Comparative Study of Three Robust Control Schemes for Single-Link Flexible Arms with Friction in the Joints

Abstract

Tip position control of flexible arms with friction in the joints is studied in this paper. The control scheme is based on two nested loops: an inner loop to control the position of the motor and an outer loop that controls the tip position. The design of the inner loop is quite straightforward. However, the the outer loop may be designed several different ways. Three methods are proposed here to control the tip position: a feedforward based control method, a method that eliminates the principal natural frequency of the beam, and a method based on the sensing at several intermediate points of the mechanical structure of the arm. In the ideal case (when there are no perturbations), these three methods produce the same dynamic behavior of the arm. This paper compares their performances when perturbations and unmodelled dynamics are present. In particular, three cases are considered, i.e.. external disturbances in the tip. unmodelled dynamics in the inner loop, and changes in the carried load. Analytic and simulation studies have been carried out to compare these methods and the results show that the third method which uses sensing at the intermediate points of the arm, had the best performance in all three cases.