Tolerance Modeling And Assembly Analysis Of Workpiece Based On Parametric Space Envelope

Manufacturing always leads to uncertainty in the size and geometry of the part,and the tolerance of which is used to control the extent to which the target geometry deviates from the ideal state in size,orientation,position and shape.The mathematical model of tolerance is the basis of related technologies such as tolerance design,tolerance analysis,tolerance inspection,etc.,which can consistently interpret the meaning of each member of the tolerance information,so as to obtain the numerical relationship of geometrical variation and shape variation between the reference element and the measured element.In addition,tolerance analysis is an important quality simulation method in the product design process.It can use information such as tolerances and geometric models to predict and analyze the quality of assembly.However,only partial tolerance semantic information can be reasonably explained under the existing model,which is not enough to simulate various complicated assembly process information,which limits the application of tolerance analysis method in engineering practice.In this paper,based on the new generation GPS framework system,workpiece tolerance modeling is taken as the research object,and further research on tolerance mathematical model,structural light measurement,point cloud registration and assembly tolerance analysis,etc.are carried out for 3D measurement,tolerance evaluation and tolerance analysis.The main research work is as follows:(1)The tolerance modeling based on the parametric space envelope is proposed.The ideal geometric features are embedded into the cubic Bezier parametric space and expressed by the control points of the parametric space.By changing the position of the control points and changing the parametric space,the size,orientation,position,and shape of the embedded ideal feature are indirectly changed.Secondly,dimensional tolerances and geometrical tolerances(GPS standards: dimensional tolerances,geometric tolerances,surface topography tolerances)are all considered as three types of tolerances: dimensional tolerances,rigid motion-related tolerances,and shape-related tolerances.And based on the tolerance requirements(constraints),the relationship between the offset of the control point along any or a given direction(reference)and the tolerances of component is established.(2)A three-dimensional measurement method based on adaptive local window M-array coding structure light is proposed.Aiming at the encoding strategy of M-array structured light,the M-array image decoding method based on adaptive local window is studied,which can solve the decoding problem of occlusion and segmentation caused by the irregularity of the feature points and the discontinuity of the depth caused by the camera shooting angle in multi-view geometry.Finally,the 3D reconstruction under multi-view geometry is realized,and the topological relationship between point cloud is also established,thereby achieving the evaluation between part geometric deviation and tolerance.The experimental results show that the method can significantly improve the decoding success rate of curved objects(98%?99.65%),and the relative error of cylindrical diameter measurement is less than 0.8%.(3)A tolerance evaluation method based on the improved ICP algorithm is proposed to improve the closest point search algorithm and matching strategy by establishing point-point,point-plane,point-footpoint(projection)directed distances.Thereby improving the accuracy of point cloud registration,so as to better judge whether the actual geometric parameters of the workpiece are in the ideal region;Aiming at the point cloud model of irregular surface workpieces such as flexible thin plates,an error metric function based on the directed distance weighting factor is proposed to solve the rigid transformation parameters of the registration between workpieces in geometric sense,so as to the determination of the assembly position of the parts during tolerance analysis.(4)An assembly tolerance analysis method based on the parametric space envelope is proposed.In the design,according to the offset synthesis of the parametric space control points,the random variation of the geometric feature deviation is reproduced or simulated,and the statistical geometric tolerance analysis is performed.Respectively,the cylindrical profile tolerance analysis and the assembly gap calculation of the workpiece by changing the dimension of the hole shaft system,the size and position of the rule piece are taken as examples to verify the feasibility of the proposed method.