This paper studies the precision motion control of single-rod hydraulic actuators with time-varying unknown inertia loads. The swing motion control of a robot arm driven by a single-rod hydraulic actuator (a scaled down version of industrial backhoe loader arm) is used as a case study. During the swing motion, the effective mass acting on the hydraulic actuator varies greatly with the swing angle and thus time-varying. Furthermore, due to the change of the load of the robot arm in operations, the effective mass is also unknown. A discontinuous projection based adaptive robust controller is constructed to take into account this strong dependence of the effective mass on the fast changing swing angle and the unknown payload of the arm. The resulting ARC controller is able to take into account not only the effect of parametric uncertainties coming from the payload and various hydraulic parameters but also the effect of uncertain nonlinearities such as uncompensated friction forces and external disturbances. The ARC controller guarantees a prescribed output tracking transient performance and final tracking accuracy while achieving asymptotic output tracking in the presence of parametric uncertainties. In addition, the zero output tracking error dynamics for tracking a large class of time-varying trajectories is shown to be globally uniformly asymptotically stable. Comparative experimental results are presented to show the effectiveness and the high performance nature of the proposed control algorithm.

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