Dynamics Prediction And Chatter Stability Analysis Of Robotic Milling System

For machining of large complex parts,the advantage of using industrial robotic milling system is greater flexibility and lower cost than traditional large machine tools.However,industrial robots’ serial structure results in its significantly less stiffness than machine tools,which easily leads to chatter in machining process.Robots’ highly pose-dependent frequency response function(FRF)give rise to obvious difference of chatter stability limitation between different working area.With prediction method of FRF under different postures of robots,chatter stability limitation can be calculated under various robot postures and cutting condition.This can be theoretical guidance and technical support in the application of robotic milling.The main contents of this thesis are as follows:1.IRB6660 robot kinematics model is obtained by means of modified D-H method,forward kinematics equation and Jacobian matrix are derived.Based on multi-body dynamics model,Newton-Euler method is used to derive robot dynamics equation,and the equation is linearized by coordinate transformation of robot links’ inertia parameters.This is the basis of following research.2.The robot stiffness matrix in the Cartesian space and in the joint space is related via Jacobian matrix.Static load experiment is designed and conducted to determine the stiffness of each joint.3.To solve the problem of robots’ highly pose-dependent FRF,FRF prediction method of robots based on linear joint model is proposed.Due to the huge difference of inertia parameters between the first three links of robot elbow and the last three links of robot wrist,the 6-DOF model of robot is simplified as a 3-DOF model,simulation is conducted to show the rationality of the simplification.Based on this 3-DOF model,one identification method of robot dynamics parameters with several impact tests is presented.Identified robot dynamics parameters can be used to predict robots’ FRF under any posture.4.With simplified 3-DOF model,joint stiffness and robot dynamics parameters,the role of robot posture,feed direction and cutting condition played in chatter stability of robotic milling process is analyzed via regenerative chatter theory and mode coupling chatter theory.Working area where has high possibility of mode coupling chatter is calculated and presented.The analysis results show that regenerative chatter is extremely easy to occur in low speed milling using robotic milling system,and for both mechanisms of chatter,changing the feed direction has a significant effect on chatter stability limitation.