Base Frame Calibration For Serial 6-dof Manipulators Based On Vision

In a multi-robot collaborative machining system,to realize the coordinated operation between the robots,it is necessary to determine the relative position of the base frame of the robot with respect to other robots and the worktable.For these industrial applications,this paper proposes a vision-based calibration method of the base coordinate system of a 6-DOF manipulator.Taking the UR5 manipulator as the research object,the position analysis based on the standard D-H notation is carried out.The forward kinematics,inverse kinematics and the reachable workspace of the manipulator are analyzed,which are simulated and analyzed based on MATLAB.To overcome the defects of standard D-H notation in error analysis,a modified D-H model is used to describe the relative pose of two adjacent links in some joints.The corresponding errors in each joint of the manipulator is derived.The error model of end-effector is established.On this basis,the error model is simulated,and the error sensitive factors of the manipulator are analyzed.To realize the calibration of the base frame of a manipulator,the positions and the poses of the target described in the camera coordinate system under different robot configurations are obtained by calibrating the camera with respect to the target installed at the end of the manipulator.Combining them with the poses of the end-effector of the manipulator obtained from its forward kinematics,the relative position of the base coordinate system described in the camera coordinate system can be calculated by solving the calibration equation AX=ZB.After calibrating all the manipulators in the multi-robot cooperation system,the pose relationship between base coordinate systems of any two manipulators in the system can be obtained.The base frame calibration method of the manipulator is simulated in MATLAB and ROS,and the result shows the effectiveness of this method.A calibration experiment platform is built,which comprises two UR5 manipulators and a CMOS digital camera,and the calibration experiments are carried out based on the proposed method.The results show the effectiveness of the method.Through the analysis on the result,it can be concluded that the accuracy of the calibration can be improved by increasing the number of the poses of the manipulator measured in a calibration session.