This paper presents a universal output-feedback tracking strategy for minim
um phase linear systems of arbitrary but known relative degree with a class
of nonlinear perturbations and output measurement noises. Some information
, such as the state dimension, the sign of high-frequency gain and the rela
tive degree of nonlinearly perturbed systems, are not known a priori. It is
proved that the tracking error is guaranteed asymptotic to the interval [-
lambda, lambda] subset of R with arbitrary prescribed lambda > 0. (C) 1999
Elsevier Science Ltd. All rights reserved.