Estimation and Compensation of Gravity and Friction Forces for Robot Arms: Theory and Experiments

This paper considers the estimation and compensation of the unknown gravity force and static friction for robot motion control. Utilizing the stability feature of PD set-point control, the estimates of gravity-related parameters and static friction can be solved from two steady state equations obtained by stopping robots at two nonsingular positions. The estimates obtained can then be used to eliminate the position error. Under a mild assumption that the mass center of each robot link is distributed on a straight line connecting two adjacent joints, the gravity force regression matrix becomes upper-triangle which can significantly simplify the algorithm. The positive experimental result obtained for practical verification is also presented.