IEEE Robotics & Automation Magazine - December 2018 - 35

robustness. Due to this demand, considerable attention is
focused on improving a multirotor's agility and enabling aerial manipulation. Advances in these areas may unlock deployment in numerous use cases. For instance, drone-based
inspection of bridges [5] may require the aircraft to collect
measurements with a sensor that must remain in contact with
the bridge structure at all times. This requires the drone to
adapt its own orientation to the bridge's geometry while controlling its position independently. Enabling a drone to reach
and hold any possible pose also gives rise to many further
applications, such as enabling uninterrupted, complex camera
motions for film shots [6]. Moreover, physical interaction
with the environment while airborne opens the door for various applications, such as contact-based inspection [7], surface
cleaning, and sensor deployment [8].
These are only a sampling of the many possible applications that would benefit if the limits imposed by the dynamics of most commercially available multirotor UAVs can be
overcome. The demand for and interest in novel multirotor
models are reflected in numerous research projects. In [9], a
small vertical takeoff and landing platform was proposed.
The system begins its flight as a multirotor, before transitioning into a fixed-wing mode for comparatively long-endurance surveillance and inspection flights. The handling of
strong winds for platforms of this size was investigated in
[10], where a fixed-wing plane was used. This model comes
at the cost of lower agility compared to a classical multirotor.
The Omnicopter presented in [11] uses intelligent rotor
placement in a cube-like structure to generate forces and
torques in any direction. It has the downside of counteracting forces that can reduce the system's efficiency and hence
its flight time; in addition, certain rotors have only limited
usefulness in a given configuration.
In [12] and [13], the Laboratory for Analysis and Architecture of Systems group at the Centre National de la Recherche
Scientifique demonstrated the advantages of omniorientational aerial platforms for physical interaction. These systems
have fixed tilted rotors and can apply forces and torques in all
directions. They have been used for aerial physical interaction
and show a high degree of robustness and dexterity. A disadvantage of these systems is the constant presence of internal
forces, which significantly reduces efficiency.
Despite these systems' ability to apply forces and torques in
all directions, they are limited to a set of possible orientations
due to limited forces achievable along the body frame's x and
y directions. The use of tilting rotors promises to address
some of these challenges.
A concept for and simulation of a quadcopter involving
tilting rotors were presented in [14] and experimentally evaluated in [15]. More approaches were evaluated in [16]
and [17]. While improving upon maneuverability, these
approaches do not offer omniorientational flight capabilities.
The work in [18] also involved tilting rotors but considered
only two distinct flight modes: one in a horizontal orientation
and the other vertical. The design concept presented in [19]
is, as far as we are aware, the most similar to that of the

proposed Voliro platform. However, this research considered
only control of orientation and did not propose any approach
for jointly controlling position and orientation. A modelbased control approach
for a quadrotor with tiltPhysical interaction with
able rotors was presented
in [20]. However, knowlthe environment while
edge of the system and
actuators model is re airborne opens the door
quired to achieve highperformance control.
for various applications,
While the previously
discussed works pushed
such as contact-based
the boundaries of multirotor agility, no tilting rotor
inspection, surface
system showed the ability
to stably achieve any concleaning, and sensor
figuration on SE(3). Our
goal is to overcome the
deployment.
limitations of common,
commercially available multirotor UAVs while preserving their advantages. The main conceptual challenges in
achieving this are the mechanical design and control of the
system. To be successful, the mechanical design must address
three challenges:
1) incorporating the hardware that enables additional maneuverability while keeping the weight low
2) placing the propellers such that they can contribute to the
system in any configuration and generate as little counteracting force as possible
3) achieving a safe design, such that a tool can be easily
mounted and the drone operated close to a given surface.
These three challenges further translate into hurdles for
designing the controller.
In this article, we present Voliro, a hexacopter with tiltable
rotors. As the rotor orientation can be fully controlled, this
system allows the control of position and orientation to be
decoupled. It can be easily manipulated through a proportional-integral-derivative (PID) controller and a simple allocation scheme, which translates the control output into motor
configurations. Moreover, the ability of the system to independently orient the thrust generated from each rotor reduces
the internal forces and, therefore, increases the efficiency in
comparison to the fixed tilted-rotor platforms described previously. A prototype system is demonstrated and controlled in
a wide range of configurations using a Vicon system to provide state estimation.
The contributions of this article can be summarized as
follows:
● To the best of our knowledge, we present the first hexacopter with tiltable rotors.
● We discuss and justify the UAV's mechanical design and
the corresponding control scheme.
● We evaluate the concept in real-world experiments on a
prototype platform.
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IEEE Robotics & Automation Magazine - December 2018

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