Precise Dimension Measurement Techniques Based On Machine Vision

It is completely non-contact to use machine vision technology for industrial production line for axis kind of parts'dimension measurement,and it is one of the key means to ensure the quality of finished. In order to increase dimension measurement accuracy of axis kind of parts and lower the cost of measurement system, lots of works have been done to deal with key technology such as camera calibration,image mosaic technology and sub-pixel edge detection technology.Finally high precise measurement of axis kind of parts dimension was achieved using machine vision inspection.Light source and lense play an important role on the quality of image just like cameras in the computer visual inspection applications.The LED light source and lense with little distortion were selected as light source and lense of imaging for inspection system,which provided high quality images during precise inspection.A simple and high precise method of calibration was given aiming at the character of the inspection system in the paper.A standard workpiece was regarded as reference.The result of the calibration compared with the Sherlock sofeware was very satisfied.In the process of image mosaic,firstly, a ratio matching method was used to find out the overlapped areas of two images.Secondly, the gray-scale of image was adjusted through a histogram matching method.Finally, image mosaic was completed with smoothing algorithm.In the process of sub-pixel edge detection,firstly,the author seted about to analyzing the response to unit step signal through ideal low-pass filter in theory,found out the relation between gray difference and sub-pixel edge locating points through computer simulating,and established a lookup table which could gain sub-pixel edge locating points according to gray difference. Secondly,the author combined the improved Pal.King edge detection method to inspect image edge, which made its edge detection precise reaching pixel level. Finally, the author applied lookup table to achieve sub-pixel level edge detection precise.A pedal of bike was used for detecting object.The theory was used to finish sequence image mosaic of the detecting object and measure the dimension of the mosaic image. The measurement results showed that using these two methods to measure axis kind of parts'dimension could meet the practical requirements, and the measurement accuracy reached 0.02mm.