Scanning measurement method of small size parts without marks
Small-size parts have a small surface area and complex structure.The traditional mark splicing method needs to manually paste marks on the surface of parts,resulting in missing the measurement data of the surface and becoming holes.The feature splicing method requires the surface of parts to have easily dis-tinguishable geometric or distance features,which are not suitable for rotating parts containing repetitive fea-tures.We propose a scanning measurement method without marks based on mechanical splicing,which does not need to paste marks or depend on the surface features of parts.Firstly,the camera calibration method based on photogrammetry is used to reconstruct the high-precision three-dimensional coordinates of the tar-get on the calibration board.By tracking the position of the coded target,the rotation matrix corresponding to different angles of the turntable is established,and the direction vector of the rotation axis and the fixed point coordinates on the axis are solved.Then the synchronous calibration of the rotation axis and the camera is completed.Secondly,based on the accurate calibration of poses of two rotation axes,the rotation mosaic mat-rix is constructed by using the turntable angle to realize the rough registration of multi-view point clouds.Fi-nally,based on the Normal Iterative Closest Point(NICP)algorithm,the fine registration of the point clouds is completed.Experimental results show that the angle error between the two rotation axes calibrated by the target tracking method is 0.023°lower than that of the traditional standard ball fitting method.After calibra-tion,the average size error of the standard ball is less than 0.012 mm.In the automatic measurement of small-size parts,the point cloud splicing effect of the mechanical splicing method after fine registration is similar to that of the mark splicing method,and the splicing stability is higher.The mechanical splicing method is suit-able for the 3D topography measurement of small-size parts where the marks cannot be pasted.
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