This article presents a robust output-feedback control scheme for the stabilization of a class of nonlinear underactuated mechanical systems in the presence of coupled disturbances. The scheme is based on the so-called coupled sliding-mode control, i.e., no transformation into the normal form is required. The proposed output approach is composed of a super-twisting controller and a high-order sliding-mode observer that robustly stabilize the origin of the system asymptotically. All of the results are proven by Lyapunov approaches. The proposed approach is validated by means of simulations and experimental results on a four-degree-of-freedom underactuated crane.
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