Design Of Ultrasonic Guided Wave Nonlinear Coefficient Detection System

The longitudinal temperature stress of high speed railway seamless rail will be greatly influenced when the ambient temperature changes.When the stress exceeds the safety threshold,it will affect the safety of train operation.As a new method to detect the temperature stress of seamless rail,the nonlinear ultrasonic testing technology has attracted more and more attention.According to the existing general nonlinear ultrasonic testing equipment with complex structure,high price and large volume,is not suitable for field use,an embedded portable detection system based on FPGA and MCU is designed in this paper,which can realize ultrasonic guided wave excitation,reception,signal processing and automatic calculation of nonlinear coefficient.Firstly,according to the one-dimensional nonlinear wave equation in solid media,the functional relationship between the two order nonlinear coefficient and the amplitude of the fundamental wave and the two harmonic amplitude under a single frequency excitation is theoretically analyzed.Based on this,the ultrasonic guided wave excitation waveform,excitation frequency and ultrasonic transducer suitable for ultrasonic guided wave nonlinear coefficient detection are determined,and the overall scheme of the detection system is designed.Secondly,in view of the requirements of the system technical specifications,the hardware design of the ultrasonic emission circuit,the ultrasonic receiving circuit,the FPGA and the peripheral circuit of the STM32 is completed.In view of the function of system detection,the software program of FPGA and STM32 is designed.Among them,the high precision synthesis and real-time acquisition function of the excitation waveform is realized through the hardware description language in FPGA,including DDS module,data buffer module and clock management module.The calculation of nonlinear coefficient and the function of human-machine interface are realized through embedded system programming in STM32,including the operation of μ C/OS-Ⅱ,the LCD touch screen drive,SD card storage,FFT and nonlinear coefficient calculation,and other related applications.The system has rich functions and high integration.The transmitting circuit can realize adjustable excitation waveform,adjustable excitation frequency and adjustable trigger interval,so as to meet the requirements of waveform and frequency in different detection environments.After conditioning,the receiving signal can achieve high precision real-time acquisition,display and storage,and intercept any wave packet of the received signal to achieve FFT,and calculate the nonlinear coefficient.Finally,the paper tests the functions and detection precision of the system through physical experiments.Compared with the US RITEC RAM-5000 SNAP nonlinear high energy ultrasonic detection system,the system detection error is less than 8%,which has certain advantages in portability and cost performance.