Stability optimization in robotic milling through the control of functional redundancies

Productivity in robotic machining processes can be limited by the low rigidity of the overall structure and vibration instability (chatter). The robot's dynamic behavior, due to changes in its posture along a machining trajectory, varies within its workspace. Chatter in robotic machining therefore depends not only on the cutting parameters but also on the robot configuration. Moreover, the robot can follow a machining trajectory in the operational space with an infinite number of possible trajectories in its configuration space. It is due to the redundancies offered by its kinematic chain. This paper deals with the optimization of robotic machining stability by controlling the robot configurations and the functional redundancies of its kinematic chain. It is shown that stability in robotic machining along a given trajectory can be ensured through the optimization of the robot configurations, without changing the cutting parameters, in order to maintain productivity performance. A multi-body dynamic model of an ABB IRB6660 industrial robot is elaborated using beam elements which can easily be integrated into the machining trajectory planning. The beam element geometry, elasticity and damping parameters are adjusted on the basis of experimental modal identifications. The present study is focused on the dynamic model-based predictions of stable and unstable zones along a robotic machining trajectory with one degree of functional redundancy. Experimental machining tests confirm the stability predictions from the numerical model.