2019 Volume 59 Issue 10 Pages 1820-1827
In optical surface inspection of steel products under hot conditions, overdetection caused by signals from harmless surface such as scale texture pattern is usually a problem. Therefore, the authors proposed a new inspection technique called the “twin-illumination and subtraction technique,” which is able to remove only harmless signals, based on the finding that most harmful defects on these products have concave shapes, whereas most harmless features that might be overdetected have flat or convex shapes. In this technique, an image in which the concave and convex parts are emphasized could be acquired from the difference between two images of the same position illuminated from two opposite directions. As an application of this technique, after laboratory tests to confirm its effectiveness, we conducted inline test for hot pipes which have poor surface. Although harmless signals from flat surfaces could be removed as expected, harmless signals from convex surfaces such as peeled scale and micro-surfaces which cause specular reflection conditions remained in addition to harmful signals from concave defects. To improve detectability, we introduced a discrimination method based on the characteristic bright/dark patterns of concave defects, a detection and decision tree judgment function using features from images. Based on the above, we constructed a prototype system which can obtain two images with a slight time difference by using strobes and dual CCD cameras. As a result, we have confirmed that the system has sufficient performance for prevention of massive rejects and is an effective technique for detection with poor surface.