Friction Compensation in Hybrid Force/Velocity Control for Contour Tracking Tasks

Nowadays robots in industrial settings are mainly used for repetitive tasks where they act as programmable devices reproducing previously recorded motions in a highly structured environment so that decision and initiative ca-pability is rarely exploited. Contour tracking is, on the contrary, an example of a complex task that requires the manipulator to continuously and autono-mously modify its path, coping with the uncertainties typical of unstructured environments (Siciliano & Villani, 1999). In many applications a robot is re-quired to follow a contour while applying a normal force; these tasks include grinding (Thomessen & Lien, 2000), deburring (Ferretti et al., 2000; Ziliani et al., 2005), shape recovery (Ahmad & Lee, 1990), polishing and kinematic cali-bration (Legnani et al., 2001). The problem of tracking (known and) unknown contours has been studied by many researchers in the last two decades. Hybrid force/velocity control (Raibert & Craig, 1981) appears to be suitable to be adopted in this context, because it explicitly controls the end-effector force in a selected direction and the end-effector velocity in the other complemen-tary directions. Actually, two kinds of hybrid force/velocity control can be implemented (Roy & Whitcomb, 2002): 1)