IEEE Robotics & Automation Magazine - December 2019 - 95

are accelerating the diffusion of robots in real environments.
The first factor is the success of Soft Robotics. Technologies
such as series-elastic actuation, variable impedance, and
teleimpedance controllers allow machines to interact safely
and effectively with humans and the environment. The sec-
ond factor is the commoditization of hardware and software
technologies that, until recently, were relegated to very spe-
cialized engineering fields, e.g., nuclear, military, and aero-
space. Examples of these technologies include virtual-reality
(VR) headsets, integrated inertial navigation units, high-
bandwidth and low-latency networking, and, in general,
affordable computational power and reliable sensing. The
third factor is the growing interest of large companies and
funding agencies, which is fostering novel humanoid robot-
ics and teleoperation developments through science compe-
titions and the awarding of prizes. Two popular examples
are the 2015 DARPA Robotics Challenge (US$8 million
prize) [28] and the recent All Nippon Airways Avatar
XPRIZE (XP) (US$10 million prize) [29], "a four-year global
competition focused on accelerating the integration of sev-
eral emerging and exponential technologies into a multipur-
pose avatar system that will enable us to see, hear, touch,
and interact with physical environments and other people
through an integrated robotic device" [30]. The focus of the
latter includes the domains of health care, services, inspec-
tion, and maintenance and demonstrates the importance of
physical-interaction capabilities, sensing integration, and
user friendliness.
Inspired by these challenges and perspectives, and
leveraging our previous experiences and contributions [4],
[7], in this article, we present ALTER-EGO. As shown in
Figure 1, the robot is a robust and versatile mobile system
with a functional anthropomorphic upper body. To oper-
ate in different working scenarios and safely perform phys-
ical human-robot interactions, ALTER-EGO is powered
by variable-stiffness actuators (VSAs), which exhibit a
stiffness behavior similar to that of human muscles [8].
Each arm mounts an anthropomorphic, synergistic artifi-
cial hand inspired by human motor synergies [7]. The
upper body is mounted on a two-wheel, self-balancing
mobile base that minimizes the robot's footprint and
increases agility. The system is equipped with sensors and
computational systems that allow the robot to work auton-
omously. Moreover, ALTER-EGO can also be used in tele-
operation mode from a pilot station mainly composed of
lightweight and wearable interfaces. Featuring an immer-
sive control mode, the system can use teleimpedance con-
trol [9] to pair the pilot's actions with the robot's mechanical
behavior, not only in terms of movements but also in terms
of intended interactions behavior.
Most of the hardware and software technologies adopt-
ed, developed, and explicitly designed for ALTER-EGO are
distributed under an open source framework and are
available on the Natural Machine Motion Initiative
(NMMI) website [31]. To the best of our knowledge, this is
the first time that variable-stiffness technology has been

built into an anthropomorphic platform with mobility
capabilities and different control modalities ranging from
autonomous to teleoperation.
Requirements Analysis
The design requirements of a robot used for assisting with the
general activities of daily living differ substantially from those
employed for industrial
or specialized machines.
The tasks required by
ALTER-EGO is powered by
the XP competition (see
Table 1) can be used to
variable-stiffness actuators,
distill a set of functional
specifications [32] to
which exhibit a stiffness
motivate and guide the
design of ALTER-EGO.
behavior similar to that of
Note that it is outside the
scope of this article to
human muscles.
propose a deterministic
approach to the defini-
tion of robot require-
ments and specifications or to propose a robot that perfectly
fits all these requirements.
Manipulation
Half of all the 28 tasks require manipulation and nearly all
require physical interaction. Furthermore, the simultaneous
presence of tasks 1) where the robot must push large, heavy
objects, 2) where finesse and precision are important, and 3)
where interaction force control is mandatory (e.g., because of
safety) suggests impedance control in the robot arms.
Locomotion
Only three locomotion tasks strictly require the use of legs,
making wheels a feasible, yet suboptimal, choice. Nonetheless,

Figure 1. ALTER-EGO: a soft, dual-arm mobile platform
equipped with variable-stiffness actuation units and soft,
underactuated hands.

DECEMBER 2019

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IEEE Robotics & Automation Magazine - December 2019

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