overall structure and realization method of the system is detailed described,on this basis, hardware and software modules of the system are designed, including the driving circuit of the ultrasonic sensors,data acquisition circuit,the inside logic modules of FPGA and the design of the upper computer's software program. Through the simulation experiment and theoretical derivation, this thesis put emphasis on the relationship between the value of the resistance in the driving circuit and the amplitude of the impulse, through the design of the data acquisition board based on FPGA, complete the ultrasonic signal data collection and transmission, design the PC software program based on VC++ 6.0, we realize the function of Wavelet de-noising, data analysising and waveform displaying.Based on the completion of the design of the system's software and hardware, laboratory environment is set up, through many experiments, we extract the features of ultrasonic signal when the rail has no defect, minor defects and larger defect, at present, the detection system has been installed on a section of a high-speed rail in Suzhou.
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