  This paper studies distributed optimal formation control with hard
constraints on energy levels and termination time, in which the formation error
is to be minimized jointly with the energy cost. The main contributions include
a globally optimal distributed formation control law and a comprehensive
analysis of the resulting closed-loop system under those hard constraints. It
is revealed that the energy levels, the task termination time, the steady-state
error tolerance, as well as the network topology impose inherent limitations in
achieving the formation control mission. Most notably, the lower bounds on the
achievable termination time and the required minimum energy levels are derived,
which are given in terms of the initial formation error, the steady-state error
tolerance, and the largest eigenvalue of the Laplacian matrix. These lower
bounds can be employed to assert whether an energy and time constrained
formation task is achievable and how to accomplish such a task. Furthermore,
the monotonicity of those lower bounds in relation to the control parameters is
revealed. A simulation example is finally given to illustrate the obtained
results.
