  We study tracking control for uncertain nonlinear multi-input, multi-output
systems modelled by $r$-th order functional differential equations
(encompassing systems with arbitrary strict relative degree) in the presence of
input constraints. The objective is to guarantee the evolution of the tracking
error within a performance funnel with prescribed asymptotic shape (thus
achieving desired transient and asymptotic accuracy objectives), for any
sufficiently smooth reference signal. We design a novel funnel controller
which, in order to satisfy the input constraints, contains a dynamic component
which widens the funnel boundary whenever the input saturation is active. This
design is model-free, of low-complexity and extends earlier funnel control
approaches. We present a simulation where the controller is compared to these
approaches.
