A task-oriented calibration method for a 5-DOF hybrid machining robot based on the equivalent system

Calibration is a practical technique to improve robot accuracy. However, the pose error of the robot with fewer than six degrees of freedom (DOF) cannot be fully eliminated via calibration. The separation of compensable and uncompensable error components remains a controversial issue. This paper presents a novel task-oriented concept for calibration, which enables the uncompensable pose error to match the unconsidered pose error during the task. From the view of the equivalent system, the pose error is attributed to configuration-dependent motion errors of the actuated and virtual joints equivalently, where the virtual joint motion error is uncompensable and the virtual joint twist is determined according to the task. The calibration methodology of a 5-DOF hybrid machining robot is then described. Calibration results show that the maximum considered position/ orientation errors can be reduced from 1.314 mm/0.101 deg to 0.118 mm/0.020 deg. Machining experiments also demonstrate the high-precision metal-cutting capacity after calibration.