Surface Inspection System for Heavy Steel Plate Using Twin Illumination and Subtraction Technique

Abstract

The surface quality of steel products is important in terms of both product appearance and material properties, requiring the installation of a surface inspection system for automatic inspection. However, it is extremely difficult to realize automatic inspection for steel products such as pipes, heavy plates and hot-rolled sheets which have an oxide film called “mill scale” on the surface, because the surface texture of the mill scale prevents detection of defects. Most of these products have been inspected visually by human operators. In order to solve this problem, the authors developed an optical surface inspection system utilizing a new technique called the “twin illumination and subtraction technique” and realized automatic inspection of rolled pipes in the hot condition by high detectability of surface convex-concave defects. This technique is able to detect only convex-concave defects and remove noise signals from the flat surface texture by illumination from two directions and subtraction. We discuss the new surface inspection system for heavy plate products by improving the proposed method. Specifically, the optimum optical system for high detectability was designed based on laboratory experiments and simulation of the reflection model, which clarified the optimal optical settings such as the incident angle and field of view. By using this knowledge in the design of the inspection system, we realized practical automatic surface inspection for heavy steel plates.