Research On Time-of-Flight Processing Technology In Ultrasonic Thickness Measurement

Because of the advantage of low cost,flexible operation,simple device and high security,ultrasonic thickness measurement technology has been widely used in non-destructive evaluation area.To attain the residual wall thickness distribution of workpiece on-machine is of key importance for target surface redesign and wall thickness error compensation.In practical production,the ultrasonic thickness measurement of large thin-walled parts is mostly manual hand-held detection,which has problems of low measurement efficiency,large labor intensity and poor consistency of measurement results.In ultrasonic measurement,the acquired echo signals are often doped with various noises and interferences,leading to the distortion and low SNR of ultrasonic echoes.In order to improve the reliability of ultrasonic thickness measurement,the accurate determination of Time-of-Flight(TOF)has always been one of the research focuses.In addition,due to the influence of cutting process and production process,the surface of the workpiece can be rough or have specific texture feature.The roughness can significantly reduce the signal to noise ratio(SNR),hence,deteriorate the accuracy of ultrasonic thickness measurement.In this paper,the adaptive time delay estimation(TDE)method is applied to determine the TOF in ultrasonic thickness measurement.Motivated by the higher requirement on accuracy of TOF determination in thickness measurement,a new VSS adaptive algorithm is proposed.Furthermore,to reduce the impact of finite sampling interval on the accuracy of TOF determination,a cubic spline fitting approach is employed to locate the maximum of impulse response sequence.Simulation experiments validated the improved performance of proposed TOF determination method.Ultrasonic thickness measurement experiments were conducted on aluminum alloy plates.Experimental results show that the proposed TOF determination method not only has the ability to maintain stability and robustness under low SNR condition,but also can significantly decrease the thickness measurement error.Based on Kirchhoff Approximation,the single scattering event is analyzed taking account of local incident angle induced mode conversion and phase variation.The effect of rough interface on the attenuation of ultrasonic pulse-echo is analyzed and the theoretical expression of the relationship between reflection factor and roughness parameters is establish in a statistical sense through ensemble averaging.Ultrasonic measurement experiment results carried out on Q235 steel specimens with different roughness verified the correctness of theoretical model.Meanwhile,the impact of roughness on ultrasonic thickness measurement accuracy was experimentally studied.Based on Labview,the software system of ultrasonic thickness measurement is developed.Ultrasonic thickness scanning measurement experiments were carried out on alloy grid plate.The three-D morphology of the grid plate workpiece is reconstructed according to the machine coordinates and thickness measurement results.The comparing between the thickness measurement results from ultrasonic and coordinate measuring machine(CMM)indicates that the ultrasonic thickness error can be kept within 0.02 mm.