Large-scale Point Cloud Processing Of Blade And Collision-free Measuring Planning

Impeller or blade is the core component of aviation, ships, nuclear power and other in-dustries, with thin-walled, difficult-to-processing and easy-to-deformation characteristics, sohow to guarantee the manufacturing accuracy is always the research frontier in the field ofdigital manufacturing. Integration of machining and measuring provides novel theory andmethod to realize precise machining of the impeller or blade with complex surface. Amongthem, large-scale point cloud processing,5-axis collision-free measuring planning, integrationplatform of software and hardware become the key problems urgently needed to solve.Therefore, the research includes point cloud simplification and smoothing of blade, shape pa-rameter extraction of blade, collision-free measuring planning of impeller.(1) Because of the blade’s complex surface with thin-walled leading and trailing edge(the thickness is less than2mm), it’s easy to lose the geometry accuracy of leading and trail-ing edge due to uneven sample for point cloud simplification and smoothing. In order to getthe point-surface distance function, the point cloud density entropy was applied to select theweighting factor which consists of local point cloud distribution characteristics and inherentfeature property. The geometric distance function between two groups of point cloud beforeand after simplification was established to avoid the insufficient sampling in thin-walledblades’edge regions with high curvature feature and sparse regions with fewer points. Anoveladaptive bilateral filter was put forward to solve the problem of point cloud’s overall shrin-kage and edge’s excessive smoothing. Experiments show that the proposed simplificationmethod ensure more balanced distribution of point cloud compared with uniform grid andhierarchical clustering method; Compared with Laplace and bilateral filtering method, adap-tive bilateral filtering method decreased the standard deviation by1order of magnitude.(2) To avoid the difficult problem of mesh reconstruction and surface parameterization,secion curve and feature parameters can be directly extracted by the blade scattered pointcloud. Well-organized point set of section curve and mean camber line was effectively ex-tracted by constructing mathematical morphology operation of the scattered point cloud. Ac-cording to the1order differential incremental of the distance function between section curveand point cloud, linear optimization model by adjusting control points of B-spline curve wasgot to limit the geometric deviation between section curve and point cloud to the tolerancerange. Combining with the principle of mean camber line, the signed point-envelope distancefunction was used to establish the distance function between the blade section curve andenvelope curve which is formed by the mean camber line and its inscribed circle. Optimaladjustment of control points of B-spline curve improved the extraction accuracy of the mean camber line. Then it was easy to extract the leading/trailing edge parameters and maximumthickness of blade. Lastly, the proposed algorithm is successfully applied to the point-sampledblade or integrated blisk.(3) Belonging to the rotating geometry model, the impeller is often measured throughcoordinate measuring machining (CMM) which leads to the second clamping and cannotachieve the impeller integration of machining-measuring. In order to take probe size andglobal accessibility into consideration at the same time, the global accessible cone of measur-ing point which was computed by graphic processing unit (GPU) was defined to get the wholevalid accessible cone along the measuring path. On the basis of the shortest and fairing mea-suring path principal, the minimum discrete surface area of the probe’s axial trace surfacealong the measuring path could get the optimal probe direction. According to the correspond-ing evaluation method, the proposed algorithm ensure effective collision-free measuring pathof the integrated impeller.(4) The preliminary integration software platform of machining-measuring was set up forimpeller or blade, including point-surface geometry modeling, blade spiral milling, colli-sion-free measuring path planning, point cloud processing and feature parameters extractionsuch as function modules. Meanwhile,5-axis collision-free site measuring planning got effec-tive application with Renishaw OMP40in the machine tool called Mikron UCP800of the na-tional key laboratory.