In this paper, the problems of stabilization and disturbance rejection in the control of class I of underactuated mechanical systems (UMS) are addressed. Based on a global change of coordinates, the original dynamic model is transformed into a strict-feedback form and a Super-Twisting (STW) controller is designed to resolve the stabilization problem. To deal with the problem of uncertainties and external disturbances in UMS, a disturbance observer (DO) is proposed for the estimation of the

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... disturbances in the aim of compensating them in the controller and improving the performance and robustness of the resulting closed-loop system. Indeed, the proposed observer-based scheme is compared with the standard STW controller. Both controllers have been implemented and validated through real-time experiments on the inertia wheel inverted pendulum. The obtained results show clearly the superiority of the proposed observer-based STW control scheme and its effectiveness in terms of external disturbance rejection.