  We propose a scalable cooperative control approach which coordinates a group
of rigidly connected autonomous surface vessels to track desired trajectories
in a planar water environment as a single floating modular structure. Our
approach leverages the implicit information of the structure's motion for force
and torque allocation without explicit communication among the robots. In our
system, a leader robot steers the entire group by adjusting its force and
torque according to the structure's deviation from the desired trajectory,
while follower robots run distributed consensus-based controllers to match
their inputs to amplify the leader's intent using only onboard sensors as
feedback. To cope with the complex and highly coupled system dynamics in the
water, the leader robot employs a nonlinear model predictive controller (NMPC),
where we experimentally estimated the dynamics model of the floating modular
structure in order to achieve superior performance for leader-following
control. Our method has a wide range of potential applications in transporting
humans and goods in many of today's existing waterways. We conducted trajectory
and orientation tracking experiments in hardware with three custom-built
autonomous modular robotic boats, called Roboat, which are capable of holonomic
motions and onboard state estimation. Simulation results with up to 65 robots
also prove the scalability of our proposed approach.
