球压试验压痕直径的视觉测量方法

Measurement of Indentation Diameter in Ball-pressure Test Based on Computer Vision Method

球压试验是测试电工、电子产品中非金属材料及固体绝缘材料耐热性能的重要方法;由于压球与压痕相接触区域不易准确识别,如何客观、准确、快速地测量压痕尺寸成为困扰质检人员已久的一大问题;根据压球与压痕相接触区域与非接触区域对垂直光照反射特性的不同,通过垂直光照下采集压痕图像可以准确地分割出接触区和非接触区;图像经过预处理后采用灰度重心法确定压痕的灰度中心位置,识别压球与压痕接触区域的分界点,采用Hough圆变换计算压痕直径的像素距离,最后根据标定实验得到直径的实际尺寸;实验结果表明,测量结果的重复性标准差<0.0016 mm,重复性限<0.003 mm,测量值与标定结果的偏差的绝对值<0.01 mm,满足压痕测量的精度要求。

Ball-pressure test is a primary approach to test the thermotolerance of non-metallic materials and solid insulating materials in electrical and electronic products.Accurate measurement of the diameter of the indentation is a key step in the ball indentation test,especially for the critical size of the indentation.The accuracy of the measurement directly determines the correctness of the test results.It is difficult to accurately identify the contact zone between the pressure ball and the indentation.Consequently,how to measure the size of indentation objectively,accurately,and quickly has become a major issue that has plagued quality personnel for a long time.The indentation image was captured under the perpendicular incident light.Since the contact zone and non-contact zone between the pressure ball and the indentation have different reflection characteristics to the perpendicular incident light,the contact zone can be accurately segmented from the non-contact zone by image processing algorithm.After the image is pre-processed,the grayscale center of gravity method is utilized to determine the gray center position of the indentation.The demarcation point of the contact area between the ball and the indentation is identified.Then,the pixel distance of the indentation is calculated through the Hough transformation.Finally,the actual size of the diameter is obtained via the calibration method.Repeatability test and accuracy comparison test results show that the repeatability standard deviation<0.0016 mm,repeatability limit<0.003 mm,absolute deviation of the measurements and the calibration results<0.01 mm,which meets the required precision of indentation measurement.