Universal integral controllers for minimum-phase nonlinear systems

We consider a single-input-single-output (SISO) nonlinear system that has a well-defined normal form with asymptotically stable zero dynamics. Using o nly knowledge of the relative degree and the sign of the high-frequency gai n, we design an output feedback integral controller that asymptotically reg ulates the output to a bounded time-varying reference signal with a constan t limit. We give regional as well as semiglobal results. We also show that, for relative degree-one and two systems, the proposed integral controller reduces to the classical PI and PID controllers, respectively.