Article
The International Journal of
Robotics Research
1–25
Ó The Author(s) 2020
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DOI: 10.1177/0278364920961452
journals.sagepub.com/home/ijr
A hip–knee–ankle exoskeleton emulator
f or studying gait assistance
Gwendolyn M Bryan
1,2*
, Patrick W Franks
1,2*
, Stefan C Klein
1,2
,
Robert J Peuchen
3
and Steven H Collins
1,2
Abstract
Lower-limb exoskeletons could improve the mobility of people with disabilities, older adults, workers, first responders,
and military personnel. Despite recent advances, few products are commercially available and exoskeleton research is
still often limited by hardware constraints. Many promising multi-joint assistance strategies, especially those with high-
torque and high-power components, have yet to be tested because they are beyond the capabilities of current devices. To
study these untested assistance strategies, we present a hip–knee–ankle exoskeleton emulator that can apply high torques
and powers that match or exceed those observed in uphill running. The system has powerful off-board motors that actuate
a 13.5 kg exoskeleton end effector worn by the user. It can apply up to 200 Nm of torque in hip flexion, hip extension, and
ankle plantarflexion, 250 Nm of torque in knee extension, and 140 Nm of torque in knee flexion, with over 4.5 kW of
power at each joint and a closed-loop torque bandwidth of at least 18 Hz in each direction of actuation. The exoskeleton
is compliant in unactuated directions, adjustable for a wide range of users and comfortable during walking and running.
When paired with human-in-the-loop optimization, we expect that this system will identify new assistance strategies to
improve human mobility. A complete computer-aided design (CAD) model of the exoskeleton and a bill of materials are
included and available for download.
Keywords
Exoskeleton design, human augmentation, locomotion, wearable robotics
1. Introduction
Lower-limb e xoskeletons ha ve the potential to improv e a per-
son’s ability to walk, run, jump, and carry loads by applying
assistiv e joint torques. They could impro v e assistance for
people with disabilities and older adults and could increase
the performance of w orkers, first responders and military per-
sonnel. Currently, exoskeletons are in a nascent stage of
dev elo pment with few products on the market (Sawick i et al.,
2020; Yan et al., 2015; Young and Ferris, 2017). Some ex os-
keletons have focused on walking assistance or rehabilitation
for people with impairments (Baunsgaard et al., 2018;
Esquenazi et al., 2012; Farris et al., 2011; Griffin et al.,
2017; Jezernik et al., 2003; Jin et al., 2015; Kilicarslan et al.,
2013; Maeshima et al., 2011; Wang et al., 2015), while
another group of devices have focused on performance aug-
mentation for walking and running.
Some performance-augmenting exoskeletons have
already shown improvements to locomotor performance,
usually measured as reductions in metabolic cost relative to
the device turned off (Young and Ferris, 2017). Of the
exoskeletons that have shown improvements during
walking, most have assisted either the ankles (Awad et al.,
2017; Collins et al., 2015; Malcolm et al., 2013; Mooney
et al., 2014; Sawicki and Ferris, 2008; Zhang et al., 2017)
or the hips (Ding et al., 2018; Seo et al., 2016; Young
et al., 2017a). A few multi-joint exoskeletons have also
reduced the metabolic cost of walking by assisting either
the hips and ankles (Lee et al., 2018; Quinlivan et al.,
2017) or the knees and ankles (Malcolm et al., 2018a).
While most of these effective exoskeletons are tethered,
some of them are mobile (Collins et al., 2015; Kim et al.,
2019; Lee et al., 2018; Mooney et al., 2014; Seo et al.,
1
Mechanical Engineering, Stanford University, USA
2
Mechanical Engineering, Carnegie Mellon University, USA
3
BioMechanical Engineering, Delft University of Technology, Netherlands
*These authors contributed equally to this work, their names are in alpha-
betical order.
Corresponding author:
Steven Collins, Mechanical Engineering, Stanford University, 450
Escondido Mall, Stanford, CA 94305, USA.
Email: stevecollins@stanford.edu
