Proceeding Paper

Investigation of a Passive Ankle Joint Exoskeleton Designed for

Movements with Dorsal and Plantar Flexion †
Nursultan Zhetenbayev 1, * , Gani Balbayev 2 , Teodor Iliev 3 and Balzhan Bakhtiyar 4

1 Department of Electronics and Robotics, Almaty University of Power Engineering and Telecommunications,
Almaty 050013, Kazakhstan
2 Institute of Automation and Telecommunication, Academy of Logistics and Transport,
Almaty 050012, Kazakhstan; g.balbayev@gmail.com
3 Department of Telecommunications, University of Ruse, 7004 Ruse, Bulgaria; tiliev@uni-ruse.bg
4 Department of Thermal Power Engineering, Kazakh Agrotechnical University, Astana 010011, Kazakhstan;
bahtyar.baljan@mail.ru
* Correspondence: n.zhetenbaev@aues.kz
† Presented at the International Conference on Electronics, Engineering Physics and Earth Science (EEPES’23),
Kavala, Greece, 21–23 June 2023.

Abstract: The ankle exoskeleton is an auxiliary device designed to restore human independence.
This paper proposes the development and initial testing of a passive ankle exoskeleton designed for
movements with dorsal and plantar flexion. The device also includes a new mechanism design with
four electric linear actuators, and the shank platform and the foot platform are connected to the ball
by a swivel joint. Mechanical tests demonstrate the ability of the prototype to function adequately in
the natural range of the ankle joint. Preliminary results show that the exoskeleton can reduce the
activation of the calf muscles on the limb on which the device is installed. In the investigation of a
passive ankle joint exoskeleton designed for movements with dorsal and plantar flexion, numerical
test results can be highlighted by focusing on parameters such as speed, acceleration, and translation
moment. These measurements provide valuable information about the performance and effectiveness
of the exoskeleton. By focusing on these numerical test results, it is possible to obtain an idea of the
performance of the exoskeleton, understand its impact on the movements of the ankle joint, and
make informed decisions for further improvements or optimization of the design.

Keywords: exoskeleton; passive ankle exoskeleton; functional testing; dorsal flexion; plantar flexion
Citation: Zhetenbayev, N.; Balbayev,
G.; Iliev, T.; Bakhtiyar, B.
Investigation of a Passive Ankle Joint
Exoskeleton Designed for
1. Introduction
Movements with Dorsal and Plantar
Flexion. Eng. Proc. 2023, 41, 17.
Every year, millions of people around the world suffer from motor disabilities caused
https://doi.org/10.3390/ by a spinal cord injury (SCI), stroke, or cerebral palsy. These people suffer from a reduced
engproc2023041017 muscular control capacity, manifested by a reduced torque generation and a lack of ability to
modulate the mechanical impedance around the joints. They often experience symptomatic
Academic Editor: Grigor Mihaylov
muscle control, for example, because of spasticity [1–3].
Published: 20 July 2023 Exoskeletons are wearable devices that augment human movement and have been
widely used in various fields, including military, healthcare, and sports.
Effective rehabilitation is a long-term process that also requires a trained physical ther-
apist to help restore joint mobility. Therefore, robotics has been involved in rehabilitation
Copyright: © 2023 by the authors.
therapy for constant patient observation and assisted motion [4].
Licensee MDPI, Basel, Switzerland.
Physiotherapy is one of the areas of medicine in which engineering works. Several
This article is an open access article
devices designed to improve and support therapy are being studied and developed there.
distributed under the terms and
Mechanics and electronics, together with physiotherapy, are developing exoskeletons,
conditions of the Creative Commons
which are electromechanical devices attached to limbs that can help the user move or adjust
Attribution (CC BY) license (https://
the movement of these limbs, providing automatic therapy with flexible and customizable
creativecommons.org/licenses/by/
4.0/).

Eng. Proc. 2023, 41, 17. https://doi.org/10.3390/engproc2023041017 https://www.mdpi.com/journal/engproc

Eng. Proc. 2023, 41, x 2 of 11

Eng. Proc. 2023, 41, 17 2 of 11

customizable programs to improve autonomy and meet the needs of each patient. Exo-
skeletons can increase the effectiveness of physiotherapy and shorten the rehabilitation
programs to improve autonomy and meet the needs of each patient. Exoskeletons can
timeincrease
of patients [5,6].
the effectiveness of physiotherapy and shorten the rehabilitation time of patients [5,6].
MostMost past robotic
past roboticdesigns
designs arearebased
basedononthe the
staticstatic platform
platform design,design,
where a where a
non-portable
non-portable device requires the patient to hold a foot on a grounded platform
device requires the patient to hold a foot on a grounded platform (usually sitting) to perform (usually
sitting) to perform[7–10].
rehabilitation rehabilitation [7–10].structures
Most of these Most of these
are not structures
only bulky, arebut
notalso
onlystationary
bulky, but in the
also field.
stationary in the field. Thus, patients must visit a hospital or an
Thus, patients must visit a hospital or an establishment where a rehabilitationestablishment wheredevice
a rehabilitation
is installed, device
despiteistheinstalled,
potential despite the potential
impairment impairment of mobility.
of mobility.
The The
Almaty ankle exoskeleton is a device
Almaty ankle exoskeleton is a device designed designed to enhance
to enhancethe function of the
the function of the
ankle joint. Ankle exoskeletons have the potential to improve mobility
ankle joint. Ankle exoskeletons have the potential to improve mobility and reduce the and reduce the risk
risk of
of injury
injury in
in individuals
individuals withwith lower
lower limb
limb impairments.
impairments.
The The
purpose of this article is to provide
purpose of this article is to provide an overview
an overview of the
of development
the development and and
testing
testing
of anofankle exoskeleton,
an ankle exoskeleton, including
including thethe
design process,
design process,testing and
testing andevaluation,
evaluation, and
andpoten-
potential
tial applications.
applications. This This article
article aims
aims to to provide
provide aa better
better understanding
understanding of of ankle
ankle exoskele-
exoskeletons
tonsandandtheir
theirpotential
potentialbenefits,
benefits,limitations,
limitations, andand challenges
challenges to to inform
inform future
future research
research and
and development
development in in this
this field.
field.
The The
virtual model
virtual of the
model of device
the devicefor rehabilitation of the
for rehabilitation of ankle of the
the ankle of third degree
the third of of
degree
freedom, capable
freedom, of performing
capable all the
of performing all necessary physical
the necessary exercises,
physical is shown
exercises, in Figure
is shown 1. 1.
in Figure

Figure 1. Virtual
Figure model.
1. Virtual model.

2. Background
2. Background
Exoskeletons
Exoskeletons havehave
beenbeen around
around forfor decades,starting
decades, startingfrom
from the
the development
development of of full-
body exoskeletons
full-body exoskeletons for
for military
militarypurposes
purposesininthe the1960s.
1960s.Since
Sincethen,
then,exoskeleton
exoskeletontechnology
tech-
nology has advanced rapidly, and a variety of exoskeletons have been developed toimprove
has advanced rapidly, and a variety of exoskeletons have been developed to im-
provemobility, strength,
mobility, and endurance
strength, in individuals
and endurance with mobility
in individuals with impairments, sports athletes,
mobility impairments,
and
sports militaryand
athletes, personnel
military[11,12].
personnel [11,12].
Exoskeletons can can
Exoskeletons be be categorized
categorized into
into twotypes:
two types:passive
passiveandand active.
active. Passive
Passive exoskele-
exo-
tons use mechanical elements, such as springs or dampers, to reduce
skeletons use mechanical elements, such as springs or dampers, to reduce the load the load on the human
on the
body during movement, while active exoskeletons use actuators to
human body during movement, while active exoskeletons use actuators to provide ex- provide external forces
to assist or replace the human muscles.
ternal forces to assist or replace the human muscles.
Ankle exoskeletons, specifically, are designed to augment the function of the ankle
Ankle exoskeletons, specifically, are designed to augment the function of the ankle
joint, which plays a critical role in human mobility. Ankle exoskeletons can be passive
joint, which plays a critical role in human mobility. Ankle exoskeletons can be passive or
or active and can aid during walking, running, and jumping. They have the potential to
active and can aid during walking, running, and jumping. They have the potential to
improve mobility and reduce the risk of injury in individuals with lower limb impairments,
improve mobility and reduce the risk of injury in individuals with lower limb impair-
such as stroke survivors or individuals with cerebral palsy.
ments, such as stroke survivors or individuals with cerebral palsy.
Research on ankle exoskeletons has grown significantly over the past decade, and
Research on ankle exoskeletons has grown significantly over the past decade, and
several prototypes have been developed and tested in laboratory and clinical settings.
several prototypes have been developed and tested in laboratory and clinical settings.
The design of ankle exoskeletons is still a challenge, as the device must provide sufficient
The design of ankle exoskeletons is still a challenge, as the device must provide sufficient
support and assistance without restricting natural human movement or causing discomfort.
Eng. Proc. 2023, 41, x 3 of 11

Eng. Proc. 2023, 41, 17 3 of 11

support and assistance without restricting natural human movement or causing dis-
comfort. Additionally, the effectiveness of ankle exoskeletons varies depending on the
individual’s gait pattern, body weight, and other factors.
Additionally, the effectiveness of ankle exoskeletons varies depending on the individual’s
gait pattern, body
3. Kinematic weight, and other factors.
Design
The four-actuator
3. Kinematic Design ankle exoskeleton is a wearable device that can help or enhance
ankle movements. The exoskeleton is designed to be placed around the tibia and foot and
The four-actuator ankle exoskeleton is a wearable device that can help or enhance
is powered by four electric motors that are connected to the ankle by transmission sys-
ankle movements. The exoskeleton is designed to be placed around the tibia and foot and
tems.
is powered by four electric motors that are connected to the ankle by transmission systems.
The exoskeleton
The exoskeleton case case is
is made
made of of PLA
PLA plastic,
plastic, which
which is is aa biodegradable
biodegradable thermoplastic
thermoplastic
material widely used in 3D printing. PLA plastic is lightweight,
material widely used in 3D printing. PLA plastic is lightweight, durable, and easy durable, and toeasy
mold,to
mold, making it the ideal material for custom-made
making it the ideal material for custom-made exoskeletons. exoskeletons.
The degree
The degree of of freedom
freedom (DOF)
(DOF) of of an
an ankle
ankle exoskeleton
exoskeleton refers refers to
to the
the number
number of of inde-
inde-
pendent directions in which it can move. Generally, ankle exoskeletons
pendent directions in which it can move. Generally, ankle exoskeletons are designed to are designed to
provide one
provide one DOF,
DOF,corresponding
correspondingto toplantarflexion
plantarflexionand anddorsiflexion
dorsiflexionmovements.
movements.
The spatial mechanism of the ankle exoskeleton involves
The spatial mechanism of the ankle exoskeleton involves a combination a combination of rigid and
of rigid
flexible components that work together to achieve the desired
and flexible components that work together to achieve the desired range of motion. The range of motion. The
mechanism typically includes a footplate that attaches to the wearer’s
mechanism typically includes a footplate that attaches to the wearer’s foot and a rigid foot and a rigid
frame that
frame that attaches
attaches to to the
the lower
lower leg.
leg. The
The footplate
footplate isis connected
connected to to the
the frame
frame byby aa hinge
hinge
joint that
joint that allows
allows forfor rotation
rotation inin the
the sagittal
sagittal plane,
plane, corresponding
corresponding to to plantarflexion
plantarflexion andand
dorsiflexion movements.
dorsiflexion movements.
According to Figure 2, it can can be calculated
calculated by the formula calculated by the Somov–
Malyshev formula, the number of degrees W𝑊ofof
Malyshev formula, the number of degrees freedom
freedom of of
thethe mechanism
mechanism forfor
thethe spa-
spatial
tial kinematic structure is determined
kinematic structure is determined as follows: as follows:
𝑊W
==6 6××(𝑛(n−−1) − 55××p 𝑝 = =
1) − 6 ×6 8× 8(−)1
(−) 1−5−× 95 =×3.9 = 3. (1)
(1)
5

Figure 2. Kinematic design with

Figure 2. with parameters.
parameters.

The
The application
application of of this
this formula
formula isis possible
possible ifif no
no additional
additional conditions
conditions are
are imposed
imposed
on
on the movements of the links that make up the mechanism (the axes of all rotational
the movements of the links that make up the mechanism (the axes of all rotational
pairs
pairs were
were parallel,
parallel, intersected
intersected atat one
one point,
point, etc.).
etc.). These
These additional
additional requirements
requirements change
change
the
the nature of the movements of the mechanism and, accordingly, change the form of
nature of the movements of the mechanism and, accordingly, change the form of its
its
structural formula.
structural formula.
In
In aa spherical
spherical mechanism,
mechanism, all all three
three kinematic
kinematic chains
chains impose
impose the
the same
same connections,
connections,
and the axes of all pairs intersect at one point. In the proposed design, there are three
and the axes of all pairs intersect at one point. In the proposed design, there are three
power screws and three kinematic mutual screws. These are the zero parameter screws. To
power screws and three kinematic mutual screws. These are the zero parameter screws.
determine the number of degrees of freedom, we apply the Dobrowolski formula:
To determine the number of degrees of freedom, we apply the Dobrowolski formula:
W = 3 × (n − 1) − 2 × p5 − p4 = 3 × (8 − 1) − 2 × 9 = 3. (2)
 𝑊 = 3 × (𝑛 − 1) − 2 × 𝑝 − 𝑝 = 3 × (8 − 1) − 2 × 9 = 3. (2)
Eng. Proc. 2023, 41, 17 4 of 11
If the last rotational pairs are replaced by spherical ones, then in this case each chain
imposes one bond. The number of degrees of freedom is determined by the Somov–
Malyshev formula:
If the last rotational pairs are replaced by spherical ones, then in this case each chain
imposes𝑊one × (𝑛 −
= 6bond. 1) −
The 5 × 𝑝 of
number − 4degrees
× 𝑝 −of3 × 𝑝 = 6 is
freedom × determined
(8 − 1) − 5 by× 6the
− Somov–
(3)
Malyshev formula: 3 × 3 = 3.

4. Hardware = 6 × (n − 1)and
W Architecture − 5Mathematical
× p5 − 4 × p4 − 3 × p3 = 6 × (8 − 1) − 5 × 6−
Modeling (3)
3 × 3 = 3.
Hardware architecture and mathematical modeling are two important aspects of
many technological
4. Hardware systems,
Architecture andincluding exoskeletons
Mathematical Modeling used for rehabilitation or other ap-
plications.
Hardware architecture and mathematical modeling are two important aspects of many
The hardware architecture of the exoskeleton is crucial for its performance, security,
technological systems, including exoskeletons used for rehabilitation or other applications.
and functionality. It includes solutions related to the type of materials used, mechanical
The hardware architecture of the exoskeleton is crucial for its performance, security,
design, placement of the sensor and actuator, power supply, and communication inter-
and functionality. It includes solutions related to the type of materials used, mechanical
faces. Hardware architecture is usually designed with specific exoskeleton requirements
design, placement of the sensor and actuator, power supply, and communication interfaces.
in mind, such as desired range of motion, strength, weight, and durability.
Hardware architecture is usually designed with specific exoskeleton requirements in mind,
To develop a mechatronic design of a rehabilitation device for ankle joint rehabilita-
such as desired range of motion, strength, weight, and durability.
tion,To
several
developmethods were used,
a mechatronic designsuch
of a as the introduction
rehabilitation device offorquality functionality,
ankle joint in-
rehabilitation,
dustrial, and structural designs. The proposed three-dimensional rehabilitation
several methods were used, such as the introduction of quality functionality, industrial, and device
consists of
structural a foot The
designs. platform andthree-dimensional
proposed elements controlled by four linear
rehabilitation deviceelectric
consistsactuators.
of a foot
platform and elements controlled by four linear electric actuators. Thanks tomovements
Thanks to this, with the help of this device, it is possible to achieve natural this, with the of
the ankle joint. In addition, this device provides an extensive workspace that provides
help of this device, it is possible to achieve natural movements of the ankle joint. In addition, the
necessary
this device movement
provides anofextensive
the lower part of the
workspace leg.provides
that Figure 3the
illustrates the
necessary comprehensive
movement of the
rehabilitation system.
lower part of the leg. Figure 3 illustrates the comprehensive rehabilitation system.

Figure 3. Hardware
Figure3. Hardware architecture.
architecture.

Given the three movements, it is very difficult to simulate the dynamics regulating
Given the three movements, it is very difficult to simulate the dynamics regulating
the operation of the ankle rehabilitation device, since the overall system is nonlinear. Due
the operation of the ankle rehabilitation device, since the overall system is nonlinear. Due
to some practical conclusions, the main task of mechatronics is to control devices using
to some practical conclusions, the main task of mechatronics is to control devices using
simplified models. Thus, they can be made more resistant to some external influences. In
simplified models. Thus, they can be made more resistant to some external influences. In
this regard, the individual dynamics of the ankle joint rehabilitation device for control-
this regard, the individual dynamics of the ankle joint rehabilitation device for control-
ling the main movements were first considered. The main reason is that during passive
ling the main movements were first considered. The main reason is that during passive
rehabilitation, certain types of exercises are performed first, which ensure the stability and
functionality of the ankle joint.
Mathematical modeling involves the use of mathematical equations and methods
to describe and analyze the behavior or characteristics of a system. In the context of
exoskeletons, mathematical modeling can be used to describe mechanical, electrical, and
control aspects of a system, as well as interactions with the human body.
 rehabilitation, certain types of exercises are performed first, which ensure the stability
and functionality of the ankle joint.
Mathematical modeling involves the use of mathematical equations and methods to
describe and analyze the behavior or characteristics of a system. In the context of exo-
Eng. Proc. 2023, 41, 17 skeletons, mathematical modeling can be used to describe mechanical, electrical,5 and of 11
control aspects of a system, as well as interactions with the human body.
Mathematical models can be used to model and predict the behavior of an exoskel-
eton,Mathematical
such as its kinematics
models can(motion),
be used to dynamics
model and (forces and
predict thetorques),
behaviorand of ancontrol algo-
exoskeleton,
rithms. These models can help in understanding the performance, stability,
such as its kinematics (motion), dynamics (forces and torques), and control algorithms. and safety of
the exoskeleton and can also be used to optimize design, evaluate
These models can help in understanding the performance, stability, and safety of the performance, and de-
velop controland
exoskeleton algorithms.
can also be used to optimize design, evaluate performance, and develop
Consider
control algorithms.Figure 4, where 𝜃 determines the angle of movement of dorsiflexion and
plantar
Consider 𝑥, 𝑦 is4, an
flexion,Figure absolute
where coordinate
θ determines angle of 𝑥′,
the system, 𝑦 ′ is a movable
movement coordinate
of dorsiflexion and
system, 𝑃 determines the beam of force, because the 0force
0 comes from some
plantar flexion, x, y is an absolute coordinate system, x , y is a movable coordinate system, part of the
weight of the ankle (the weight of the entire foot is not provided, since
P determines the beam of force, because the force comes from some part of the weight of the patient relies
only on, for
the ankle (theexample,
weight ofif the
we entire
take the
footpatient’s sitting position),
is not provided, since the then it also
patient describes
relies only on, the
for
stiffness
example,ofif the
we tibial–ankle
take the patient’s 𝑑 is a constant
joint, sitting position),distance, 𝜏 is the torque
anddescribes
then it also transmitted
the stiffness of the
by the motorjoint,
tibial–ankle to adjust
d is athe angular
constant orientation.
distance, and τ is the torque transmitted by the motor to
adjust the angular orientation.

Figure 4.
Figure Angle describing
4. Angle describing dorsiflexion
dorsiflexion and
and plantarflexion
plantarflexion movements
movements θ.
θ.

For the control of the angular position during the dorsiflexion and plantarflexion
For the control of the angular position during the dorsiflexion and plantarflexion
movements of the foot platform, only individual dynamics are provided, not for combined
movements of the foot platform, only individual dynamics are provided, not for com-
movements. The mathematical model that governs the dynamics of motion can be obtained
bined movements. The mathematical model that governs the dynamics of motion can be
by applying Newton’s second law:
obtained by applying Newton’s second law:
.. .
J θ𝐽𝜃++cθ𝑐𝜃
==τ 𝜏−−Fd,
𝐹𝑑, (4)
where J𝐽 isisthe
where themoment
momentofofinertia andc 𝑐is is
inertiaand thethe coefficient
coefficient of of friction.
friction.
It is recommended to use the PID controller to realize
It is recommended to use the PID controller to realize the desired the desired position
position 𝜃 ac-
θ according
cording to predetermined
to predetermined trajectories.
trajectories.
.
τ 𝜏==Jυ𝐽𝜐++c𝑐𝜃
θ; ; (5)
.. ∗ . . ∗ Z t
∗
υ=θ − α θ − θ − α ( θ − θ ) − α (θ − θ ∗ )∗dt. (6)
𝜐 = 𝜃 ∗ − 2𝛼 𝜃 − 𝜃 ∗ − 𝛼1 (𝜃 − 𝜃 ∗ ) − 𝛼 0
0 (𝜃 − 𝜃 )𝑑𝑡. (6)
The dynamics of a closedsystemwith a controller controlling the trajectory proposed
The dynamics of a closed .∗
system with a controller controlling the trajectory pro-
above, the error of which e = θ − θ , is given as follows:
posed above, the error of which 𝑒 = 𝜃 − 𝜃 ∗ , is given as follows:
Z t
.. .
e 𝑒++α2𝛼e +
𝑒+α1𝛼e +
𝑒+α0𝛼 𝑒𝑑𝑡
edt = =−−𝐹𝑑;
Fd; (7)
0

... .. .
e 𝑒⃛++α𝛼2 e𝑒++α𝛼1 e 𝑒−−α𝛼 ==0.0.
0e 𝑒 (8)
(8)
The parameters α0 , α1 , and α2 are chosen so that the characteristic polynomial (3) is the
Hurwitz polynomial to ensure that the model dynamics are globally asymptotically constant.
The desired trajectory of motion is given by the following Bezier polynomial:
   
θ ∗ (t) = θi + θ f − θi σ t, ti , t f µ5p ; (9)
 The parameters 𝛼 , 𝛼 , and 𝛼 are chosen so that the characteristic polynomial (3)
is the Hurwitz polynomial to ensure that the model dynamics are globally asymptotically
Eng. Proc. 2023, 41, 17 constant. 6 of 11
The desired trajectory of motion is given by the following Bezier polynomial:
𝜃 ∗ (𝑡) = 𝜃 + 𝜃 − 𝜃 𝜎 𝑡, 𝑡 , 𝑡 𝜇 ; (9)
 
2
σ t,𝜎ti ,𝑡,t f𝑡 , 𝑡= γ=1 −
𝛾 γ−2 µ𝛾p 𝜇+ γ
+3𝛾µ p𝜇− − . ++γ𝛾6 µ𝜇5p ;;
. .⋯ (10)
(10)

t− 𝑡−
ti 𝑡
µ p 𝜇= = , , (11)
(11)
t f −−
𝑡 ti 𝑡
where 𝜃 = 𝜃 ∗ (𝑡 ) and 𝜃 = 𝜃 ∗ 𝑡 are the initial and final necessary positions, thus the
 
where θi = θ ∗ (ti ) and θ f = θ ∗ t f are the initial and final necessary positions, thus the
rehabilitation process is carried out by slowly and smoothly moving from the initial po-
rehabilitation process is carried out by slowly and smoothly moving from the initial position
sition to the next position.
to the next position.
5. Experimental
5. Experimental TestTest Bed
Bed for
for Prototype
Prototype
The four
The four electric
electric drives
drives in inthe
theexoskeleton
exoskeletonprovide
providethe necessary
the necessary force to to
force help or in-
help or
crease ankle movement. The motors can be controlled by a microcontroller,
increase ankle movement. The motors can be controlled by a microcontroller, which can which can
adjust the
adjust the speed
speed and
and direction
direction of of rotation
rotation of
of the
the motors
motors depending
depending on on the
the movement
movement of of
the user.
the user.
As shown
As shownininFigure
Figure 5, the
5, the ankle
ankle jointjoint rehabilitation
rehabilitation systemsystem
that wethat weimplemented
have have imple-
mented consists mainly of a PC (personal computer), a microcontroller,
consists mainly of a PC (personal computer), a microcontroller, an activated exoskeleton, an activated ex-
oskeleton, and a device for recording myoelectric signals (EMG). The
and a device for recording myoelectric signals (EMG). The PC has three functions: it PC has three func-
tions: it interacts
interacts with the with the operator
operator (who may (who
be amay be a therapist
therapist or even theor even thehimself)
patient patient himself)
using a
using a graphical
graphical interface,interface,
registersregisters and processes
and processes the patient’s
the patient’s myoelectric
myoelectric signals,signals, and,
and, finally,
finally, communicates
communicates with thewith the microcontroller
microcontroller viaconnection
via a serial a serial connection
to transmitto commands
transmit com-or
mands sensory
receive or receive sensory information.
information. The microcontroller
The microcontroller generates
generates the the control
necessary necessary con-
signals
trolthe
for signals forand
drives the monitors
drives and monitors
their actual their actual
position position
using using
sensors on sensors
them. The on them. The
actuators
actuators
affect affect the exoskeleton
the exoskeleton of the patient.of the patient.

Figure5.
Figure 5. Technological
Technological control
controlscheme
schemeof
ofthe
theankle
ankleexoskeleton
exoskeletonfor
forrehabilitation.
rehabilitation.

The
The exoskeleton cancanbebeused
usedtoto rehabilitate
rehabilitate after
after an an
ankleankle injury,
injury, to help
to help people
people with
with mobility
mobility impairments,
impairments, or toor to improve
improve athletes’
athletes’ performance.
performance. The exoskeleton
The exoskeleton cancus-
can be be
customized
tomized forfor each
each individual
individual user
user to ensure
to ensure convenient
convenient fit and
fit and optimal
optimal performance.
performance.
As shown in Figure 6, the description is a prototype of an ankle exoskeleton with four
electric linear actuators, consisting of the following components:
• Lower body: This component provides structural support and contains exoskele-
ton components.
 As shown in Figure 6, the description is a prototype of an ankle exoskeleton with
four electric linear actuators, consisting of the following components:
• Lower body: This component provides structural support and contains exoskeleton
Eng. Proc. 2023, 41, 17 components. 7 of 11
• Rear drive mounting: The rear drive mount serves as the attachment point for the
rear of the exoskeleton, providing stability and alignment.
• • Rear
Front-wheel drive mounts:
drive mounting: The rearFront-wheel
drive mountdrive servesmounts fix and position
as the attachment point the front
for the of
rear
ofthe
theexoskeleton,
exoskeleton,contributing to stability
providing stability and and alignment.
alignment.
• • Front-wheel driveActuator
Electric Linear mounts: (S1):
Front-wheel
Locateddriveat themounts fix and
front, this position
electric theactuator
linear front of cre-
the
exoskeleton, contributing to stability and alignment.
ates linear motion to aid or resist certain ankle movements.
• • Electric
ElectricLinear
linearActuator
actuator (S1):
(S2): Located
An electricat the front,
linear this electric
actuator locatedlinear actuator
at the creates
rear provides
linear motion to aid or resist certain ankle
actuation of the movements of the ankle joint. movements.
• • Electric
Electriclinear
Linear actuator
Actuator(S2): AnLocated
(S3): electricon linear actuator
the right side,located at the linear
this electric rear provides
actuator
actuation of the movements of the ankle joint.
is associated with a certain movement of the ankle and promotes assistance or re-
• Electric Linear Actuator (S3): Located on the right side, this electric linear actuator is
sistance.
associated with a certain movement of the ankle and promotes assistance or resistance.
• • Electric
ElectricLinear
Linearactuator
actuator(S4):
(S4):Located
Locatedon onthetheleft
leftside,
side,this
thiselectric
electriclinear
linearactuator
actuator isis
responsible for actuating the movements of
responsible for actuating the movements of the ankle joint. the ankle joint.
• • Installation
Installationofofthethe right
right drive:
drive: This
This component
component refers
refers to the
to the attachment
attachment mechanism
mechanism for
for attaching the components of the right drive
attaching the components of the right drive of the exoskeleton. of the exoskeleton.
• • Left
Leftdrive
driveattachment:
attachment:Similarly,
Similarly,thisthiscomponent
componentisisan anattachment
attachment mechanism
mechanism for for
attachingthe
attaching thecomponents
componentsofofthe theleft
leftdrive
driveofofthe theexoskeleton.
exoskeleton.
• • Foot
Foothousing:
housing:ThisThiscomponent
componentprovides
providesthe thesupport
supportand andstructure
structureofofthe
theuser’s
userʹsfoot
foot
inside
insidethe
theexoskeleton.
exoskeleton.

Figure6.6.Experiment
Figure Experiment test1—battery,
test bed: bed: 1—battery, 2—shank
2—shank platform, platform,4—ball
3—actuators, 3—actuators, 4—ball joint,
joint, 5—microcontroller,
5—microcontroller, 6, 7—drivers,
6, 7—drivers, 8—PC, 9—foot platform. 8—PC, 9—foot platform.

Theexoskeleton
The exoskeletonprovides
providesactuation
actuationand
andcontrol
controlofofthe
themovements
movementsof ofthe
theankle
anklejoint
joint
using
using four
four electric
electric linear
linear actuators.
actuators. The front drives
drives (S1
(S1 and
and S2)
S2) provide
provide forward
forward and
and
backward movement, and the side drives (S3 and S4) facilitate lateral movements.
backward movement, and the side drives (S3 and S4) facilitate lateral movements. This This con-
figuration provides coordinated and targeted assistance or resistance to certain movements
of the ankle joint.

6. Experimental Testing of a Prototype

After the design and development of the four linear electric drives, their performance
was tested and evaluated to ensure that they met the required specifications. The testing
and evaluation process typically involves several key steps:
 ware.
• Load testing: After completion of functional testing, the next step is to
testing. This includes applying the required loads and effort to linear
to ensure that they can handle the required workload.
Eng. Proc. 2023, 41, 17 8 of 11
• Speed testing: In addition to load testing, speed testing is also requ
that linear electric drives can reach the required speed. This includes
• under different
Functional testing: The loads
first stepand conditions
is to perform to ensure
functional testing tothat
ensureit that
meets the requ
each of
the linear electric drive’s functions correctly. This includes testing the engine, gearbox,
tions.
and other mechanical components as well as control electronics and software.
•• Durability
Load check:
testing: After To ensure
completion the durability
of functional of linear
testing, the next electric
step is to performdrives
load and
withstand
testing. long-term
This includes applying use, it is also
the required loads necessary to test
and effort to linear their
electric durability.
drives to
ensure that they can handle the required workload.
multiple uses of actuators to simulate long-term use and identify any
• Speed testing: In addition to load testing, speed testing is also required to ensure that
rability
linear electricproblems.
drives can reach the required speed. This includes speed testing under
• Safetyloads
different check: Finally, toa ensure
and conditions safetythatcheck is the
it meets also necessary
required to ensure the s
specifications.
• Durability check: To ensure the durability of linear electric drives and their ability
electric actuators. This includes checking for potential hazards suc
to withstand long-term use, it is also necessary to test their durability. This includes
shockuses
multiple or mechanical failure long-term
of actuators to simulate and ensuring
use andthat theany
identify actuators meet all ne
potential dura-
standards
bility problems. and regulations.
• Safety check: Finally, a safety check is also necessary to ensure the safety of linear
Overall,
electric the testing
actuators. and checking
This includes evaluation process
for potential is crucial
hazards such asto ensure that
electrical
drives
shockmeet the required
or mechanical specifications
failure and ensuring that theand are meet
actuators safe all
and reliable
necessary in use.
safety
standards and regulations.
identified during testing should be addressed before the drives are put in
andOverall,
used in the testing and evaluation process is crucial to ensure that linear electric
real applications.
drives meet the required specifications and are safe and reliable in use. Any problems
The
identified possibility
during of be
testing should dorsiflexion and
addressed before the plantarflexion positionsandof the a
drives are put into production
pends
used onapplications.
in real several factors, including the anatomy of the joint, the integr
The possibility of dorsiflexion and plantarflexion positions of the ankle joint depends
rounding ligaments, and the strength and flexibility of the muscles involve
on several factors, including the anatomy of the joint, the integrity of the surrounding
Figure
ligaments, 7 shows
and the strength the movements
and flexibility of the of the platform
muscles involved. that the exoskeleton u
itate the 7ankle
Figure shows joint during of
the movements testing of the
the platform thatdevice, whichused
the exoskeleton vary depending on s
to rehabilitate
the ankle joint during testing of the device, which vary depending on several factors, includ-
including the specific goals of rehabilitation, the condition and needs of t
ing the specific goals of rehabilitation, the condition and needs of the person undergoing
dergoing rehabilitation,
rehabilitation, as and
as well as the design wellcapabilities
as the design and capabilities
of the exoskeleton itself. of the exoske

Figure 7. The results of testing the device in the position of the back and plantar flexion of the ankle
joint speed.

Judging by the graph, movements in all directions have peaks of maximum values of
350 deg/s2 in 12 s.
During device testing, the speed of the exoskeleton can be adjusted to achieve specific
rehabilitation goals, such as improving range of motion, strength, balance, or gait. It is
 the rehabilitation program improves.
It is important to note that the appropriate speed for the rehabilitation of the ankle
joint with an exoskeleton during the testing of the device should always be determined
by a qualified medical professional or therapist who will consider the specific needs and
Eng. Proc. 2023, 41, 17 condition of the person and follow the established protocols and recommendations 9 of 11 for
rehabilitation to ensure safety and effective rehabilitation results.
Figure 8 shows the acceleration of the exoskeleton used for ankle rehabilitation.
During testing,
important thespeed
that the deviceof may vary depending
the exoskeleton is set aton several
a level thatfactors,
is safe andincluding
suitable the
for aspecific
person
goals of undergoing rehabilitation
rehabilitation, to avoid
the condition andany potential
needs riskperson
of the or discomfort.
undergoing rehabilitation,
as wellTheasexoskeleton
the design has
andadjustable settings
capabilities thatexoskeleton
of the allow the user to adjust the speed individ-
itself.
ually and gradually over time as the human condition
Acceleration can affect the comfort, safety, and effectiveness improves. The speed ofmay
thestart at a
rehabilitation
slower pace and gradually increase as the person’s ability to tolerate and benefit from the
process.
rehabilitation program improves.
During testing of the device, the acceleration of the exoskeleton can be adjusted
It is important to note that the appropriate speed for the rehabilitation of the ankle
depending
joint with an onexoskeleton
the person’s condition,
during progress,
the testing of the and
devicecomfort
shouldlevel,
always asbewell as any specific
determined
protocols or guidelines for rehabilitation.
by a qualified medical professional or therapist who will consider the specific needs and
Exoskeletons
condition usedand
of the person for follow
ankle the
joint rehabilitation
established protocols canandhave adjustable acceleration
recommendations for
rehabilitationwhich
parameters, to ensure safety
allows and effective
individual andrehabilitation
progressiveresults.
adjustments to be made over time
Figure 8 shows
as a person’s the acceleration
condition improves.ofAcceleration
the exoskeletoncan used for ankle
start rehabilitation.
at a lower During
level and gradually
testing, the device may vary depending on several factors, including
increase as a person’s ability to tolerate and benefit from a rehabilitation program the specific goals of im-
rehabilitation, the condition and needs of the person undergoing rehabilitation, as well as
proves.
the design and capabilities of the exoskeleton itself.

Figure 8. The results of testing the device in the position of the back and plantar flexion of the ankle
Figure 8. The results of testing the device in the position of the back and plantar flexion of the ankle
joint acceleration.
joint acceleration.
Acceleration can affect the comfort, safety, and effectiveness of the rehabilitation process.
During testing of the device, the acceleration of the exoskeleton can be adjusted
depending on the person’s condition, progress, and comfort level, as well as any specific
protocols or guidelines for rehabilitation.
Exoskeletons used for ankle joint rehabilitation can have adjustable acceleration pa-
rameters, which allows individual and progressive adjustments to be made over time as a
person’s condition improves. Acceleration can start at a lower level and gradually increase
as a person’s ability to tolerate and benefit from a rehabilitation program improves.
The translational motion relative to the angle is shown in Figure 9, which shows 14 s
of 60 Newton-seconds. The translational moment, also known as the moment arm or lever
arm, of an exoskeleton used for ankle joint rehabilitation during device testing refers to the
distance between the axis of rotation of the ankle joint and the line of force. It determines
the torque or rotational force applied to the ankle joint, which can affect the biomechanics
and effectiveness of the rehabilitation process.
The translational moment can be set in such a way as to provide an appropriate level
of load and stimulation of the ankle joint without causing discomfort or risk of injury.
 The translational motion relative to the angle is shown in Figure 9, which shows 14 s
of 60 Newton-seconds. The translational moment, also known as the moment arm or
lever arm, of an exoskeleton used for ankle joint rehabilitation during device testing re-
fers to the distance between the axis of rotation of the ankle joint and the line of force. It
Eng. Proc. 2023, 41, 17 determines the torque or rotational force applied to the ankle joint, which can affect the
10 of 11

biomechanics and effectiveness of the rehabilitation process.

Figure 9. The
Figure 9. Theresults
resultsofof testing
testing thethe device
device in position
in the the position
of theof theand
back back and plantar
plantar flexion
flexion of of the ankle
the ankle
joint—translation moment.
joint—translation moment.

7. Conclusions
The translational moment can be set in such a way as to provide an appropriate level
In conclusion,
of load the design
and stimulation of theand development
ankle of four
joint without lineardiscomfort
causing electric drives involves
or risk of injury.
several key steps, including specification of requirements, component selection, mechanical
system design, electrical system integration, prototype development, testing, and produc-
7. Conclusions
tion. The testing and evaluation process is critical to ensure that the linear electric drives
meet Intheconclusion, the design
required specifications andand
are development
safe and reliableoftofour lineardrives
use. These electric
have drives
a wideinvolves
several
range of key steps,
potential includingacross
applications specification of requirements,
various industries, component
including industrial selection, me-
automation,
robotics, aerospace
chanical and defense,
system design, medical
electrical devices,
system automotive, prototype
integration, and entertainment. As tech- testing,
development,
nology
and continues toThe
production. advance, theand
testing use of linear electric
evaluation drives is
process is likely
criticalto increase,
to ensure providing
that the linear
efficient, precise, and reliable linear motion for various applications. It is important to
electric drives meet the required specifications and are safe and reliable to use. These
note that for the rehabilitation of the ankle joint using an exoskeleton during testing, the
drives have a wide range of potential applications across various industries, including
devices should always be determined by a qualified medical professional or therapist who
industrial
will considerautomation,
the specificrobotics,
needs andaerospace
conditionand defense,
of the personmedical
and follow devices, automotive, and
the established
entertainment.
protocols and recommendations for rehabilitation to ensure safe and effective rehabilitationdrives is
As technology continues to advance, the use of linear electric
likely
results.toDuring
increase,
the providing
rehabilitationefficient,
process,precise,
it may beand reliabletolinear
necessary motion
regularly for various
monitor and ap-
plications. It is important to note that for the rehabilitation of the ankle joint using an
adjust the parameters of the translational moment to optimize the benefits and safety of
ankle rehabilitation
exoskeleton during using an exoskeleton.
testing, the devices should always be determined by a qualified
medical professional or therapist who will consider the specific needs and condition of
Author Contributions: Conceptualization, N.Z., T.I. and G.B.; methodology, N.Z.; software, N.Z. and
the person and follow the established protocols and recommendations for rehabilitation
G.B.; data analysis, N.Z., G.B. and B.B.; data interpretation, N.Z. and G.B.; writing—preparation, N.Z.
to
andensure safe and effective
T.I.; writing—editing, N.Z. and rehabilitation
T.I.; visualization,results. Duringhave
N.Z. All authors theread
rehabilitation process, it
and agreed to the
may be necessary
published version of thetomanuscript.
regularly monitor and adjust the parameters of the translational
moment to optimize
Funding: This thebeen
research has benefits
funded and safety
by the of ankle
Ministry rehabilitation
of Science and Higherusing an exoskeleton.
Education of the
Republic of Kazakhstan, Grant No. AP14972221.
Author Contributions: Conceptualization, N.Z., T.I., and G.B.; methodology, N.Z.; software, N.Z.
Institutional Review Board Statement: Not applicable.
and G.B.; data analysis, N.Z., G.B., and B.B.; data interpretation, N.Z. and G.B.; writ-
ing—preparation, N.Z. and Not
Informed Consent Statement: T.I.;applicable.
writing—editing, N.Z. and T.I.; visualization, N.Z. All authors
have read and agreed to the published version of the manuscript.
Data Availability Statement: Data are obtained in the article.
Conflicts of Interest: The authors declare no conflict of interest.
Eng. Proc. 2023, 41, 17 11 of 11

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