  This paper tackles the problem of robots collaboratively towing a load with
cables to a specified goal location while avoiding collisions in real time. The
introduction of cables (as opposed to rigid links) enables the robotic team to
travel through narrow spaces by changing its intrinsic dimensions through
slack/taut switches of the cable. However, this is a challenging problem
because of the hybrid mode switches and the dynamical coupling among multiple
robots and the load. Previous attempts at addressing such a problem were
performed offline and do not consider avoiding obstacles online. In this paper,
we introduce a cascaded planning scheme with a parallelized centralized
trajectory optimization that deals with hybrid mode switches. We additionally
develop a set of decentralized planners per robot, which enables our approach
to solve the problem of collaborative load manipulation online. We develop and
demonstrate one of the first collaborative autonomy framework that is able to
move a cable-towed load, which is too heavy to move by a single robot, through
narrow spaces with real-time feedback and reactive planning in experiments.
