Importance of resolution of joint encoders during robotic hard-on-hard tasks

This paper presents the importance of the resolution of joint encoders during a hard-on-hard contact task typified by robotic drilling. Experiments have been conducted with a PUMA 560 manipulator in contact with a stiff environment since the manipulator can be regarded as being in a quasi-static configuration during drilling. Experimental results reveal that velocity feedback which is important for the stability of a control strategy for a hard-on-hard contact task is inoperative because of encoder quantisation. Velocity feedback is the only source of damping for stabilising the system when there is not enough passive damping (nonlinear friction) in a force control system. We conclude that high quality manipulator joints with low passive damping need high resolution encoders for velocity feedback, a requirement for the stabilisation of a force control system making a hard-on-hard constrained contact