On the output feedback control of passive nonlinear systems with input perturbations

Two problems concerned with output feedback control of passive nonlinear systems under input affine perturbations are considered. The first one is the problem of input-to-state stabilization of passive system with respect to "perturbation" input with semiglobal ISS gain assignment. The second problem is ultimate boundedness control of nominally passive nonlinear systems with structured uncertainties satisfying a matching condition. The key assumption is output-to-state stability of the system. It is shown that under this assumption both problems are solvable by almost smooth static output feedback.