A new definition of the minimum-phase property for nonlinear systems, with an application to adaptive control

We introduce a definition of the minimum-phase property for general smooth nonlinear control systems. The definition does not rely on a particular choice of coordinates in which the system takes a normal form or on the computation of zero dynamics. It requires the state and the input of the system to be bounded by a suitable function of the output and derivatives of the output, modulo a decaying term depending on initial conditions. The class of minimum-phase systems thus defined includes all affine systems in global normal form whose internal dynamics are input-to-state stable and also all left-invertible linear systems whose transmission zeros have negative real parts. We explain how the new concept enables one to develop a natural extension to nonlinear systems of a basic result from linear adaptive control.