Abstract— This paper presents design principles for
comfort-centered wearable robots and their application in a
lightweight and backdrivable knee exoskeleton. The mitigation of
discomfort is treated as mechanical design and control issues and
three solutions are proposed in this paper: 1) a new wearable
structure optimizes the strap attachment configuration and suit
layout to ameliorate excessive shear forces of conventional
wearable structure design; 2) rolling knee joint and double-hinge
mechanisms reduce the misalignment in the sagittal and frontal
plane, without increasing the mechanical complexity and inertia,
respectively; 3) a low impedance mechanical transmission reduces
the reflected inertia and damping of the actuator to human, thus
the exoskeleton is highly-backdrivable. Kinematic simulations
demonstrate that misalignment between the robot joint and knee
joint can be reduced by 74% at maximum knee flexion. In
experiments, the exoskeleton in the unpowered mode exhibits 1.03
Nm root mean square (RMS) low resistive torque. The torque
control experiments demonstrate 0.31 Nm RMS torque tracking
error in three human subjects.
Index Terms—Contact modeling, force control, mechanism
design, misalignment mitigation, prosthetics and exoskeletons
I. INTRODUCTION
N the last two decades, exoskeletons have been heralded as
one type of promising assistive device for performance
augmentation of healthy individuals [1]–[9] and medical
rehabilitations of patients with disabilities [10]–[15]. Metabolic
reduction has been considered the primary metric for device
evaluation and its feasibility has been successfully
demonstrated in walkers [16]–[19] post-stroke patients with
paretic limbs [20], load carriers [21], and joggers [22]. From a
Index Terms—Contact modeling, force control, mechanism design,
misalignment mitigation, prosthetics and exoskeletons, modeling and control,
wearable robotics, nonlinear control, mechanism design, modeling and control,
wearable mechanism.
* indicates corresponding author. Junlin Wang, Xiao Li, Tzu-Hao Huang,
Shuangyue Yu, Yanjun Li and Hao Su are with Lab of Biomechatronics and
Intelligent Robotics (BIRO), Department of Mechanical Engineering, The City
University of New York, City College, NY, 10023, US (E-mail:
hao.su@ccny.cuny.edu). Junlin Wang and Xiao Li are with also with BrainCo
Inc.
Tianyao Chen is with Department of Biomedical Engineering, Catholic
University of America, Washington, DC, 20064, US.
Alessandra Carriero is with the Department of Biomedical Engineering, The
City University of New York, City College, 10031, US.
Mooyeon Oh-Park is with the Burke Rehabilitation Hospital, NY, 10605,
US.
Digital Object Identifier (DOI): see top of this page.
design perspective, wearable robots are typically composed of
actuators, transmissions, and wearable structure. Most
exoskeletons with electric actuators are generally classified in
terms of wearable structures as either rigid, or soft or flexible
designs. Rigid exoskeletons (e.g. ReWalk or Ekso Bionics) rely
on rigid materials to deliver torque in perpendicular to the
musculoskeletal structure. Soft exosuits [23], [24] use cable
transmission and textile-based wearable structures to deliver
power from the actuator to the human through linear forces
along the musculoskeletal structure. This innovation minimizes
the joint misalignment issue with great metabolic reduction
benefit [16]. However, it has limitations due to high-pressure
concentrations [25] and the absence of weight-support
functionality [26]. Flexible exoskeleton designs [26], deliver
torque-type assistance (instead of linear force) with flexible
structures by combining the advantages of rigid exoskeletons
and soft exosuits.
The challenges of widespread adoption of this technology,
however, arise from the manifestation (and need for resolution)
of the discomfort due to excessive weight, or restricted range of
motion, or high-pressure concentration; as well as the difficulty
to develop a synergistic control that can mechanically assist
human and physiologically adapt to human performance.
Comfort and risk mitigation [27], [28] have been identified as
two of the key features to allow individuals to safely and
independently ambulate or use exoskeletons.
We propose to use shear force produced by the exoskeleton,
joint misalignment, and actuator backdrivability as the
quantitative measurement for comfort. Our contribution of this
paper includes: 1) a structural analysis and design of a knee
exoskeleton that ameliorates excessive shear forces; 2) a
mechanism design that reduces joint misalignment and
minimizes the distal weight; 3) a novel lightweight, compact,
and highly-backdrivable actuation system. The overall weight
of the exoskeleton prototype is 3.2 kg and its on-board battery
can power walking assistance for 1 hour. Our exoskeleton
design is intended to augment human capability by providing
moderate levels of assistance at optimal timing of walking gait
cycles as this methodology has been proved to be effective and
efficient [25]. Normalized peak knee torques of 80 kg
able-bodied individuals during walking and sit-to-stand are
typically reported as 40Nm and 80Nm respectively. The knee
exoskeleton in this paper aims to provide walking assistance.
The peak output torque is 16 Nm, which is equivalent to 40% of
peak biological knee moment of an 80 kg healthy individual
