Research On Precise Positioning Of Parts Of Automatic Image Measuring Instrument

Image measurement system has the advantages of real-time on-line detection,non-contact measurement and high reliability,and has been more and more widely used in the field of visual measurement.However,in the process of repeated parts or batch measurement,the use of positioning datum or positioning fixture to assist positioning results in low measurement efficiency and can not meet the needs of high-efficiency automated measurement.To a large extent,it has become one of the bottlenecks of its application in the field of automated measurement.Therefore,aiming at this problem,this paper carries out the research on visual guidance of image measuring instrument,aiming at realizing the automatic positioning of batch parts,which is an important prerequisite for subsequent automatic measurement.The main contents of this paper are as follows:(1)Aiming at the problem of low measurement efficiency caused by the small field of view of the camera of the existing image measuring instrument and the relying on positioning fixture for the repeated measurement of parts,the global image of parts is obtained by adding a large field of view module,and the vision guidance of the measuring platform is carried out to realize the automatic positioning of parts;the corresponding image measuring system is built to provide the basis for the follow-up research.(2)Aiming at the problem of large distortion in the large field of view positioning module introduced,the mathematical model of lens distortion is established through detailed analysis of the camera imaging process;the internal parameters of the camera are obtained by Chessboard lattice and Zhang's calibration method,and the distortion image is corrected.On this basis,according to the projection model of large and small field of view cameras,using the same marking point,the mathematical model of the relative position of the optical axis of two cameras is established.Finally,the experimental verification is carried out.The experimental results show that the calibration method of the relative position of large and small field of view cameras is feasible.(3)In order to locate the target accurately,the image measurement system uses the global image information of the large field of view module to guide the vision of the measurement platform.According to the obvious characteristics of the gray contrast of the global image,the threshold segmentation is realized by the double-peak method,the sub-pixel contour pointsare obtained by polynomial interpolation,and the precise coordinates of the circle are obtained by the least square fitting method.The positioning accuracy is guaranteed.On this basis,the conversion method of feature point pixel coordinates to machine tool coordinates is elaborated in detail.The experimental results show that the method is feasible.(4)Based on the in-depth analysis of the source of positioning error,a comprehensive error compensation method is proposed.Through the analysis of the positioning error,the distribution rule and the main factors affecting the error distribution are found out,and the corresponding error compensation mathematical model is established.The positioning error of the image measurement system is corrected by the error compensation model,so that the guiding positioning accuracy of the measurement platform can be improved ideally.On this basis,in order to further compensate the errors,the image-based visual servo technology is introduced to form the closed-loop control of the image measurement system.Finally,the precise position alignment of the positioning parts with the image center of the small field camera is realized.