H-infinity optimised control of external inertial actuators for higher precision robotic machining

Serial-link industrial robots that feature small footprints and large working volumes have been used widely in various applications. However, their low-stiffness-induced vibration hinders application in precision robotic machining processes. In robotic milling, the spindle may experience chatter, leading to unsatisfactory surface finishes. Moreover, the chatter frequency varies, depending on the robot pose and its underlying stiffness and inertia characteristics, which is likely to be outside of the controlled robot system bandwidth. One solution is to use inertial actuators close to the source of excitation on the robot to generate forces that counteract the vibration. This paper advances matters by employing optimised H∞ control strategies for improved effectiveness and robustness to perform active vibration suppression. The strategies are independent of tool paths and take account of different robot poses, hence the variability of the robot dynamic characteristics. Performance is assessed against that of standard velocity feedback controllers in eccentric mass experiments and milling tests. The experimental results show that within a relatively large work-plane (500 mm × 500 mm), H∞ controllers greatly improve machining capability and reduce machining errors by up to 85%.