IEEE Robotics & Automation Magazine - December 2017 - 67

Exoskeleton Background
Exoskeletons, long an idea of science fiction, have the potential
to change day-to-day life for countless individuals, particularly
those with mobility issues. While approximately 70% of people
with spinal cord injury paraplegia use a manual wheel-chair
[1], being seated for extended periods causes a variety of other
medical concerns, e.g., degradation of bone density [2], muscle
atrophy [3], and pressure sores [4], in addition to requiring
special infrastructure adaptations such as ramps and lifts to
conduct daily life activities. To address this, commercial exoskeletons such as ReWalk [5], Ekso [6], and Indego [7] have
made significant strides forward since the first exoskeleton
prototypes. Indeed, the recent 2016 Cybathlon illustrated the
incredible progress that exoskeletons have made in recent
years, with many now capable of ascending and descending stairs and ramps. However, while the Cybathlon included
some tasks of daily living, they were presented in an idealized
situation, far from what one would encounter in the real
world; realistically speaking, handrails aren't always available,
steps are of different heights, and surrounding crowds interfere
with crutch placement. While several of the pilots were able to
complete most of the challenges, the Cybathlon thoroughly
demonstrated how far exoskeletons have to go before pilots are
able to walk with the speed and ease of an able-bodied person.
A variety of exoskeletons have been developed for improving mobility. The ReWalk was one of the first such devices to
show the potential for restoring ambulation for those limited
to a wheelchair. Three exoskeletons have been approved by
the U.S. Food and Drug Administration for use as rehabilitation devices on flat ground: ReWalk [5], Ekso [6], and Indego
[7]. Each of these devices has demonstrated sit-to-stand capabilities, although ReWalk is leading the way by providing
users the ability to ascend and descend stairs, as well.
All three platforms feature motors at the hips and knees to
power the leg motions. Each executes a position trajectory at
the command of the pilot, triggered by either a body tilt or
button press, to perform the walking motion. The ReWalk and
Ekso can also prematurely stop this stepping motion if ground
contact is detected. Additionally, the ReWalk can command
different step types through a wrist-mounted interface. While
walking and standing, however, all of the balance and stability
is provided by the pilot through the use of forearm crutches.
Overall, the mobility capabilities of even these most advanced
devices are limited to rehabilitation centers or home use.
We previously developed the Mina v1 [8] and NASA X1
exoskeletons [9] (the latter in collaboration with NASA). Similar to the three commercial devices, these two exoskeletons
feature actuators at the hips and knees: harmonic drive
reduced dc motors for Mina v1 [8] and custom rotary series
elastic actuators on X1. Series elastic actuators were also used
by the University of Twente, The Netherlands in the design of
lower-extremity powered exoskeleton (LOPES), a gait-training
exoskeleton that operates by setting joint impedances for the
hips and knees using custom Bowden-cable series elastic actuators to adjust the user's gait [10]. For purposes of rehabilitation,
controlling the impedance allows the user to adjust the amount

of assistance the device provides, leading to potentially effective
therapies. Additionally, impedance control is now the standard
actuation approach for legged robotics, as it enables compliant
interactions with the environment. For mobility assistance, impedance control thus offers the potential for powered exoskeletons
to provide locomotion on par with humanoid robots.
Despite trying to restore upright sagittal plane mobility,
powered exoskeletons have differed from their biological
counterparts in one critical way: powered exoskeletons typically lack ankle actuation. Able-bodied walking relies on the
motion and forces exerted by ankle plantar flexion and dorsiflexion. During flat walking, approximately 40-50% of positive power is provided by the ankle joint [11]. It has been
found, however, that as much as 40% of the positive work
done by the entire leg during walking comes from energy
stored in the ankle muscle-tendon system [11]. Despite this,
powered ankle plantar flexion is rare in powered exoskeletons. Indeed, the most common powered exoskeletons
(ReWalk [5], Ekso [6], and Indego [7]) do not have powered
ankles but instead use a passive, spring-loaded joint.
While there have been many powered exoskeletons that
include powered ankles, these are typically ankle-only exoskeletons for data collection [11] or specific orthotic purposes [12]
or, alternately, augmentation exoskeletons [13]. To explore the
effects of ankle plantar flexion and dorsiflexion on exoskeleton systems, our new exoskeleton, Mina v2 (pictured in Figure 2), includes powered hip flexion/extension, knee flexion,
and, notably, powered ankle dorsi/plantar flexion.
The Powered Exoskeleton Competition in the Cybathlon
consisted of a variety of tasks, including a slalom course, ascending and descending a ramp that is not compliant with the Americans with Disabilities Act (ADA), navigating a tilted path,
ascending and descending a set of stairs, and traversing a set of
stepping stones, in addition to sitting down and standing up
from a seat. In this article, we present the strategies that we
developed for accomplishing these tasks. Our approach uses
powered ankle flexion to reduce pilot effort as much as possible by including powered toe-off motions in the trajectories.
Unlike many other exoskeleton gait designs, ours provides a
walking gait utilizing a transfer phase that includes a large
degree of toe-off. To increase the flexibility of the walking gait,
we also developed a unique method for finding joint-position
trajectories based on defining Cartesian waypoints for the
swing foot. We believe that the use of toe-off motion combined

Figure 1. IHMC's pilot, Mark Daniel, walking with the Mina v2
exoskeleton in Zürich, Switzerland.

DECEMBER 2017

IEEE ROBOTICS & AUTOMATION MAGAZINE

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