Global practical output tracking via output-feedback for cascade systems with more uncertainties

This paper considers the global practical tracking problem by output-feedback for a class of uncertain cascade systems with zero-dynamics and unmeasured states dependent growth. In contrast to the closely related works the systems investigated are substantially different, because of the simultaneous existence of zero-dynamics, unknown growth rate, and unknown time-varying control coefficients. This makes the problem much more difficult to solve, and for which, new solution scheme should be proposed to control design. Mainly motivated by our recent works, by skillfully combining the methods of universal control and dead zone with backstepping technique, and with the help of the pattern of performance analysis proposed in our tracking work, we successfully propose a new adaptive tracking control design scheme for the system studied. It is shown that the designed controller guarantees that all states of the resulting closed-loop system are globally bounded and the tracking error remains prescribed arbitrarily small after a finite time.