| Summary: | 碩士 === 元智大學 === 工業工程與管理學系 === 92 === Steels ball are important elements in a precision machine, especially as components of stainless steel bearings and automotive parts. The quality of steel balls makes a great impact on the operating condition and the life cycle of the composed product. Traditionally, the inspection of steel balls in the manufacturing process requires highly experienced operators with their naked eyes; this method is out of time nowadays. In this research, we propose a machine vision method to tackle the deficiency of manual vision inspection and to achieve high effectiveness and efficiency of automatic inspection.
Owing to the high reflectance and the spherical surface of steel balls, the inspection lighting must be selected appropriately to acquire an optimum image. A ring-shaped LED light along with a dome-shaped unite reflective board is employed to eliminate the reflectance of light and enhance surface texture of the inspection steel ball. The resulting inspection region is a ring-like area under the experiment framework. In this thesis, we use two image enhancement techniques (contrast stretching and gray-level slicing) to stress the difference of gray-level information between the defect and normal areas. According to the contrast stretching and gray-level slicing results, we use Entropy method and gray-level statistics in a neighborhood window, respectively, to compute a threshold value for distinguishing defect areas from the background. Since the surface roughness consists of irregularities such as large scaled crack, dent, cuts and highly concentrated small defects, we compute the maximum defect blob area and average defect blob area as two quantitative measures to determine whether the steel ball under inspection is good or bad. In order to improve the inspection rate, multiple steel balls inspected in the same image is also evaluated using the proposed method. Experimental results have shown that the proposed two inspection methods are able to detect the defects of steel ball effectively for both single and multiple steel ball inspections.
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