A tool for designing a periodic motion in underactuated systems, by generating a self-excited oscillation of a desired amplitude and frequency, driven by a variable structure control is reviewed. The main purpose of this paper is to illustrate the capabilities of the two-relay controller to induce oscillations in dynamical systems. In this paper, we overview three methods to set the frequency and amplitude of oscillation: describing function, LPRS, and Poincare´ maps. Theoretical and practical open problems are also discussed.
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