  In this paper we study an event based control algorithm for trajectory
tracking in nonlinear systems. The desired trajectory is modelled as the
solution of a reference system with an exogenous input and it is assumed that
the desired trajectory and the exogenous input to the reference system are
uniformly bounded. Given a continuous-time control law that guarantees global
uniform asymptotic tracking of the desired trajectory, our algorithm provides
an event based controller that not only guarantees uniform ultimate boundedness
of the tracking error, but also ensures non-accumulation of inter-execution
times. In the case that the derivative of the exogenous input to the reference
system is also uniformly bounded, an arbitrarily small ultimate bound can be
designed. If the exogenous input to the reference system is piecewise
continuous and not differentiable everywhere then the achievable ultimate bound
is constrained and the result is local, though with a known region of
attraction. The main ideas in the paper are illustrated through simulations of
trajectory tracking by a nonlinear system.
