Research On The Key Techniques Of Nurbs Surface Merging And Fitting In Reverse Engineering

With the rapid development of computer technology, reverse engineering combining with computer has been applied to fields of aerial navigation, aerospace, machine, architecture, mold design, medical instrument, organ reforge, precision measure of the complicated product, etc. Comparing with the traditional engineering, reverse engineering can design and reconstruct complex free surface, and it can repair and redesign the existing product efficiently. So the study of reverse engineering has academic and practical meaning. But it is especially in the field of the free surface construct the technique of the reverse engineering isn't perfect. On base of the relate research all over the world, the paper deduced the general subdivision theory, obtained the really G1 smooth connecting condition of the surfaces base on the piecewise NURBS surface joint technique and analyzed the precision influence factors of NURBS surface fitting. Finally, the paper made a deeper exploration and research for the perfect and development of the reverse engineering.Firstly, the subdivision algorithm which is based on the uniform B-spline theory is analyzed, the basic theory, influencing factor and experimental result of the other subdivision algorithm is concluded and summarized, the general subdivision algorithm is proposed. By observing practical surface construction, analyzing the influence factors of the every weight to holding the features on the subdivision algorithm, how to select the weights in order to enhance the precision about the free surface is obtained.The G1 smooth merging theory of the piecewise Bezier surface is analyzed, the Bezier curve is a special NURBS curve is considered, the idea of resort to Bezier surface G1 continuous condition for solving the G1 continuous condition of NURBS surfaces is presented. G1 continuous condition of the NURBS is deduced and realized. Linear system of G1 continuous condition is solved by the COM components about Matlab.The existing grid generation theory is studied, incapability of grid generation that these theory to the four sided region constructed by four discontinuous broken line is found. A rapid parametrization algorithm which is used to spatial four-sided region on triangular mesh is presented in the paper. By comparing two kinds of parametrization projects of the spatial four-sided region, directly grid generation algorithm is chose. By coordinates transform and point project to the basic plane, the problem of grid generation about spatial four-sided region changed to planar four-sided region grid generation problem. A planar two-directional flexing grid generation algorithm is presented which can quickly parametrizing the planar four-sided region, in fact, a spatial four-sided region is parametrizated.Comparing two NURBS surface fitting methods, in order to enhance the fitting precision and obtain more detail information of the NURBS surface fitting, two-directional fitting method is adopted to fitting the NURBS surface. For the points on the common boundaries and corners is restricted base on the G1 smooth connection condition. So by fitting every patches and a whole G1 smooth surface is obtained. A read-write interface is developed in the paper.By analyzing the precision influencing factors about piecewise NURBS fitting, relationship of the number of the triangular mesh, grid number of every patches and whether well distinguished sharp features of the triangular mesh to precision is considered and influencing trend is obtained.Finally, practical application of reverse engineering is illuminated by using reverse engineering to an aerial part and common process and practical operating methods is concluded by using reverse engineering to complex surface construction.