Vibro-impact dynamics of material removal in a robotic grinding process

This paper studies the vibratory dynamics governing material removal performed by a compliant robot. The objective is to understand whether metal removal at a target rate or to a target depth of cut is possible given the unavoidable significant and sustained vibrations inherent to the process. Robotic grinding by the SCOMPI robot developed at Hydro-Québec’s research institute for field maintenance work on hydroelectric equipment is studied. A test rig is developed to investigate the instantaneous process of material removal. The setup employs the methodology of angular analysis to detect and locate the discrete cutting events with respect to the instantaneous angular position of the spindle. Test results confirm that the grinding wheel exhibits cyclic impacting oscillations while removing material. This vibro-impact behavior is explained by the nonsmooth nature of the system arising from the substantial difference between process stiffness and stiffness of the robot. Grinding force and power are formulated based on the impacting dynamics of material removal (impact cutting). Parameters of the impact-cutting model are determined experimentally. The impact-cutting model is found well-suited to predicting the grinding power required to remove metal at a target rate. The conclusion is reached that metal removal to a target depth and with acceptable surface waviness is possible despite high-amplitude vibro-impacts between the grinding wheel and workpiece.