IEEE Robotics & Automation Magazine - December 2018 - 69

within the physical limits of the actuators. Such a design is
gaining popularity in the literature [18]-[20] mainly because
multidirectional thrust aerial vehicles are, by design, particularly well suited for physical interaction tasks. Indeed, they can
exert a decoupled set of forces and torques on their environment that are independent of the contact point position. In
particular, this means that they can resist external disturbances
while completing a manipulation task without having to
change their orientation, which could jeopardize the manipulation task itself.
The second novel feature of the OTHex is an aperture of
85° between two bars of its skeleton [Figure 2(b)]. This configuration is preferred over regular hexagonal positioning of
actuation units to facilitate the manipulation of a long object,
e.g., a beam. Thanks to this unique design, the object can pass
through the aperture, allowing for a wider variety of beam
manipulation tasks.
The extra payload of the OTHex AM can be used to assist
the GM in the cooperative manipulation task. For example,
only 7 N at the end of the bar (25% of its extra payload) are
sufficient to cooperatively transport a 3-m bar of weight that
fits the considered applications.
Finally, we designed a lightweight mechanical system consisting of a passive revolute joint with two grippers, which
endows the OTHex with the grasping and manipulation capabilities required to perform physical interaction. A passive joint
is preferred over an actuated one to reduce system complexity
and weight; this solution is viable thanks to the full actuation of
the OTHex. In the studied use case, the OTHex acts as a flying
assistant following the GM motion; hence, a 1-DoF passive
joint is sufficient to accomplish the task.
The OTHex control architecture is articulated over three
main components (a low-level controller, a wrench estimator,
and an admittance filter), details of which can be found in our
previous work [15].
To guarantee the robustness of the physical aerial interaction, software compliance was necessary. We preferred the
admittance over the impedance framework for two main reasons: 1) the availability of a precise pose controller for the
OTHex and 2) the ease of enabling/disabling the software
compliance during pose-based tasks. In practice, we did not
experience the tuning drawbacks of the admittance framework with respect to inconsistencies for steady-state commands with different virtual inertia.
Haptic Interface
Human operators can interact with robotic systems via various interfaces, such as touch displays, game pads, and haptic devices. A haptic device (or haptic interface) is a robot
that works on the master side of a teleoperation system. The
human operator can move the master robot by applying
force to its handle, and, reversely, the haptic interface can
apply forces to the human operator, called haptic cues or
haptic feedback. Haptic feedback is used to provide the
human operator with information about the state of the
remote system.

In our test bed, we used an Omega.6 haptic device with 6
DoFs. The three translations are actuated by independently
controlled dc motors, and the three rotations are passive. The
device communicates with the main personal computer via the
Universal Serial Bus 2.0 and can be controlled at up to 4 kHz
(the faster the control loop, the better the force rendering).
System Autonomy Framework
Following the discussion on system autonomy in the technical report, the Tele-MAGMaS has three distinct operation
modes: 1) fully autonomous, 2) teleoperated, and 3) shared
control. In the following sections, we review these approaches
and show how the human operator can assist or intervene in
each approach by means of a haptic device.

Table 2. The main specifications of the OTHex.
Parameter

Value

Weight (without battery)

2.48 kg

Extra payload

2.9 kg

Actuation unit first tilt angles

35°

Actuation unit second tilt angles

-10°

Autonomy (on battery)

15 min

Maximum lateral force (hovering)

(a)

(b)
Figure 2. (a) The OTHex (hovering) with its main features
highlighted: one of the tilted motors ①, the electronics case
②, the passive joint ③, and the two coordinated grippers ④.
(b) A comparison of the spatial location of the propellers for a
standard hexarotor configuration (left) and for the OTHex. The
OTHex configuration is such that an aperture of 85° is left to
facilitate the manipulation of a long object (green area).

december 2018

IEEE ROBOTICS & AUTOMATION MAGAZINE

IEEE Robotics & Automation Magazine - December 2018

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - December 2018