IEEE Robotics & Automation Magazine - December 2018 - 67

Robotic object handling has been tackled using (mobile)
ground manipulators (GMs). The rich literature on GMs
proposes solutions with single or multiple robots, e.g., for
cooperative transportation of large objects [1], offshore
robotic sensing and manipulation [2], or cooperative assembly [3]. The use of GMs presents two major drawbacks: 1)
typical small industrial manipulators have limited joint
torques, resulting in poor maximal admissible Cartesian
torques at the end effector (EE); and 2) GMs have a rather
small workspace around their base, reducing their manipulation capabilities. These limitations can be particularly problematic for long objects if the GM cannot grasp them by their
CoM because doing so demands high torques at the EE and
a large workspace.
An emerging approach is the use of aerial manipulators
(AMs) for construction and large load handling. This has
been made possible by recent developments in unmanned
aerial vehicle (UAV) control, especially the latest progress in
the field of physical interaction, e.g.; see [4] for a panorama of
the field. Their use has been demonstrated in cooperative
load transportation using cables [5], [6] (splitting the overall
payload among members), multirobot assembly [7], [8], and
aerial manipulation [9]-[11]. A major drawback of these
platforms is their limited payload.
In this article, we present a novel class of heterogenous systems to tackle the problem of manipulating long objects that
cannot be grasped close to their CoM. Such systems go
beyond the limitations of previous approaches by leveraging
the combined advantages of both AMs and GMs. The small
payloads of AMs are compensated for by the strength of the
GM, whereas the limited workspace and poor Cartesian
torque at the GM's EE is balanced by the virtually unlimited
workspace and the favorable lever provided by AMs. The
AMs act as flying assistants, supporting the GM from the air.
Tele-MAGMaS
The proposed class is called Tele-MAGMaS, where "MAGMaS" stands for Multi Aerial Ground Manipulator System
and "Tele" reflects the teleoperation capabilities; in fact, such
systems also allow for remote human operation at different
autonomy levels. The presence of a human operator having
superior intelligence and cognitive capabilities is necessary in
most realistic applications to cope with unknown/partially
known environments and possibly unpredicted events. In the
proposed Tele-MAGMaS scheme, the human operators are
also provided with haptic feedback to enrich their telepresence and improve their performances.
Loosely related to the Tele-MAGMaS concept, a theoretical scheme for cooperative transportation between an AM
and a mobile cart was presented for the planar case in [12].
Another scheme with the GM and tethered UAVs is presented in a simulation in [13]. Unlike these examples, the TeleMAGMaS concept considered in this article consists of two
real-world manipulators [aerial and ground with any number
of degrees of freedom (DoF)] that have to perform a real
manipulation task.

This article highlights the key components of Tele-MAGMaS and demonstrates the practical implementation of the
described approach in software and hardware for aerialground comanipulation. Theoretical groundings are discussed in our previous work [14], [15]. To the best of our
knowledge, this is the first time a flying assistant has been
implemented. In particular, for the first time, a cooperative
manipulation task between a ground industrial manipulator
and an AM has been robustly demonstrated.
The design, architectural, and experimental aspects raise
many scientific and technological challenges. Indeed, cooperative manipulation implies an exchange of wrenches that
needs to be handled carefully to ensure the system stability.
Additionally, experimental validation demands robustness to
model imperfections and other nonlinearities. Based on our
experience, we felt the need to develop our own fully actuated
AM, one able to resist external force disturbances without
changing its orientation and with an extra aperture for object
manipulation. The proposed system is overactuSuch systems go
ated with respect to the
task, and leveraging rebeyond the limitations
dundancy in the system is
not straightforward. In
of previous approaches
particular, attention to all
actuation constraints is
by leveraging the
necessary during cooperative manipulation. Also,
combined advantages
the integration of the
human operator needs to
of both AMs and GMs.
be handled with care so
that no provided commands can push the system beyond its limits and into unstable configurations. This
requires careful analysis of the system behavior and the construction of safety margins around the operator behavior.
The high complexity of the Tele-MAGMaS required a
modular software design to facilitate the development process. This required the careful design of the full software
architecture described in this article, which implements a
control framework that permits the different modalities of
1) full autonomy, 2) teleoperation, and 3) shared control of
the system. This architecture allows the system to cope
with the different complexity levels of various environments by leveraging (when needed) the cognitive abilities
of a human operator.
Robotic Subsystems
In this section, we present the three robotic systems that compose the Tele-MAGMaS; the complete system depiction can
be found in the technical report in the IEEE Xplore multimedia material. The GM is a state-of-the-art industrial manipulator, and the AM is a custom platform, called the open tilted
hexarotor (OTHex), specifically designed for bar lifting. Finally, the haptic interface is a commercial delta manipulator with
force feedback capabilities. The three robotic systems have all
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