Robust velocity tracking with only sign-observation

This paper presents a simple strategy for velocity tracking in mechanical systems using only the sign information of the tracking error. The presented method indeed applies to a more general case of constrained measurement that preserves sign. The error convergence as well as the closed-loop stability is rigorously shown in terms of both smooth and nonsmooth versions of Lyapunov argument. Analysis based on relay feedback system is also provided. The practical significance of the proposed control method is demonstrated through its application to the actuator servo in a hard disk drive in the absence of position information. Both simulation and experimental results show that, in spite of simplicity, the proposed method achieves the desired error regulation quite robustly to unmodelled dynamics.