Research On Toolpath Planning Method For Robotic Surface Milling

With the advantages of large working space,high degree of freedom and high processing flexibility,industrial robot has become an important embodiment in intelligent manufacturing,and shows unique advantages compared with traditional NC machining in machining process.However,due to the weak stiffness and low accuracy of the industrial robot,the problems of low machining accuracy and poor machining quality are very prominent in the complex surface milling with large cutting force.It is the research hotspot and difficulty to achieve high precision and high efficiency robot milling.In view of this problem,this work studies the two main problems of robot stiffness deficiency and cutting force fluctuation in complex parts robotic milling.Firstly,the stiffness model of robot milling system is established,and the stiffness evaluation index for robot milling is proposed in this work.Then,the method of robot posture optimization and workpiece setup planning for stiffness optimization is proposed,so that the robot can maintain a high stiffness posture for machining.On this basis,the toolpath planning of robot milling for complex surface is completed,so as to achieve the goal of improving the machining accuracy of robot milling.Then,the material removal rate prediction method for milling is proposed.On this basis,an efficient toolpath planning method which makes the cutting force stable is proposed,so as to improve the machining efficiency of robot milling.The main research contents of this paper are as follows:(1)Stiffness characteristic analysis and modeling of robot milling system.To solve the problem of stiffness evaluation in robotic surface milling,this paper first establishes the kinematic model of robot milling system,and obtains the accurate kinematic model of robot through kinematic geometric parameter calibration.Then,on the premise of identifying the joint stiffness matrix of the robot,the Cartesian stiffness matrix of the robot end is established.On this basis,the stiffness evaluation index which can reflect the robot's deformation,caused by the force,during milling is proposed,laying a theoretical foundation for the subsequent stiffness optimization and toolpath planning.(2)Robot posture optimization and toolpath planning method based on the stiffness performance.To address the problem of insufficient stiffness in robot milling,an optimization method of robot posture and a toolpath planning method based on region segmentation are proposed.Firstly,a robot posture optimization model is established to optimize the stiffness index.This model considers the influence of tool orientation and robot redundancy on the robot stiffness,and is constrained by joint limit,singularity and collision.Then,for the problem that the robot stiffness can not be fully improved due to the limitation of robot kinematics performance,a posture-based surface subdivision method is proposed in this study.In this method,the workpiece is divided into multiple processing sub-regions by spectral clustering algorithm,which breaks through the limitation of joint speed on robot posture optimization and makes the robot obtain higher stiffness.Finally,the feed direction of toolpath is optimized according to the tangential stiffness of each sub-region to further improve the stiffness.Through the verification,this method can improve the robot stiffness during milling,so as to improve the machining accuracy.(3)Workpiece setup planning method based on stiffness threshold.To address the problem of low accuracy of some machining area casused by poor workpiece' orientation,a method of workpiece setup planning based on stiffness threshold is proposed.Firstly,under the constraints of collision,singularity and robot joint,the optimization model of workpiece position and orientation for stiffness improving is established and solved by particle swarm optimization.Then,for complex freeform surfaces,to obtain the minimum number of posture changes of the robot and workpiece under the premise of meeting the limit of stiffness threshold,a workpiece setup planning method based on minimum set covering method is proposed.Through the verification,this method can obtain the minimum number of posture changes of the workpiece under the premise of meeting the limit of stiffness threshold,so as to reduce the machining error.(4)Milling toolpath planning method for cutting force stability.To address the problem of low machining efficiency caused by large fluctuation of cutting force in robot milling,a milling path planning method for stability optimization of cutting force is proposed.Firstly,based on the analysis of the correlation between cutting force and material removal rate,an efficient calculation method of material removal rate based on pixel method is proposed.On this basis,the threshold selection method and the contour fitting algorithm are used to extract the critical machining region in the milling process.Then,by combining the advantages of variable radius trochoidal toolpath in cutting force stability and the advantages of contourparallel toolpath in efficiency,a RVTR-CP toolpath planning method is proposed.Control algorithms are developed to ensure that the connection between trochoidal trajectories with neighboring contour-parallel toolpaths is geometrically continuous,and under very consistent material removal rate transition as well.Through verification,this method can obtain more stable cutting force and higher machining efficiency than the existing CAM software.(5)Application case of robot machining for large complex parts.Firstly,based on the above research,the robot milling software module is developed by combining the methods of stiffness-based robot posture optimization,workpiece setup planning,cutting force stability optimization and milling toolpath planning.Then,aiming at the large and complex casting parts with large size,high hardness and complex processing area,the robot milling processing examples are verified by combining the above optimization method.