  A leader-follower system is developed for cooperative transportation. To the
best of our knowledge, this is the first work that inter-UAV communication is
not required and the reference trajectory of the payload can be modified in
real time, so that it can be applied to a dynamically changing environment. To
track the modified reference trajectory in real time under the
communication-free condition, the leader-follower system is considered as a
nonholonomic system in which a controller is developed for the leader to
achieve asymptotic tracking of the payload. To eliminate the need to install
force sensors, UKFs (unscented Kalman filters) are developed to estimate the
forces applied by the leader and follower. Stability analysis is conducted to
prove the tracking error of the closed-loop system. Simulation results
demonstrate the good performance of the tracking controller. The experiments
show the controllers of the leader and the follower can work in the real world,
but the tracking errors were affected by the disturbance of airflow in a
restricted space.
