On-line Compensation Of Machining Errors In Robot Milling Based On Static Stiffness Model

With the development of industrial automation,the accuracy and reliability of industrial robots are constantly improving,and it is gradually applied to the field of milling processing due to its advantages of flexibility,adaptability,wide processing range and high cost performance.However,due to its weak rigidity,the milling accuracy of industrial robots is lower than that of CNC machines.Therefore,modeling and analyzing the stiffness characteristics of industrial robots,compensating for errors in the milling process of industrial robots,and improving the accuracy of milling of industrial robots are of great significance for industrial robots to be better applied in the field of milling.Consider the parallel four-bar link of the ABB IRB6660 industrial robot,using the DH method to establish its kinematics model.Based on the kinematics model,the Jacobian matrix is constructed using the vector method,and the Jacobian calculator of the ABB IRB6660 robot and the six-joint robot of the universal model is developed.Considering the change of the joint stiffness of the robot,the joint stiffness is functionally fitted through the joint stiffness identification data,and the static stiffness model of the joint considering the joint stiffness change is established.The Z-direction stiffness distribution of the same height in the workbench space is calculated based on the static stiffness model,and the calculation results are verified.A feedforward robot milling error compensation algorithm is proposed with the only input of real-time cutting force.Based on the milling force prediction model,the relationship between cutting force and depth of cut is fitted.Considering the coupling relationship between compensation and deformation,a quadratic iterative method for single point compensation calculation is proposed.The criterion will be set when the second iteration method is applied to the continuous milling process.The interrupt mechanism is used together with the correction generator to realize the robot milling error compensation algorithm in the robot controller.The correctness and stability of the continuous milling machining error compensation process are verified under the cutting conditions of constant cutting depth,gradient cutting depth and sudden cutting depth.The overall scheme of robot milling machining error compensation is developed.The computer collects real-time force information through the force sensor installed at the end of the robot,communicates with the robot controller through Ethernet through the PC Interface option,and performs online error compensation outside the position loop.Use C# combined with ABB PC SDK to write the software to realize the error compensation algorithm,modify the robot post-processing program,add the interrupt mechanism,cooperate with the correction generator,and compensate the machining error by the absolute trajectory offset of the robot.Finally,the feasibility and accuracy of the robot milling error compensation scheme are verified under the conditions of constant cutting depth and variable cutting depth.