A metrological study of the small displacement behaviour of a SCARA type pick and place robot

SCARA (selective compliance assembly robot arm) robots are frequently used for light pick-and-place type assembly tasks, and typically offer repeatability of +or-0.025 to +or-0.05 mm (+or-1 to 2 mils). Metrological data taken at a variety of positions, scattered throughout the horizontal plane of the robots´ Cartesian workspace, indicates that the robot typically fails to respond to very small bands of displacement commands which correspond to the angular bands of physical motion within which the position feedback encoders present discrete values for displacement. An analysis is presented of the small-displacement behavior of a SCARA robot within the range of its repeatability specification, showing that it is basically dependent on the digitized resolution of the axial-joint-position feedback encoders.<>