A simple iterative scheme for learning gravity compensation in robot arms

The set-point regulation problem for robot manipulators under gravity is usually solved by either model-based compensation or PID control. The former cannot be applied if an unknown payload is present or when model parameters are poorly esti-mated, while the latter requires fine and lengthy tuning of gains in order to achieve good performance on the whole workspace. A simple iterative scheme is proposed for generating exact gravity compensation at the desired set-point, without the knowl-edge of the robot dynamic model. The control law starts with a PD action on the joint error, updating at discrete instants an additional feedforward term. Global con-vergence of the scheme is proved under a mild condition on the proportional gains. Simulations are presented for a 3R rigid robot arm moving in a vertical plane. 1