Calibration Method For Modular Collaborative Robot Considering Joint Stiffness

With the increasing demand for flexibility and intelligence in modern manufacture,modular collaborative robots show a good application prospect in many industrial fields because of its simple structure and high load/self-weight ratio and other advantages.However,this kind of robots mostly use the harmonic reducer as the main transmission structure of the joint,its stiffness is relatively low,under the influence of robot's gravity and external load,the joint deformation seriously affects the positioning accuracy of the robot.The traditional the kinematic calibration method which only considers the geometric parameter errors is no longer applicable.In addition,the stiffness of the harmonic reducer is highly nonlinear,so it is difficult to solve the joint stiffness parameters by linear identification method.For this reason,a kind of new calibration method is proposed in order to improve positioning accuracy of the modular collaborative robot.Firstly,the kinematic model of modular collaborative robot is established based on D-H method,including forward and inverse kinematics,and the mathematic model is verified using MATLAB.Secondly,geometric error mapping model is established according to the link differential motion.Through static analysis,the joint torque model is established considering gravity of the robot arms and external load.After piece-wise linearizing the stiffness characteristic curve of the harmonic reducer,the deformation of the robot joint angles have been solved,thus,the stiffness error model of the robot has been established.Then,the end position errors of in different configurations of robot have been measured using Laser Tracker.And component of the position errors caused by joint flexible deformation is predicted based on the stiffness error model,then it is separated from the comprehensive errors to get the end positioning error component caused by geometric parameter errors.Afterwards,using the least squares identification method,the geometric error parameters of the robot have been identified based on the geometric error component.Next,geometric error parameters of the robot have been corrected in controller,meanwhile,the joint angle errors caused by the joint flexible deformation have been compensated in joint space.Finally,the calibration experiment of the modular cooperative robot has been carried out.The experimental results show that the positioning accuracy of the robot prototype can be increased by about 70% using the calibration method proposed in this paper.