In this paper output tracking problem of time-varying systems is inves
tigated. A system under consideration contains time-varying parameters
which may change very fast and may be subjected to high-order nonline
ar disturbances. A robust controller requiring only input-output measu
rement is proposed. The control guarantees at least stability of unifo
rm ultimate boundedness which can be made arbitrarily close to asympto
tic stability through choosing a design constant. The resulting contro
l is continuous, uniformly bounded, and designed by a recursive mappin
g procedure. The only information of unknown systems required by the r
obust control approach are the bounding function on the magnitude of n
onlinear disturbances and the bounds on the parameters and their deriv
atives; which represent the distinct feature of the proposed result. T
hat is, time-varying parameters are not restricted to be either slow t
ime-varying or of known structure. (C) 1997 Elsevier Science Ltd.