Robust, digital, nonlinear control of magnetic-levitation systems

This paper presents a robust, adaptive, nonlinear controller for a class of magnetic-levitation systems, which includes active-magnetic bearings. The controller is analytically and experimentally shown to be superior to a cla ssical linens control system in stability, control effort, step-response pe rformance, robustness to parameter variations, and force-disturbance reject ion performance. Using fin adaptive backstepping approach, a Lyapunov funct ion is generated along with an adaptive control law such that the nonlinear , closed-loop, continuous system is shown to guarantee stability of the equ ilibrium and convergence of the parameter estimates to constant values. The control system error coordinates are proven to be bounded in the presence of a bounded force disturbance input. The novelty of this controller is tha t it is digitally implemented using Euler integrators with anti-windup limi ts, it is single-input-single-output requiring only a measurement of the po sition of the levitating object, and it is designed to adaptively estimate not only the uncertain model parameters, but also the constant forces appli ed to the levitating object ill order to ensure robustness to force disturb ances. The experimental study was conducted an a single-axis magnetic-levit ation device. The controller is shown to be applicable to active-magnetic b earings, under specific conditions, as well as any magnetic-levitation syst em that can be represented in output-feedback form.