The Blade Surface Inspection Based On The Improved Convex Hull Algorithm

Blade surface is generally complex surface, which has the technical characteristics ofvariable cross-section, strong distortion and deformation. The manufacturing process of bladeis complicated. The quality of multi-axis CNC machining of the blades plays an importantrole on the performance of aircraft engines and nuclear power turbines. How to fast andefficiently inspect the blades in situ of the machine tools has become the vital technicalproblem in the field of the aviation and nuclear power. In this paper, a certain aircraft engineblade is studied as an example. The methods on how to fast and precisely extract theparameters (including the characteristic parameters and profile parameters) of the aircraftengine blade surface from the measured point cloud data is studied. The main researchcontents are as follows:By using the coordinate measuring machine and flexible measuring arm, the fast acquireof the blade surface measured point cloud is studied. The data preprocessing methods on pointcloud denoising and point cloud reduction are also analyzed. On the basis of the achievementof the research group, the coarse registration based on the characteristics of the blade surfaceand the fine registration based on the adaptive distance function are studied. The principalcomponent analysis is used to accurately extract the normal vector of the cross-section of theblade, which provides the raw data for subsequent extraction of blade surface parametersprecisely.According to the discrete distribution characteristics of the measured point cloud, animproved convex hull algorithm based on the flat rectangular area etching method is proposed.The qualitative discriminant functions of the convex hull regional distribution of the point andthe directed straight line are defined, and the computational efficiency of the improvedconvex hull algorithm is analyzed. The proposed improved convex hull algorithm is used tosort the convex hull data of the blade-section, on the basis of which, the fast and efficientextraction algorithm of the characteristic parameters (such as the trailing edge, the chord, themiddle arc, the maximum thickness, etc.) and the profile parameters (such as the type lineerror, the tilt error, the curvature error, the distortion error, etc.) of the blade surface.Moreover, this proposed method is compared with the traditional method. The stability ofsurface parameter extraction is analyzed with the existence of Gaussian noise. Theimplemented experiments verify that the method of surface parameter extraction based on theimproved convex hull algorithm has advantages on both computational accuracy andcomputational efficiency.By using the proposed blade surface parameter extraction algorithms, the blade surface parameter extraction software module V1.0is developed based on the GUI environment ofMATLAB. This software module has the technical characteristics of human-computerinteraction, simple implementation and easy to spread. The machining experiments and fieldmeasurements of a certain aircraft blade have been implemented on the Mikron CNC machinetool at the state key lab of digital manufacturing equipment and technology. The fastextraction and precisely inspection in situ of the blade surface parameters can be easilyachieved by using the developed software module.