| Industrial robots,equipped with high-performance spindles and end effectors for the milling operation,are widely used in the aerospace,energy and marine industry,since they are flexible,reconfigurable,multi-sensor and intelligent.Because of the high machining requirements of large complex parts,robot milling operations are increasing day by day.Robot milling of marine propeller has become a new machining method instead of manual grinding and CNC machining.It is a common problem to improve the robot stiffness and obtain the robot tool tip dynamic characteristics when milling the large complex parts with the robot.Due to the poor stiffness of the robots and the change of dynamic characteristics with the tool tip position,which affects the machining quality of the robot,the main research methods are to optimize the robot's machining stiffness based on the stiffness performance index and accurately predict the robot's tool tip frequency response.However,in view of the current research,there are few quantifying indexes to evaluate the stiffness performance of the robot,and the prediction of the tool tip frequency response cannot meet the requirements.In this dissertation,the force-deformation law of the robot,the changes of robot stiffness with the posture and milling conditions and the effects of dynamic changes within the workspace on the machining are studied by the experiments and modeling.The related theoretical methods are applied to the process optimization,and the main contents are as follows:The force-direction stiffness performance indexes for robot milling are proposed,and the distribution characteristics of stiffness performance indexes in the workspace,task plane and machining surface are analyzed.Based on the force deformation characteristics of milling process,considering time-varying cutting force in robot milling,the influence of normal deformation on machining accuracy and the influence of stiffness difference in different directions on modal coupling characteristics for robot milling path,the overall stiffness performance index,normal stiffness performance index and the principal stiffness performance index of robot are proposed.Based on the milling conditions,the distribution of the overall stiffness performance index in Cartesian space for different posture,the influence of different joint angles on the area which is fit for machining,the distribution of normal stiffness performance index of work plane with different height and the influence of different redundant angle on the distribution of normal stiffness performance index on the machining plane,and the influence of different feed direction on the distribution of principal stiffness performance index on the given parametric surface are analyzed.In this dissertation,a novel model based on binary tree method is proposed to predict the frequency response of the tool tip of a robot at any posture.A large number of hammering experiments are carried out,and the results show that the dynamic characteristics of the robot tip vary with the posture.For the robot composed of links and rotating joint components,based on the frequency response function obtained from hammering experiments at joint angle of 0 ° or 90 °,a prediction model of the frequency response of the holder of a single degree of freedom robot at any posture is established.Using the properties of binary tree,the model is extended to the prediction of the holder frequency response of the six degree of freedom robot,and the tool tip frequency of the robot at any posture is established considering the coupling effects between the holder and the tool.Thus,a prediction model of the tool tip frequency response of a 6-DOF robot at any posture is proposed based on the RCSA method.The laser displacement sensor and acceleration sensor are used to measure the low frequency and high frequency response of the tool tip respectively,which verifies the correctness of the frequency response prediction model.This dissertation analyzes the characteristics of the frequency response of the tool tip of different robot postures,and explores the influence of the joint angle on the dynamic stiffness.The optimization models of tool axis,redundant angle and feed direction are proposed for marine propeller milling with a robot.The efficiency and accuracy of propeller machining are very important.Based on machining stability analysis,the contact area between tool and workpiece is established,and the maximum cutting depth and cutting area of different tool axis are calculated.The optimization model of material removal rate in robot milling is proposed through tool axis calculation,which can shorten the machining cycle of robot milling.After determining the tool axis and the tool center point,a redundant angle optimization model is proposed to reduce the normal deformation using the normal stiffness performance index.Milling with low speed is easy to cause mode coupling chatter in the machining of propeller blade root area.The state of two-dimension mode coupling chatter of robot milling in the principal direction coordinate system is deduced.The influence of feed direction during machining on mode coupling chatter state is analyzed,and the optimization model of robot feed direction in chatter free milling is proposed.The process validation of robot milling for marine propeller is realized.A robot milling system for the marine propeller is built.The structure of the machining system is introduced.The coordinate system of the machining system is constructed.The method of calibrating the coordinate system using laser tracker is proposed.The robot milling CAM software is developed to realize the tool path planning of robot multi axis machining,including trajectory planning,robot redundant angle,tool axis,feed direction optimization,machining performance evaluation and machining process monitoring.In the propeller factory,the robot milling validation of multi marine propellers was carried out. |
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