  This study aims to design a motion/force controller for an aerial manipulator
which guarantees the tracking of time-varying motion/force trajectories as well
as the stability during the transition between free and contact motions. To
this end, we model the force exerted on the end-effector as the Kelvin-Voigt
linear model and estimate its parameters by recursive least-squares estimator.
Then, the gains of the disturbance-observer (DOB)-based motion/force controller
are calculated based on the stability conditions considering both the model
uncertainties in the dynamic equation and switching between the free and
contact motions. To validate the proposed controller, we conducted the
time-varying motion/force tracking experiments with different approach speeds
and orientations of the surface. The results show that our controller enables
the aerial manipulator to track the time-varying motion/force trajectories.
