IEEE Robotics & Automation Magazine - December 2018 - 57

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Multimodal
Aerial Locomotion
An Approach to Active Tool Handling

By Han W. Wopereis, Wilbert L.W. van de Ridder, Tom J.W. Lankhorst, Lucian Klooster,
Evyatar M. Bukai, David Wuthier, George Nikolakopoulos, Stefano Stramigioli,
Johan B.C. Engelen, and Matteo Fumagalli

he research focus in aerial robotics is shifting
from contactless inspection toward interaction
and manipulation, with the number of potential
applications rapidly increasing [1]. Eventually,
aerial manipulators, i.e., unmanned aerial
vehicles (UAVs) equipped with manipulators, will likely
take on hazardous maintenance tasks now performed by
humans. For this to happen, aerial manipulators must be
able to perform all the different operations required in
these maintenance routines.
Many such operations demand that the aerial manipulator
either move or position a tool precisely on the surface of a
static object, while generating sufficient contact force on the
tool for correct operation. Examples of such operations are
cleaning, grinding, and taking measurements. These operations present two main challenges:
1) They require the contact forces to be sufficiently large and
persistent to perform meaningful operations on the surface.
2) They require that the tool can be moved over the surface
on which the operation takes place.
Digital Object Identifier 10.1109/MRA.2018.2869527
Date of publication: 12 October 2018

These challenges are even more problematic when the aerial platform is underactuated, as is the case for the vast majority of commercially available aerial platforms. For this class of
UAVs, contact constraints alter the dynamics in interaction,
thus introducing additional coupling terms between the linear and rotational dynamics [2]. These constraints, in combination with the anisotropy of the torque limits, complicate the
task of both applying relevant contact force and moving the
tool on the surface.
Regular multirotors-multirotors with coplanar rotors-
typically use classical control approaches to maintain stability
up to certain interaction forces [3] (the boundaries for stability are assessed in [4]). This approach has been adopted in [5]
to apply a quadrotor as a three-dimensional (3-D) force effector. Typically, in scenarios that require substantial interaction
forces, contacts placed widely apart are used to constrain the
rotational dynamics, while contact force is generated by additional rotors [6], tilting rotors [7], or full-body rotation [8].
However, the use of widely spaced contacts restricts the system's motion capabilities.
Instead of regular multirotors, fully actuated multirotors
can be used as the floating base platform for the aerial
manipulator, because these do not suffer from the intrinsic
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