Development of On-Line Lamb Wave Testing Technique using Real Time Digital Signal Processing

Abstract

Lamb wave testing technique has been used for on-line defect inspection of steel strips. However, conventional Lamb wave testing technique has the following two large problems. One problem is the existence of the wide dead zones which appear in near and far field. Another one is the misdetection due to parasitic echo or electrical noise. We have developed the method which solves these problems by using the signal processing technique. We designed and constructed the new system which collected the received signals and change it into the two-dimensional signal. It distinguish defect signals from the other signals by the two dimensional signal characteristics and the if-then rule logic. This method does not require the inspection gate. As a result, the dead zone and the misdetection can be reduced. This paper describes the measurement principle, the algorithm of the signal processing and the distinction logic. The results showed that the dead zone was reduced to 100mm from the strip edge.