State-space characterization of Youla parametrization for nonlinear systems based on input-to-state stability

This paper deals with the state-space parametrization of all nonlinear stabilizing controllers via detectable kernel representations. We adopt ISS as the operator stability, and give the characterization of detectable kernel representations based on it. We then give a unified result of Youla parametrization of nonlinear systems, which is valid both locally and globally in its state-space realization, while the existing result in conventional input/output approach is valid only in global setting, namely, it is only applicable when a global stabilizing controller and a global state observer are found. This result tells us when the existing state-space parametrizations give all the stabilizing controllers.