Universal output regulation for nonlinear systems in output feedback form

This paper deals with global output regulation via output feedback of a class of uncertain nonlinear systems subjected to a class of unknown disturbances. Both the uncertainty in the system model and the uncertainty in the exosystem are tackled concurrently. The disturbances generated from a unknown linear exosystem are completely compensated. The order of the exosystem is assumed known and the eigenvalues are distinct. The system is assumed in the format of the minimum-phase output feedback form, with no knowledge of the values of the system parameters, including the high-frequency gain. No other assumptions are needed in the control design. A new formulation of state estimation is introduced to ensure the regulation is a global one with respect to state variables. The passivity of the exosystem is exploited to design a new auxiliary error, involving both the unknown parameters of the reformatted exosystem and those of the system, which makes it possible to group the all the unknown parameters in a format suitable to adaptive control design. A Nussbaum gain is introduced to adaptive control to deal with the unknown high frequency gain and a number of control coefficients are also made adaptive so that the regulation is global with respect to unknown frequencies in the disturbances.