IEEE Robotics & Automation Magazine - December 2018 - 58

challenges posed by underactuation [9]. In this case, rotors
can be placed in different noncoplanar configurations [9],
[10], or mechanisms can be designed that adjust the orientation of the rotors with respect to the frame fixed to the
body [11]. However, these systems have different drawbacks: they require additional mechanisms, adding weight
to the system, and they use noncoplanar configurations,
which means some rotors may not fully contribute to the
generated output thrust.
Rarely has a UAV been shown to successfully operate a
tool upon the surface of a static object. Such UAV-driven
approaches, however, while applying only limited interaction
forces, have demonstrated that active movement over a surface is possible [12], [13].
In this article, we introduce, as an alternative to UAVdriven locomotion, the concept of multimodal aerial locomotion for active tool handling in aerial manipulation
(Figure 1). Demonstrations have shown that multimodal
locomotion can provide additional locomotion capabilities in
situations where a single modality would be insufficient to
accomplish the task [14], [15]. We present a novel platform
that exploits two locomotion modalities to achieve precise
tool positioning and persistent contact force. To create this
platform, we combined an aerial manipulator with a wheeled
end effector. Three actuated omnidirectional wheels were
placed on this end effector to allow active locomotion of the
tool, which required sufficient friction with the surface to
function correctly. To guarantee this friction, a controller
based on our previous work [2] was applied. The scope of
this controller is twofold: 1) it provides the contact force necessary to ensure wheeled locomotion and tool operation, and
2) it stabilizes the attitude of the UAV subject to the interaction forces.
We evaluated the approach in experiments in which the
end effector, with a cleaning brush attached, is used to clean
off a drawing made by a marker on a flat vertical surface.
The results of these experiments demonstrate effective
locomotion of both the aerial and wheeled systems, thus
validating our approach to multimodal locomotion for
active tool handling.

Multimodal Aerial Locomotion Approach
The main task attempted in the experiments described in this
article resembles a maintenance operation on a wind turbine-the cleaning of a confined region on its static surface. To
demonstrate the multimodal locomotion approach, we considered a challenging scenario: moving an end effector on a
vertical surface. For the sake of simplicity but in keeping with
the general requirements of related tasks, we used a flat
rigid surface and assumed knowledge of the position and
orientation of the surface at the region of interest.
Within this scenario, our multimodal locomotion approach
for performing the required surface operation task used an
aerial manipulator with an end effector, on which the brushing
tool was mounted along with wheels that allowed for active
repositioning on the surface. This is sketched in Figure 2.
The multimodal locomotion system consists of a regular
multirotor, a 1-degree-of-freedom (DoF) manipulator, and a
custom-designed end effector. The multirotor constitutes
the aerial-locomotion system, which we define as a system
that is not constrained to a static environment. The multirotor functions as the base for the manipulator. On one
extremity, the manipulator contains a 1-DoF joint, which
connects to the multirotor and decouples the pitch of the
multirotor. On the other extremity, a flexible joint connects
the manipulator and the end effector, decoupling their relative orientation. The end effector, by means of an omnidirectional driving unit, performs the ground locomotion,
which we define as the locomotion on a static environment.
This design enables the operator to reposition the tool and
control its operation.
The symbiosis between the aerial-locomotion system and
the ground-locomotion system is the key element that allows
the task to be accomplished. The aerial-locomotion system
carries the tool to the desired location, while accuracy in positioning the tool is achieved by the ground-locomotion system. A normal force between the surface and the wheels is
required to generate friction for the ground-locomotion system to work. To clean a vertical surface, this normal force can
be achieved only by the aerial system pushing the groundlocomotion system onto the surface.

ψb
→
pbe

ψe
µ

ψw

Figure 1. A photo demonstrating the multimodal locomotion
system performing a cleaning operation on a vertical surface.

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IEEE ROBOTICS & AUTOMATION MAGAZINE

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December 2018

Figure 2. An illustration of the aerial manipulator showing the
inertial (world) frame W w, the body-fixed frame of the multirotor
W b, and the end-effector frame W e . The red, green, and blue
axes represent (xt , yt , zt ), respectively.
IEEE Robotics & Automation Magazine - December 2018

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