  This paper considers the tracking control problem for an unknown nonlinear
system with time-varying bounded disturbance subjected to a prescribed
performance and input constraints. When performance and input constraints are
specified simultaneously for such a problem, a trade-off is inevitable.
Consequently, a feasibility condition for prescribing performance and input
constraints is devised to address such difficulties of arbitrary prescription.
In addition, an approximation-free controller with low complexity is proposed,
which ensures that the constraints are never violated, provided that the
feasibility condition holds. Finally, simulation results corroborate the
effectiveness of the proposed controller.
