A New Approach to Control Single-Link Flexible Arms, Part II: Control of the Tip Position in the Presence of Joint Friction

Abstract

This report presents a new way to control the tip position of single-link flexible arms when friction is present in the joint. In order to minimize the influence of the nonlinear components of this friction, the control scheme is composed of two nested loops: an inner loop that controls the motor position, and an outer loop that controls the tip position. It is demonstrated that proper design of the inner loop of this control scheme eliminates the effects of friction in controlling the tip position, and may significantly simplify the design of the tip position controller. Three control schemes are proposed for the outer loop. All of them are based on an hybrid feedforward-feedback control scheme. The first and second schemes use only tip position feedback while the third one uses sensing of positions at several points of the beam. Performances of the three schemes are compared under the following disturbances: a) mator position perturbations, b) unmodelled dynamics, and c) changes in the payload. Experimental results are presented for the two arms described in the Pari I; the three tip position control schemes are compared in both arms; and. finally, conclusions are drawn.