  This paper shows a strategy based on passive force control for collaborative
object transportation using Micro Aerial Vehicles (MAVs), focusing on the
transportation of a bulky object by two hexacopters. The goal is to develop a
robust approach which does not rely on: (a) communication links between the
MAVs, (b) the knowledge of the payload shape and (c) the position of grasping
point. The proposed approach is based on the master-slave paradigm, in which
the slave agent guarantees compliance to the external force applied by the
master to the payload via an admittance controller. The external force acting
on the slave is estimated using a non-linear estimator based on the Unscented
Kalman Filter (UKF) from the information provided by a visual inertial
navigation system. Experimental results demonstrate the performance of the
force estimator and show the collaborative transportation of a 1.2 m long
object.
