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AbstractTo overcome the different requirements of torque-
velocity characteristics for walking, running, stand-to-sit, sit-to-
stand, and climbing stairs, we propose a novel concept for actuator
design, namely, a series elastic actuator with two-motor variable
speed transmission. The two-motor variable speed transmission
can be adjusted in real-time to realize variable torque-velocity
characteristics. A novel lightweight wearable hip exoskeleton
driven by a series elastic actuator with two-motor variable speed
transmission, named SoochowExo, has been developed in this
paper for use in the elderly population. The weight of the whole
hip exoskeleton is 2.85 kg (excluding batteries), including two
actuators and the frame. The proposed hip exoskeleton can match
the weight of the state-of-the-art hip exoskeleton while offering
suitable torque and velocity for sitting-to-standing, walking,
running on level ground, and climbing stairs. The benchtop tests
and the preliminary human subject tests further confirm the
design.
Index TermsHip Exoskeleton, Variable Speed Transmission,
Series Elastic Actuator, Locomotion modes, Multiple Tasks.
I. INTRODUCTION
GING is accompanied by physical performance and sense
reduction due to muscle weakness and motor unit loss [1].
The major concerns for senior citizens are handicaps related to
sitting-to-standing, climbing stairs, and walking. The hip
exoskeleton [2-9] has become a rising research topic in recent
years due to the small inertia added to the leg and the device’s
ability to assist elderly people in walking, sitting-to-standing,
and climbing stairs. However, the current hip exoskeletons still
have limitations, such as wearability, weight, and assistive
performance.
Human hip joints are very important for many activities that
are classified as activities of daily living (ADL), including
walking, running, sitting-to-standing, climbing stairs, and the
real-time adjustment of the step width to avoid falling [10, 11].
Furthermore, the hip joint has different characteristics for the
different types of motion. In particular, the velocity and torque
requirements for walking and sitting-to-standing are opposite.
The joint needs the high torque and slow velocity of the hip
flexion/extension (HFE) joints during sitting-to-standing,
whereas it needs a high velocity with small torque for walking
and running [12, 13]. To satisfy the requirement of high torque
This work was supported in part by the National Key R&D Program of China
(2020YFC2007804), the Natural Science Foundation of the Jiangsu Higher
Education Institutions of China (19KJA180009), the Natural Science
Foundation of Jiangsu Province (BK20191424), the Jiangsu Frontier Leading
assistance for sitting-to-standing, most exoskeletons have been
designed using high-reduce radio transmission or high-power
motors [14-16]. This design will inevitably increase the inertia
and weight. To improve the wearability, in addition to
improving the actuator’s ability, the hip exoskeleton should be
as light as possible, and the structure should be slimly designed.
To reduce weight, most hip exoskeletons adopt low-power
motors or low reduce radio gear to meet the high-velocity
requirement for walking and running. However, this will
compromise the loss of the ability of high-torque assistance. Su
proposed a quasi-direct drive actuation using a custom high
torque density motor and low ratio gear transmission for a hip
exoskeleton that demonstrates mechanical versatility for being
lightweight (3.4kg overall mass), highly-backdrivable (0.4 Nm
back drive torque) with high nominal torque (17.5 Nm) and
high control bandwidth (62.4 Hz) [17]. However, the nominal
torque of 17.5Nm is far away from the human biological hip
moment requirement of the sitting-to-standing.
An exciting concept, namely, a dual reduction actuator for
the knee joint of the flexible exoskeleton GEMS L-Type, has
been recently introduced [18]. The two reduction radios were
fixed and switched through a clutch. And the exoskeleton has
to stop when the reduction radios switch. Yang, et al. [19] and
Rouse, et al. [20] designed hip and knee exoskeletons so that
the gear ratio can be changed during movement. Vanderborght,
et al. [21] designed a variable stiffness actuator that used
another motor to change the actuator stiffness. Horst, et al. [22]
designed a lead screw and belt-based gear variable actuator for
Orthotics. Jang designed an active-type twisted string actuator
(TSA)-based on continuously variable transmission (CVT) for
the robot application [23]. Lee designed a new actuator with
continuously variable transmission using parallel dual-motors
and a planetary gear for mobile robots [24]. Tran developed a
powered knee prosthesis with actively variable transmission for
multiple tasks [25, 26]. However, there has been comparatively
little work on developing a hip exoskeleton with variable speed
transmission to meet the different torque-velocity for the
different gait tasks.
In this paper, a lightweight, compact, compliant robotic hip
exoskeleton is developed for the elderly population and the
population with lower-limb impairments. To overcome the
different requirements of torque-velocity characteristics for
walking, running, stand-to-sit, sit-to-stand, and climbing stairs,
Technology Fundamental Research Project (BK20192004D), and the
Distinguished Professor of Jiangsu province.
T. Zhang, C. Ning, L. Yang and M. Wang are with the Robotics and
Microsystems Center, College of Mechanical and Electrical Engineering,
Soochow University, Suzhou 215000, China. (e-mail: zhangt.hit@gmail.com).
Design and Validation of a Lightweight Hip
Exoskeleton Driven by Series Elastic Actuator
with Two-Motor Variable Speed Transmission
Ting Zhang, Member IEEE, Chuanxin Ning, Yang Li, Meng Wang
A
This article has been accepted for publication in IEEE Transactions on Neural Systems and Rehabilitation Engineering. This is the author's version which has not been fully edited and
content may change prior to final publication. Citation information: DOI 10.1109/TNSRE.2022.3201383
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we propose a novel concept for actuator design, namely, a series
elastic actuator with two-motor variable speed transmission.
The speed transmission ratio can be adjusted in real-time to
realize variable torque-velocity characteristics. Furthermore,
two-motor variable speed transmission can adapt to elderly
people’s personalized motion characteristics due to age and
physical condition.
The main contributions of this paper are as follows:
1) introduce the concept of the series elastic actuator with
two-motor variable speed transmission to the hip exoskeleton
design to meet the different requirements of torque-velocity
characteristics for walking, running, stand-to-sit, sit-to-stand,
and climbing stairs;
2) develop a novel lightweight, wearable hip exoskeleton
driven by series elastic actuator with two-motor variable speed
transmission, named SoochowExo, to promote the independent
living of the elderly and the population with lower-limb
impairments.
The paper is structured as follows. The hardware and
controller design are described in Sections II and III. Section III
and V present the bench-top testing and the preliminary
human subject tests, which show the performance of the
SoochowExo. The discussion and conclusion are described in
Sections VI and VII, respectively.
II. HARDWARE DESIGN
A. Design Objectives
The main objective of the hip exoskeleton is to assist the
elderly and the population with lower-limb impairments in
sitting-to-standing, walking, running, and climbing stairs. The
actuator requirements of torque and speed are different for
different locomotion modes, such as walking, running, stand-
to-sit, sit-to-stand, and climbing stairs [13, 27, 28]. The
actuators of hip exoskeleton are required to working in
distinctively different torque-speed load conditions. To achieve
this, a series elastic actuator with two-motor variable speed
transmission is designed to meet the different requirements for
the assistive torque and velocity. The peak torque and velocity
are designed to meet the velocity requirement, gain higher
torque output, and reduce the weight. To ensure a wide range of
speeds, the actuator’s requirement of the hip exoskeleton firstly
is to have high velocity, and then can offer large assistive torque
for the sitting-to-standing and slow walking. The design
requirements of the hip exoskeleton are summarized in Table I.
The hip exoskeleton was designed to offer assistive torque of
60 Nm with the velocity of 2.3rad/s at the highest transmission
ratio, while output torque of 12Nm with the velocity of 5.6rad/s
at the lowest transmission ratio.
Table I summarizes the hip exoskeleton design goals.
target
actual
Total active DOFs
Hip flex. /ext.
Hip flex. /ext.
Speed transmission ratio
range
6:1 ~ 32:1
6:1 ~ 32:1
Peak torque range (Nm)
12 ~ 60
12 ~ 62
Peak velocity range (rad/s)
5.6 ~ 2.3
5.9 ~ 2.8
Actuator
SEA
SEA
Weight (excluding the power
supply system)
As light as possible
2.85 kg
B. Design Overview
Fig. 1 shows the overall system of the hip exoskeleton. This
hip exoskeleton is designed to deliver assistive torque to both
HFE joints. The hip exoskeleton includes the flexible suit and
two module actuators that can connect to the flexible suit. The
hip exoskeleton consists of a pair of series elastic actuators with
two-motor variable speed transmission for the HFE joints, and
a pair of 3D-print customized frames. The layout of the hip
exoskeleton is designed to minimize the extra inertia on the
wearer’s limb and maximize wearing comfort. The interface
between the hip exoskeleton and the wearer consists of four
parts: a belt to conform to the wearer’s waist shape, two braces
for both thighs, and a suspender. The two actuators, the
embedded control system, and the battery are located close to
the back, which is in the proximity of the center of mass of the
wearer. The overall hip exoskeleton system weight without a
battery is 2.85 kg. The material and weight for different parts of
the hip exoskeleton are shown in Table II.
(a) Frame of the hip exoskeleton
(b) System overview of the hip exoskeleton
Fig. 1 System overview of the proposed hip exoskeleton: SoochowExo
The hip exoskeleton includes the following core elements:
1) Two-motor variable speed transmission: To meet the
assistance of various tasks, such as level-walking and sitting-
to-standing, the transmission ratio of the HFE actuators of the
hip exoskeleton could continue to change adaptively. The series
elastic actuator with two-motor variable speed transmission
uses a novel transmission system that adapts the actuator torque
and speed to the demand of different ambulation tasks. For
example, the speed transmission ratio adjusts to high for tasks
that require high torque and low speed, such as sitting-to-
standing tasks, and adjusts to low for tasks that require high
speed with low output torque, such as walking and running.
2) Series elastic actuator (SEA): The actuator is designed
based on the SEA concept presented in our previous works [7].
The SEA configuration can realize accurate interaction force
control and complaint interaction with the wearer, as well as the
This article has been accepted for publication in IEEE Transactions on Neural Systems and Rehabilitation Engineering. This is the author's version which has not been fully edited and
content may change prior to final publication. Citation information: DOI 10.1109/TNSRE.2022.3201383
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torque sensor, which measures the displacement under the load.
Table II The material and weight for different parts of the hip exoskeleton
Item
Quantity
Material
weight
Actuator with link
2
aluminum alloy
0.875
Brace
2
Plastic, textile
0.15
Suspender
1
textile
0.1
Frame
2
plastic
0.2
Belt
1
fabric
0.2
Circuit
1
-
0.1
Total
2.85kg
C. The principle of the two-motor variable speed transmission
The principle of the two-motor variable speed transmission
for the hip exoskeleton is based on the planetary gear
mechanism. A planetary gear includes four parts: sun gear, ring
gear, planet gears, and carrier. There have two motion control
inputs, one is the sun gear and the other is the ring gear rotation,
to adjust the rotation speed of the carrier and the planet gears,
as shown in Fig. 2.
Fig. 2 Basic concept of two-motor variable speed transmission
When the ring gear is fixed ( ), the velocity relation
between the input 1 and output is:
(1)
When the sun gear is connected with input 1 and the ring gear
is connected with input 2, the output velocity can be
expressed as
(2)
where ,
, and is the number of sun gear teeth, the
planetaries teeth, and the ring gear teeth, respectively.
Fig. 3 shows the concept of the planetary gear-based two-
motor variable speed transmission in our design. Two planetary
gear subsystems are series-connected together in the two-motor
variable speed transmission. The basic principle is that two
inputs contribute to one output. The first planetary gear’s
reduction ratio is constant, while the second planetary gear’s
reduction ratio is determined by two inputs and . The main
input comes from the output of the first planetary gear and is
connected to the sun gear of the second planetary stage. The
second input is connected to the ring gear. A worm and gear
driven by the small-power motor were applied at the input
to change the reduction ratio of actuators and can also provide
a self-locking function.
Fig.3 Schematic diagram of the transmission system
When the small-power motor is powered off ( ), the
two-motor variable speed transmission worked in the highest-
transmission ratio mode. The speed transmission ratio is:
(3)
(4)
(5)
When the rotated in the opposite direction to the main
drive motor, then Two motor variable speed transmission
worked in the low-transmission ratio mode. The speed
transmission ratio is computed as follows:
(6)
(7)
(8)
(9)
(10)
where , are the number of sun gear teeth and ring gear
teeth of the planetary gear 1, respectively, , are the number
of sun gear teeth and ring gear teeth of the planetary gear 2,
respectively, , is the number of worms and worm gear
teeth, respectively, is proportion factor of the velocity
between the main motor and secondary motor.
D. Fully-integrated SEA-with-two-motor-variable-speed-
transmission
Fig. 4 shows the CAD of the SEA-with-two-motor-variable-
speed-transmission. The system consists of the main motor,
secondary motor, two planetary gear modules, two motor
variable speed transmission, and strings. The main motor is
directed connected with the sun gear of the first planetary gear
module using a coupler. The carrier of the first planetary gear
module was mounted next to the two-motor variable speed
transmission. The two-motor variable speed transmission
includes a sun gear, ring gear, three planet gears, a carrier, a
worm-and-gear, and a secondary motor. The output of the first
planetary gear module is connected with the sun gear of the
two-motor variable speed transmission. The ring gear of the
two-motor variable speed transmission connected with the
second motor through a worm-and-gear.
Input 1
Planetary Gear Carrier
Output
Input 2
Input 1
Input 2
Output
Input 2
Output
This article has been accepted for publication in IEEE Transactions on Neural Systems and Rehabilitation Engineering. This is the author's version which has not been fully edited and
content may change prior to final publication. Citation information: DOI 10.1109/TNSRE.2022.3201383
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Fig. 4 CAD of the SEA-with-two-motor-variable-speed-transmission
Fig. 5 shows the fabricated fully-integrated SEA-with-two-
motor-variable-speed-transmission with dimensions of Ø90
mm × 51mm. The overall actuator weight is 680 g (actuator
with link weight is 875 g), including the main motor and second
motor, two planetary gear-based two-motor variable speed
transmission systems, and two high-resolution encoders.
Furthermore, the motor driver and STM32 microcontroller-
based embedded control system are integrated into the actuator
as a module.
To meet the small dimension, lightweight, high power
density ratio requirements of hip exoskeleton design, a custom
high torque density motor (Our motor was manufactured by
Unitree Robotics) was adopted into our design to work as the
main motor of the SEA-with-two-motor-variable-speed-
transmission. The customized high torque density motor (280
g), which can output nominal torque of 2 Nm with a normal
speed of 2100 RPM works as the main input of the two-
motor variable speed transmission.
The second input of the two-motor variable speed
transmission is driven through a small power DC motor (EC10,
Maxon, Sachseln, Switzerland), a harmonic gear drive (HDUC-
5-80-1U-CC, Harmonic Drive, Limburg, Germany), and a
worm-and-gear with a reduction ratio of 75:1 (MÄDLER,
modulus of 0.5 mm). This motor and transmission ratio
configure can ensure the maximum velocity of the second input
make the output in the (5) equal to zero, then the SEA-
with-two-motor-variable-speed-transmission work at the
lowest-transmission mode.
Considering assistive torque and velocity requirements
during level running and sitting-to-standing, we chose the gear
parameters , , , , , and . The relative ratio between
the maximum low- transmission and high- transmission modes
for the hip exoskeleton was set to 4. The transmission ratio
reached 32:1 in the highest-transmission mode, and the
resulting output torque reached 64 Nm with a nominal velocity
of 5.6 rad/s. In the low-transmission mode, we set the small-
power motor motion at maximum speed, resulting in the
minimum reduction ratio for the HEF joint being 6:1. The
resulting nominal velocity was up to 25 rad/s with an output
torque of 16 Nm. The lowest-transmission mode allows the
wearer to run up to 2.5 m/s with 20% biological torque
assistance.
Fig. 5 Fully-integrated SEA-with- two-motor-variable-speed-transmission
This article has been accepted for publication in IEEE Transactions on Neural Systems and Rehabilitation Engineering. This is the author's version which has not been fully edited and
content may change prior to final publication. Citation information: DOI 10.1109/TNSRE.2022.3201383
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Furthermore, the series elastic actuator concept was also used
in the design. There is a customized torsion spring serving as an
elastic element sandwich between the output of the
transmission and the torque output. This SEA configuration can
realize accurate torque control and complaint interaction, as
well as the torque sensor [8, 29]. The designed stiffness of the
monolithic disc-shaped torsion spring is 400 Nm/rad. The
maximum of the deformation and torque is 10° and 60Nm,
respectively. The torsion spring, with a high-resolution
quadrature kit encoder (Avago AEDA-3300 Series, Broadcom
Limited, South Carolina) is needed to measure the deflection; it
also operates as a torque sensor with a resolution of 0.03 Nm.
III. CONTROLLER DESIGN
A. Embedded System
The hip exoskeleton includes the onboard and off-board
custom electronic boards. The onboard custom electronic
circuit is devoted to controlling and driving the motors (two
motors for the HFE joint, and one motor for the step-width
adjustment joint), collecting data on the encoders, and CAN bus
communication. The chip of the onboard embedded system is
STM32 32-bit Arm Cortex MCUs. The onboard embedded
system is integrated into the modular joint, as shown in Fig. 5.
The off-board and battery are placed at the back of the
exoskeleton. One DSP chip (TMS320F2812)-based embedded
system operates controllers that read all sensor data to compute
high-level control algorithms. The high-level controller
communicates with a motor controller that is integrated into the
actuators through a CAN bus. There are two IMUs on both
thighs to estimate the acceleration of hip flexion/extension and
adduction/abduction. There is one more IMU placed at the back
to measure the angles, velocity, and acceleration of the upper
body.
B. Low-Level Torque Control of the SEA-with-two-motor-
variable-speed-transmission
For the low-level controller, the controller performs closed-
looped torque tracking control of the SEA-with-two-motor-
variable-speed-transmission. The torque sensor data measured
by the deformation of the torsion spring are fed back to the low-
level controller. The torque control is transformed into the
torsion spring’s deflection control:
(11)
where is the torsion spring’s deflection, and is the stiffness
of the torsion spring.
The torsion spring’s deflection control is based on the
proportional-integral differential (PID) controller. Reference
torque signal is the input and is converted into a command
spring deflection angle for the feedback term. The output of the
PID torque controller is the desired velocity . Then we can
get the control command of the main motor and secondary
motor through:
(12)
C. High-Level Assistive Control
The hip exoskeleton works in various modes, such as level
walking (LW), stair ascent (SA), stair descent (SD), sitting-to-
standing, and standing-to-sitting. The relationship between the
assistive timing and the gait cycle is different for the various
locomotion modes inspired by the human hip joint biological
torque [30, 31]. The control goal is to offer effective assistive
torque to adapt to the different locomotion modes. Starting from
the heel strike (), the controller generates the desired
assistive torque. The assistive torque profile is a mixture of four
halves of minimum jerk curves joined together at their tops, as
shown in Fig. 6.
Fig. 6 Parameterization of the hip torque profile
The assistive torque profile is:
when ,
(13)
when ,
(14)
when ,
(15)
when ,
(16)
otherwise (17)
where is the assistive torque;
, , and
are the hip angles corresponding to gait events of the
heel strike, heel off, and toe-off, respectively; and are
the flexion and extension peak torques, respectively; is the
hip flexion angle corresponding to the assistive torque at the
peak timing; and is the hip joint angle. The gait cycle begins
and ends at the heel-strike event. is maximum flexion
angle, which is an average value based on previous strides
measured through IMUs [32]. The proposed assistive torque
profile mimics the human biological hip moment profile. Over
a gait cycle, the assistive torque profile can generate an assistive
torque for both hip flexion and extension. Three parameters,
, and , are adjusted in real-time to adapt to the
different locomotion modes (LW, SD, SA, running) and the
This article has been accepted for publication in IEEE Transactions on Neural Systems and Rehabilitation Engineering. This is the author's version which has not been fully edited and
content may change prior to final publication. Citation information: DOI 10.1109/TNSRE.2022.3201383
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different speeds. is defined as follows:
(18)
where is a constant, which is to determine the hip flexion
assistive torque onset timing (increasing from toe-off and end
at the maximum flexion angle) in the gait cycle. And will be
set as different values for the LW, running, SD, and SA. The
onset timing is determined by the gait events. and are
scaled for the different locomotion modes. Additionally,
and are also scaled for different walking speeds and
benefits from the actuator being able to actively vary the
reduction ratio. The three key parameters (, and ,) can
dictate the assistance profile that is offset timing, assistance
duration, and assistance magnitude.
IV. BENCHTOP EXPERIMENTS
A. Backdrivability and zero-torque control performance of the
SEA-with-two-motor-variable-speed-transmission
We conducted benchtop testing and quantified the
performance in terms of the torque and velocity capabilities and
backdrivability. The fully integrated SEA-with-two-motor-
variable-speed-transmission was fixed during the benchtop
experiments. The output shaft of the actuator was manually
pushed. The torque and angle data are measured through a
spring-based toque sensor and encoders.
Fig. 7 Backdrivability Experiment. (a) The actuator worked at the highest-
reduction ratio mode (the stiffness of the SEA), (b) closed-looped zero torque
control
In the first test, the actuator was working in the highest-
reduction ratio mode, both motor M1 and M2 are power off. Fig.
7 (a) shows the relationship between the back-drive torque and
the angle. The proposed hip exoskeleton’s passive compliance
is due to the elasticity introduced by the customized torsion
spring. In the second test, SEA-with-two-motor-variable-
speed-transmission worked at close-looped zero torque control
to testing the active compliance. The back-drive torque of the
fully integrated SEA-with-two-motor-variable-speed-
transmission within 0.1 Nm for the different motion frequencies,
as shown in Fig. 7 (b).
B. Closed-loop Torque Control Bandwidth with Different
Reduction Ratio
To test the torque and velocity characteristics of the fully
integrated series elastic actuator with two-motor variable speed
transmission, the actuator was velocity-controlled motion with
different loadings. The peak torque can reach 12 Nm when the
actuator operates in the lowest-transmission ratio mode (6:1),
and the normal velocity is more than 5.9 rad/s. The actuator is
operated with a peak torque of 62 Nm and a normal velocity of
2.8 rad/s when operating in the highest-transmission ratio mode
(32:1).
Fig. 8 Step response of the torque control
Table III The performance of the actuator with different transmission ratios
6: 1
12: 1
24: 1
32: 1
Peak Torque (Nm)
12
23
46
62
Peak velocity (rad/s)
5.9
5.18
3.75
2.8
Cutoff frequency (Hz)
43.75
29.17
19.44
12.52
Transmission ratio adjusting
time (increasing) (ms)
→
13±4
28±5
21±3
Transmission ratio adjusting
time (decreasing) (ms)
11±4
24±3
17±5
←
Step response tests were performed for the SEA-with- two-
motor-variable-speed-transmission with the output of the
device rigidly fixed. The output of the SEA-with-two-motor-
variable-speed-transmission was blocked while step responses
were tested (i.e., 10 Nm, 20 Nm, 40 Nm, and 60 Nm), as shown
in Fig. 8. Table III summarized the performance of the actuator
with different speed transmission ratios. For the SEA-with-
two-motor-variable-speed-transmission, the cutoff frequency
of the closed-loop torque control is 43.75 Hz, 29.17 Hz, 19.44
Hz, and 12.52 Hz at 10 Nm, 20 Nm, 40 Nm, and 60Nm,
respectively. We also test the adjustment time of the speed
transmission ratio increasing from 6:1 to 12:1, 24:1, and 32:1,
and then decreasing from 32:1 to 24:1, 12:1, and 6:1 five times.
The adjustment time is 13±4, 28±5, and 21±3s for increasing
from 6:1 to 12:1, from 12:1 to 24:1, and from 24:1 to 32:1,
respectively. And the adjustment time is 17±5, 24±3, and 11
±4s for decreasing from 32:1 to 24:1, from 24:1 to 12:1, and
from 12:1 to 6:1, respectively.
C. Continual Torque control of the SEA-with-two-motor-
variable-speed-transmission
In this experiment, the proposed SEA-with-two-motor-
variable-speed-transmission torque and velocity characteristics
This article has been accepted for publication in IEEE Transactions on Neural Systems and Rehabilitation Engineering. This is the author's version which has not been fully edited and
content may change prior to final publication. Citation information: DOI 10.1109/TNSRE.2022.3201383
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are examined in the condition that the speed transmission ratio
continually adjusting, while outside of SEA-with-two-motor-
variable-speed-transmission is interacted with an external
motion. A high-power motor (300W) with a reduction gear of
200:1 was used as external motion. A commercial torque sensor
is connected between the SEA-with-two-motor-variable-speed-
transmission and external motion to measure the load torque.
Test 1: The SEA-with-two-motor-variable-speed-
transmission is controlled to track a torque increased from 0Nm
to 60 Nm within the 60 s, while the external motor is braked.
Test 2: The SEA-with-two-motor-variable-speed-
transmission is controlled to track a torque increased from 0Nm
to 60 Nm within the 60 s, while the external motor with a fixed
speed of 3 rad/s.
Test 3: The SEA-with-two-motor-variable-speed-
transmission is controlled to track a fixed torque of 10 Nm,
while the external motion is increased from 0 rad/s to 10 rad/s
within the 60 s.
Fig. 9 Continual torque control. Note: The M2 velocity in the plots is the
harmonic gear drive output side’s velocity.
The SEA-with-two-motor-variable-speed-transmission
output torque, the velocity and current of the main and
secondary motor, and external motion velocity were recorded.
Fig. 11 shows the test results, from top to the bottom (Fig. 9 (a)
- Fig. 9 (c)) is the results of Test 1, Test 2, and Test 3. For Test
1, as shown in Fig. 9 (a), we can see that only the main motor
M1 motion. The SEA-with-two-motor-variable-speed-
transmission worked at the highest-transmission ratio mode.
The motor current increased with torque increasing. For Test 2,
as shown in Fig. 9 (b), the secondary motor M2 began motion
at T1 and stopped at T2 to adjust the reduction ratio to meet
both the torque and velocity requirements. For Test 3, as shown
in Fig. 9 (c), the secondary motor M2 began motion at T1 to
adjust the reduction ratio.
V. PRELIMINARY HUMAN SUBJECT TESTS
Five healthy subjects (age 28.56 y.o., weight 78.59.9 kg,
height 1.760.26 m) participated in the preliminary human
subject tests. All subjects signed an informed consent form. The
experiments were approved by the Ethical Committee of
Soochow University. The hip exoskeleton was powered by a
lithium-ion battery (400 g), which can support the device in
operation for 1.5 hours. The actuator, electric circuit, and
battery were fixed in the bag, which was fixed at the wearer’s
back.
The hip joint angle and assistive torque of the hip
exoskeleton were recorded to analyze the performance of the
assistance. The hip joint angles and assistive torque were
measured by an encoded spring-based torque sensor integrated
into a fully integrated series elastic actuator with actively
variable reduction ratio transmission. The hip angles during the
free motion were performed without wearing the hip
exoskeleton (no exo) and were measured through an inertial
motion capture system (Perception Neuron 2.0, Noitom
Technology Ltd.). The heel-strike event, which was detected
through two loading cells placed at each shoe, was used to
normalize the gait periods.
A. Sit-to-stand test
The subjects wore the hip exoskeleton and performed sit-to-
stand trials with different peak torque assistances. The subjects
were asked to keep the same speed to sitting-to-standing at each
trial. For the sitting-to-standing, the assistive torque profile is:
(19)
where is the peak assistive torque, is the hip
flexion angle when the human stands, and is the hip flexion
angle when the human sits. We set to 5 degrees; the hip
exoskeleton will work in zero torque mode when the hip flexes
and is smaller than 5 degrees, which allows the subject to freely
move. The peak assistive torque was set as 50 Nm, 30 Nm, 10
Nm, and 0 Nm (power off) for the four test groups.
Fig. 10 shows the HFE joint angle, assistive torque under the
peak assistive torque was set as 50 Nm, 30 Nm, 10 Nm, and 0
Nm (power off). There were no significant changes in the hip
HFE joint angle on the peak assistive torque was set as 50 Nm,
30 Nm, 10 Nm, and 0 Nm (power off). We can see from Fig. 8
that the assistive torques start from the beginning sitting on the
chair and are gradually reduced to zero with standing. The
assistive torques show that the hip exoskeleton can output
assistive torque up to 50 Nm.
This article has been accepted for publication in IEEE Transactions on Neural Systems and Rehabilitation Engineering. This is the author's version which has not been fully edited and
content may change prior to final publication. Citation information: DOI 10.1109/TNSRE.2022.3201383
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Fig. 10 Sit-to-Stand transition experiment
B. Different locomotion modes assistance tests
The subjects walked on a treadmill for 10 min at a fixed speed
of 1.2 m/s, ran on a treadmill for 10 min at a fixed speed of 2.2
m/s, and ascended and descended a stair (four floors, 230 stairs)
under three conditions: wearing the hip exoskeleton with
assistance (assist on), wearing the hip exoskeleton without
assistance (assist off), and without wearing the hip exoskeleton
(no exo) [35]. Participants were asked to keep the selected
cadences constant during the ascending stair and descending
stairs. We set as different values for LW, SA, SD, and
running based on the human biological hip moment [12, 13].
Table IV Assistance parameters for the different locomotion modes
Speed
LW
0.2Nm/kg
-0.2Nm/kg
65%
1.2 m/s
SA
0.3Nm/kg
-0.5Nm/kg
20%
-
SD
0.4Nm/kg
-0.4Nm/kg
30%
-
Running
0.1Nm/kg
-0.1Nm/kg
20%
2.2m/s
and are the flexion and extension peak torque, respectively, is a
constant, which is to determine the hip flexion assistive torque onset timing
(increasing from toe-off and end at the maximum flexion angle) in the gait cycle.
Fig. 11 shows the average kinematic and assistive force
delivered by the hip exoskeleton under three conditions: assist
on, assist off, and no exo. Fig. 11 shows that the assistive torque
begins at the heel-strike event, smoothly increases to the
negative peak and ends at heel-off, then increases to the positive
peak and ends at maximum hip flexion for all locomotion
modes. Table V summarized the performance of hip
exoskeleton assistance for the different locomotion modes. The
peak torque reached 0.22 Nm/kg, 0.12 Nm/kg, 0.53 Nm/kg,
0.42 Nm/kg and 0.21 Nm/kg and 0.12 Nm/kg, 0.32 Nm/kg, and
0.42 Nm/kg at extension and flexion for walking, running, SA,
and SD, respectively. The assistive torque up to peak flexion
torque reached 20%, 20%, 30%, and 40% for the walking,
running, SA, and SD, respectively, while up to extension torque
at 20%, 20%, 30%, 40%. In summary, the hip exoskeleton only
introduced minimum changes to gait kinematics. The assistive
force tracked the designed profiles for different locomotion
modes. Furthermore, the assistive torque could also adapt to
different subjects’ independent walking speeds.
Table V Performance of assistance for the different locomotion modes
LW
Running
SA
SD
Angle (deg)
(-14, 23)
(-12, 18)
(4, 39)
(-14, 37)
Peak torque
(Nm/kg)
0.22
0.12
0.53
0.42
Peak velocity
(rad/s)
2.43
5.48
2.98
3.21
Torque
tracking error
(RMSE)
0.189
0.234
0.201
0.209
Fig.11 The results of the different locomotion modes assistance tests
VI. DISCUSSION
The design of hip exoskeletons for elderly individuals is
challenging in terms of the weight of the exoskeleton, the
compromise between force and velocity, the comfortable
This article has been accepted for publication in IEEE Transactions on Neural Systems and Rehabilitation Engineering. This is the author's version which has not been fully edited and
content may change prior to final publication. Citation information: DOI 10.1109/TNSRE.2022.3201383
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wearer–exoskeleton interface, and assistance performance. The
hip joints work together on many activities, such as sitting-to-
standing, walking, running, and climbing stairs. There are
diverse characteristics of the different activities. The torque and
velocity requirements for walking/running and sitting-to-
standing are opposite. The motivation of this paper is to address
this opposite and to avoid using a high-power motor to reduce
the weight of the hip exoskeleton.
Benefitting the planetary gear’s multiport characteristics, a
series elastic actuator with two-motor variable speed
transmission is designed for our hip exoskeleton, which can
support different speed transmissions. In the lowest-
transmission ratio mode, the transmission ratio for the HFE
joints is 6:1. Based on our benchtop tests, the peak velocity is
more than 5.9 rad/s with an output torque of 16 Nm, which
allows the wearer to run at 2.5 m/s, as shown in Fig. 11. In
contrast, in the highest-transmission ratio mode, the
transmission ratio for the HFE joints becomes 32:1. The
resulting peak torque can reach 60 Nm, which can assist sitting-
to-standing with 25% of the biological hip flexion torque, as
shown in Fig. 10. Based on the two-motor variable speed
transmission and custom high torque density motor, the fully
integrated SEA weighing 870 g can generate assistive torque up
to 60 Nm in the highest- transmission ratio mode and can allow
the hip exoskeleton to run at a high speed of 2.5 m/s in the
lowest-reduction mode. These performances are achieved by
adopting a high torque density motor and two-motor variable
speed transmission rather than using a high transmission ratio
or high-power motor due to their inherent heavy and bulky
characteristics.
The proposed SEA-with-two-motor-variable-speed-
transmission can continuously adjust the speed transmission
ratio similar to [26], and not only discretely adjust between
tasks like [25, 26], and [18]. Different from the two same motor
parallel arranged input to the planetary gear in [24], there has a
high-power motor work as the main input to output the drive
torque, and a low-power motor with the worm-and-gear as
secondary input work to adjust the speed transmission ratio.
Different from [24], the reduction ratio of the proposed SEA-
with-two-motor-variable-speed-transmission is reduced with
the increasing velocity of the secondary motor. Furthermore,
there have two planetary gears series connections as the
variable speed transmission. These configure reduced the axial
dimension and the weight of the actuator. Besides, our two-
motor variable speed transmission connected with a torsion
spring assemble as a series elastic actuator to realize accurate
interaction force control and complaint interaction with the
wearer, as well as the torque sensor.
The proposed wearable hip exoskeleton with the two-motor
variable speed transmission series elastic actuator is to promote
the independent living of the elderly and the population with
lower-limb impairments. The SEA-with-two-motor-variable-
speed-transmission can meet the individual needs of the elderly
because different elderly people and people with lower-limb
impairments have different assistance level requirements.
Furthermore, our exoskeleton also has the potential to reduce
joint loadings of able-bodied populations for regular activities.
The motivation of this paper is to propose an actuator design
concept for the hip exoskeleton. The speed transmission ratio
adjusts rang can choose based on the user’s requirement.
Whatever how our effort, the hip exoskeleton still introduces
nonnegligible mass. Currently, the hip exoskeleton focuses on
reducing the weight to assist energy-efficient walking, which
hasn’t a frame to support the weight. A biomechanics study [34]
showed that the energy expenditure increased the smallest when
additional loading was applied to the waist compared with the
thigh, shank, or foot. The energy expenditure will not
significantly increase when the 4 kg mass loading is at the waist.
The proposed hip exoskeleton is 2.85 kg (excluding batteries).
And this weight is light than the most of hip exoskeletons [4, 6-
9, 15, 16]. Our hip exoskeleton with the rigid frame can connect
with the weight support frame, such as Kim designed bio-
inspired knee joint [35] or Lenzi designed self-aligning
mechanism [36], to balance the weight of the exoskeleton, it
will be beneficial for the elderly to wear the hip exoskeleton
long time.
Several limitations related to the device’s design and controls
still limit the possibility of adoption without extra work. There
is a tradeoff between the actuator’s backdrivability and the two-
motor variable speed transmission. The backdrivability of the
SEA-with-two-motor-variable-speed-transmission depends on
the stiffness of the customized torsion spring when both motor
M1 and M2 power off. The back-drive torque is big than [17,
37]. A simplistic control scheme was used in this paper to
evaluate the performance of the hip exoskeleton during steady-
state walking, running, SA, SD, RA, and RD. The peak timing
and peak force were fixed. Previous studies [38] have shown
that online peak force and timing optimization can improve gait
performance and reduce energy expenditure. The load cells at
the shoes were used in the controller to detect the gait event and
introduce cables at the shoes and legs. In the future, detecting
gait events based on IMUs placed on the thighs will eliminate
cable issues. With the design of the hip exoskeleton now
validated, additional elderly people and people with lower-limb
impairments clinical validation can conduct investigate clinical
outcomes in the further [39].
VII. CONCLUSIONS
In this paper, a novel concept for actuator design, namely, a
series elastic actuator with two-motor variable speed
transmission is introduced in the hip exoskeleton design. The
reduction ratio can be adjusted in real-time to meet the different
requirements of torque-velocity characteristics for walking,
running, stand-to-sit, sit-to-stand, and climbing stairs. The
design, control, and experimental validation of a novel
lightweight hip exoskeleton for elderly individuals and people
with lower-limb impairments are presented in this study. The
proposed lightweight hip exoskeleton driven by SEA-with-two-
motor-variable-speed-transmission can application in elderly
people and people with lower-limb impairments, as well as the
able-bodied populations for regular activities.
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This article has been accepted for publication in IEEE Transactions on Neural Systems and Rehabilitation Engineering. This is the author's version which has not been fully edited and
content may change prior to final publication. Citation information: DOI 10.1109/TNSRE.2022.3201383
This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/
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This article has been accepted for publication in IEEE Transactions on Neural Systems and Rehabilitation Engineering. This is the author's version which has not been fully edited and
content may change prior to final publication. Citation information: DOI 10.1109/TNSRE.2022.3201383
This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/
