  This article studies the collaborative transportation of a cable-suspended
pipe by two quadrotors. A force-coordination control scheme is proposed, where
a force-consensus term is introduced to average the load distribution between
the quadrotors. Since thrust uncertainty and cable force are coupled together
in the acceleration channel, disturbance observer can only obtain the lumped
disturbance estimate. Under the quasi-static condition, a disturbance
separation strategy is developed to remove the thrust uncertainty estimate for
precise cable force estimation. The stability of the overall system is analyzed
using Lyapunov theory. Both numerical simulations and indoor experiments using
heterogeneous quadrotors validate the effectiveness of thrust uncertainty
separation and force-consensus algorithm.
