A tracking problem is considered
in the context of a class $\mathcal{S}$ of multi-input,
multi-output, nonlinear systems modelled by controlled functional
differential equations. The class contains, as a prototype, all
finite-dimensional, linear, m-input, m-output, minimum-phase
systems with sign-definite “high-frequency gain". The first control
objective is tracking of reference signals r by the output y of
any system in $\mathcal{S}$: given $\lambda \geq 0$, construct a
feedback strategy which ensures that, for every r (assumed bounded
with essentially bounded derivative) and every system of class
$\mathcal{S}$, the tracking error $e = y-r$ is such that, in the case
$\lambda >0$, $\limsup_{t\rightarrow\infty}\|e(t)\|<\lambda$ or, in
the case $\lambda=0$, $\lim_{t\rightarrow\infty}\|e(t)\| = 0$. The
second objective is guaranteed output transient performance: the
error is required to evolve within a prescribed performance funnel
$\mathcal{F}_\varphi$ (determined by a function φ). For
suitably chosen functions α, ν and θ, both
objectives are achieved via a control structure of the form
$u(t)=-\nu (k(t))\theta (e(t))$ with $k(t)=\alpha (\varphi
(t)\|e(t)\|)$, whilst maintaining boundedness of the control and
gain functions u and k. In the case $\lambda=0$, the feedback
strategy may be discontinuous: to accommodate this feature, a
unifying framework of differential inclusions is adopted in the
analysis of the general case $\lambda \geq 0$.