I. Introduction
Thickness measurement of products is an important part in industrial processes, since thickness is a key factor affecting the pass rate of the products. For example, in the metal industrials, the thickness of the rolled steel determines the pass rate of the steel products; in the process of material composition, the thickness of the layers have a major influence on the physical and chemical properties of the products. With the development of science and technology, non-destructive testing (NDT) technologies were used by more and more factories to control the product quality and adjust production process [1]–[3]. As one of the NDT technologies, traditional ultrasonic testing has been used to determine the thickness. the inner defects and the residual stress in metal plate for a long time[4]–[9].