Pose Parameter Calibration And Smooth Path Planning For Robotic Milling Aircraft Skin Parts

The thin-walled aviation parts represented by skin parts have the characteristics of complex shape(free-form surface design),large structure size(the skin of the fuselage can be up to 10 m),and weak rigidity of the thin wall(thickness only 2 mm).At present,manufacturing companies generally use the naked eye positioning/manual trimming method to remove the edge machining allowance of the skin blank.There are problems such as large human error,low accuracy of contour machining,and uncontrollable seam clearance after assembly.Vision-guided robotic machining has technical advantages such as precise visual positioning,flexible operation in a large range,and multi-robot collaborative operation,which provides a new approach to solve the above problems.However,the diversity of the measurement-processing system components,the non-rigidity of the actual assembly deformation of the skin,and the discreteness of the point cloud data required for the generation of the processing path make it difficult to calibrate and compensate for the errors between the components,and to map the surface calculate the edge machining allowance,and to control the smoothness of the machining path under the constraints of stiffness/dexterity.They have become the main problems which restrict the application of vision-guided robotic machining technology.To this end,this paper has carried out deep researches on system pose calibration and error compensation,surface reconstruction based on measuring points and edge machining allowance calculation,tool trajectory generation and robot posture optimization.The main research results include:(1)A mathematical model AXB(28)YCZ for simultaneous calibration of the transformation matrices between the multiple units of the robot measuring-machining system is established.Two simultaneous calibration methods of closed solution and numerical iterative solution are proposed to efficiently solving the transformation matrices between the hand-eye coordinate systems,the base coordinate systems,and the end-tool coordinate systems of the robot measuring-machining system.Five unsolvable lemmas of the calibration equation AXB(28)YCZ are proposed and proved mathematically.The optimal conditions for selecting the end poses of the machining robot and the measuring system in calibration are defined,which solves the singular problem of solving the calibration equation caused by improper selection of the robot end poses in practical applications.(2)The quantitative expression of the differential motion of the robot end caused by the kinematic errors of each joint and the calibration errors of each unit’s pose parameters is derived.Define the generalized kinematic error vector composed of the pose errors to be identified including the joint kinematic error vectors and the pose error vectors generated by the calibration of pose parameters between units.A linear identification and accurate compensation method for pose error among multi-units considering kinematic error compensation is proposed,which solves the essential problem that the calibration accuracy of robot measuring-machining system is limited by kinematic errors of robot joints.In the simulation,the calibration error after compensation is reduced from 0.317 mm to 0.043 mm(3)Define the boundary-skeleton constraint conditions for the surface reconstruction of the vacancy area to be assembled.A fast surface reconstruction method under the boundary constraint of the inner contour with an arbitrary shape is proposed.Furthermore,a model of surface deformation and optimization considering external forces,surface stiffness,constraints boundary and inner narrow skeleton characteristic is established.The accuracy reconstruction of surface mesh model for the vacancy area to be assembled is achieved.An efficient calculation method of surface’s normal form with geodesic distance as inherent geometric feature descriptor is proposed.A surface non-rigid mapping method based on normal form matching and fine projection is proposed.The precision calculation of edge allowance of skin parts under pure bending deformation is realized.(4)A tool path generation method based on dual NURBS curve least-squares fitting is proposed using the mesh model of the skin surface and the points of edge machining allowance as inputs.The ruled surface of the initial tool trajectory is obtained.Then,the tool path smoothing-deviation synchronization optimization model is further established,which solves the jitter problem of tool path points and tool axis vectors in the robotic machining path generated directly from the measuring points.The stiffness and dexterity synchronization optimization objective function is established,and the intelligent optimization algorithm is used to solve the optimal robot poses.Finally,a robotic machining path that satisfies the requirements of smoothness,rigidity and dexterity is generated.On the basis of the above research,the experiments of dual-robot system calibration,edge machining allowance calculation and robotic machining path generation are carried out by relying on the robot measuring-machining system built by our team,which verified the effectiveness of the proposed system calibration and machining path planning methods.Moreover,the proposed methods are applied to edge milling of a cover skin sample,edge milling of flat tail skin samples and milling of gaskets samples for wing and fuselage assembly.The machining errors of skin samples and the thickness errors of the gasket samples can meeting the requirements of application.