Variable structure control of a robot with elastic links

The authors consider the problem of controlling the endpoint of a two-link planar robot with a flexible forearm. The control strategy which imposes the torque on the two joints of the robot is based on the sliding mode technique. The main advantage of this choice is to obtain a control law which is robust with respect to both parameter variations and external disturbances. To develop the proposed strategy, two reference state trajectories (one for each link) are chosen. The corresponding sliding surfaces are then defined in terms of the error between the system state variables and the reference ones. The coefficients which characterize the sliding surfaces are determined in such a way that the equivalent motion is asymptotically stable irrespective of the robot geometric configuration. The algorithm that is obtained has the advantage that the terms which act to reduce the sensitivity to parameter variations and the effects of nonlinearities appear explicitly in the control law. Experimental results obtained by digital simulation have confirmed the satisfactory behaviour of the robot motion with the proposed control law