Research On Error Measurement Of Irregular Contour Based On Machine Vision

In order to measure the error of workpieces,a high-accuracy contour measurement algorithm that can be used for arbitrary shapes is proposed in this thesis.The implementation of the algorithm includes three steps: contour extraction and representation,contour matching and error calculation.The main work of this thesis is described as follows:(1)Edge extraction and representation.First,the sub-pixel-level edge of the workpiece is extracted from the region of interests and represented as a contour.If the extracted contour is registered with the standard contour directly,the large number of contour points would cost too much time,so the contour should be simplified hereafter.In this step,we improve the DCE algorithm to simply the contour while remaining enough key points at the same time.In other words,while ensuring the basic structure of the contour is not changed,the efficiency of contour matching is enhanced.(2)Contour matching.A coarse to fine matching strategy is applied to register the extracted contour with the standard one.In the coarse matching stage,we use the shape context descriptor to describe the simplified contour points to match with the standard contours,that is obtaining the rigid transformations(translation and rotation)between the contours and gradually register with the standard contours.Since only a part of the contour points is used in coarse matching,the results still have a large error.So based on the coarse matching position,the ICP algorithm is used in fine matching stage with all the contour points.In this step,we comprehensively consider the curvature and the normal vector of the contour points,and giving little weight to possible noise points or outliers.(3)Error calculation.Finally,the errors in every contour point is calculated.In order to reduce the error caused by the discrete of contour,this thesis proposes an error calculation method based on neighboring line segments.In this thesis,we use the calibration board and the workpiece to experiment the proposed algorithm.The contour error of each location in the contour can be measured and the accuracy can up to 0.5 pixels.