  We study the problem of sequential task assignment and collision-free routing
for large teams of robots in applications with inter-task precedence
constraints (e.g., task $A$ and task $B$ must both be completed before task $C$
may begin). Such problems commonly occur in assembly planning for robotic
manufacturing applications, in which sub-assemblies must be completed before
they can be combined to form the final product. We propose a hierarchical
algorithm for computing makespan-optimal solutions to the problem. The
algorithm is evaluated on a set of randomly generated problem instances where
robots must transport objects between stations in a "factory "grid world
environment. In addition, we demonstrate in high-fidelity simulation that the
output of our algorithm can be used to generate collision-free trajectories for
non-holonomic differential-drive robots.
