Mechanical Design of Knee and Ankle Exoskeleton to Help Patients

with Lower Limb Disabilities

Ignatius Deo Putranto

, Eka Budiarto

, Kidarsa, Lydia Anggraeni

Swiss German University, Alam Sutera, Indonesia

Bioteknik, Bandung, Indonesia

ideoputranto4@gmail.com, eka.budiarto@gmail.com, lydia@kidarsa.com

Keywords: Exoskeleton, Motorized, Knee, Ankle, Mechanical.

Abstract: In the medical world, exoskeleton in this project refers to an orthosis, which is applied externally to the

user’s body. This project aims to develop an exoskeleton with an affordable cost especially for assistance

and rehabilitation. This project mainly cover around the mechanical design of the product, covering of two

primary joints of the lower body part: knee and ankle. The overall size of the exoskeleton referring to limb

segments is limited to a select range of subject proportion. Each joint is to be motorized and equipped by

certain mechanism such as the application of linear actuator and four-bar linkage mechanism for the knee

and ankle joint, that enhances the efficiency with respect to speed and power transmission during actuations,

as well as the ability to deliver a smooth human locomotion. The types of body movement discussed is

around the sagittal plane which are flexion and extension for knee joint, including the dorsiflexion and

plantar flexion of the ankle joint. In order to help the patient regain the ability to move as mentioned, the

exoskeleton is made especially for external use with existing limbs for the lower body part to move, as well

as to endure the subject’s weight.

1 INTRODUCTION

The development of exoskeleton is one of the most

progressive topics in this decade, with each step that

aims to produce the most accurate lifelike motion

that makes the user feel as if it is part of their body

(Chen et al, 2015). Essentially, a human walk or

moves within the sagittal and frontal plane. The

application of exoskeleton is primarily utilized on

limbs which handles the person’s mobility and

stability. The means of an external actuations that

produce a movement on specific limbs, such as the

leg and arm, enhances the power of the

corresponding joints. Some of the applications are

widely used in the medicinal branch of orthotics that

aids people in moving their limbs as a form of

rehabilitation during their recovery. To be specific,

this project will prioritize on the support for the knee

and ankle joint.

In this current project, the main actuator to

simulate the knee and ankle joint will be using a

motor-powered linear actuator mechanism. To

achieve a more flexible and convenient design the

joint mechanism should mainly be considered, with

the fact that an actual knee joint has a slight

displacement that affects the shank (Wang et al,

2011). The hypothesis of this project is that the

mechanical design which includes linear actuator

and four-bar linkage could produce an efficient in

terms of strength and speed motion for the

exoskeleton to mimic human gait in terms of joint

angular motion.

Putranto, I., Budiarto, E. and Kidarsa, L.

Mechanical Design of Knee and Ankle Exoskeleton to Help Patients with Lower Limb Disabilities.

DOI: 10.5220/0009061600160027

In Proceedings of the 11th National Congress and the 18th Annual Scientiﬁc Meeting of Indonesian Physical Medicine and Rehabilitation Association (KONAS XI and PIT XVIII PERDOSRI

2019), pages 16-27

ISBN: 978-989-758-409-1

Table 1: Subject’s Inclusion and Exclusion Criteria.

Inclusion

Exclusion



This exoskeleton was aimed towards more flaccid,

hypotonic lower extremity function.

 The subject is unable to stand and walk on his own

due to weakness or partial paralysis of leg due to

Spinal Cord Injury, Spinal Muscular Atrophy, Spina

Bifida, or Polio.



Good trunk control.

 Preferably, the subject should be able to stand on

their knees and take a couple of steps on their knees.



Pain in ankle and/or knee joint when articulated.



Fragile skin integrity and skin vulnerability.



Poor trunk control.

 Moderate to excessive spasticity.



Complete paralysis of lower extremity.



Fragile bone such as in the case of osteogenesis

imperfecta.

2 METHODS

The design overview of the knee and ankle is

composed of a joint that connects thigh and shank

body part. The four-bar linkage mechanism is

applied for the knee joint as it is efficient and

simulates the joint motion and muscle contractions

in the knee (Widjanarko, 2018).

The scope for this project is to implement a

motorized actuator in that case such as a DC Motor

with a rotary to linear output conversion by the

means of lead screw. The chosen actuator is

expected to be able to handle the maximum load

subjected to it through calculation and testing. For

the knee joint specifically, the actuation will be

conducted using four-bar linkage mechanism.

The production process starts with the subject

measurement with provided inclusion and exclusion

criteria. However, the subject measures are only

gathered to specify the user requirement, and is not

intended for direct testing. Followed by gait cycle

analysis to measure the maximum and minimum

angular range of motion on each joint, also to

compute the needed torque and speed respectively.

2.1 Subject Measurement

Before proceeding to the next process, subject

measurement is crucial to determine the limitation

scope that includes load, body segment lengths and

the range of adjustments that the final product must

satisfy. The subject is not limited by a certain range

of age; however, the subject must satisfy the criteria

shown in Table 1. The reason for these exclusions is

to prevent unexpected injuries or complication on

the subject, if the exoskeleton would be tested on a

real subject.

The data will be compared between real life

measurement and the calculation using the

referenced body segment ratio (Winter, 2009). The

reference will be useful to estimate the segment

center of mass, weight ratio and to compare the

length differences of each segment.

Table 2: Body Segment Length.

Referring to Table 2, the subject was measured on

April 8, 2019, with the height of 118 cm and

weighing 27 kg. The ailment should also be noted,

which is Spinal Muscular Atrophy, affecting the

lower limbs. The body segment weights are shown

in Table 3.

Table 3: Body Segment Weight.

Body Segment

Weight (N)

Thigh

26.487

Shank

12.316

Foot

3.841

Upper Body (HAT)

179.582

2.2 Gait Cycle

Figure 1: Human Walking Gait (Bonnefoy-Mazure, 2015).

Mechanical Design of Knee and Ankle Exoskeleton to Help Patients with Lower Limb Disabilities

In general, humans walk and run in a common

pattern. A single movement sequence as seen in

Figure 1 above is called as the human gait cycle

which is divided into two phases; the swing phase

and the stance phase. These phases are indicated

when both feet are on the ground (stance phase) and

when one foot is lifted from the ground (swing

phase). Considering the swing phase, in which one

foot is floating, means that the other limb will act as

a support before the person reach their next footing

during the swing phase. The cycle continues by the

switching between the left and right limb

alternatively, performing the walking or running

pattern (Kharb et al, 2011).

Figure 2: Joint Angle Progression Graph (Whittle, 2007).

The joint angle progression will refer to Figure 2.

The graph will be reverse engineered and estimated

with 0.5º tolerance. According to this data, the

minimal and maximum angle for each knee and

ankle joint respectively are 0º until 47.5º and -22º

until 8º. The polarity of the ankle angle refers to the

dorsiflex (+) and plantar flexion (-).

2.3 Actuation Speed Calculation

The actuation speed also plays a role on the overall

machine performance. Each joint speed could be

estimated using the data from Figure 2 in a single

gait cycle.

Figure 3: Walking Steps Parameter (Kharb et al, 2011)

The parameter as shown in Figure 3 such as the

stride length will be used to calculate the cycle time

(s) and walking speed (m/s) of a person. And the

calculation for these two variables are as follows:

   



















     

















  

  





Each person has a distinct stride length on each

gait cycle, even with the same walking speed.

Therefore, the controlled variable in this calculation

are the walking speed = 1.4 m/s (AVG human

walking speed) and stride length = 0.8 m. From

these values, we could acquire the cycle time from

the equation (3), cycle time = 0.571.

Regarding the chosen subject is a child,

approximately nine years old, the controlled

parameters will be halved, however according to

equation (1), the cycle time remains the same. The

angular speed parameters are determined from the

graph in Figure 2, which states the gait cycle

progression (%) and joint angle (º). The range is

determined from the steepest slope, which means the

fastest speed possible.

2.3.1 Required Time and Angular Speed

Equation





 



 



   





KONAS XI and PIT XVIII PERDOSRI 2019 - The 11th National Congress and The 18th Annual Scientiﬁc Meeting of Indonesian Physical

Medicine and Rehabilitation Association











 







 

From equation (4) and (5), the minimum required

velocity of each joint is provided in the Table 4.

Table 4: Joint Velocity.

2.4 Material Selection

Stainless steel pipes will be the primary material

used for the frame, and PETG for motor mountings.

Stainless steel was chosen due to its corrosion

resistance and high tensile strength. While the

remaining concern is the weight, the material will

take form of a pipe to reduce volume. As for the

motor mountings, PETG was chosen due to its

manufacturing capability through 3D printing that

could produce a more complex and flexible design.

The stainless-steel type that will be used as the

exoskeleton frame is 304, as it is one of the most

common types available in general stores. Even

though stainless steel is widely known for its high

corrosion resistance, the design and manufacturing

process should always be monitored (Vaghani,

2014).

For 3D printed material, the PETG was chosen

as it has high ultraviolet resistance in comparison to

ABS and PLA. This property would allow the

exoskeleton for outdoor use, with minimum

probability of deformation.

2.5 Four-Bar Linkage

Four-bar linkage mechanism is efficient, provides a

smooth motion and is known to be similar with the

actual knee muscle mechanism (Figure 4).

Figure 4: Human Knee Joint.

Figure 5: Four-Bar Linkage Properties 1 (Widjanarko,

2018).

Represented as a model in Figure 5, the knee

muscles that crosses together are link a and b. In the

actual contraction, however, link a and b changes

length because of the flexing. By implementing this

mechanism, linear displacement and angular

displacement does occur during the locomotion.

Such that it simulates the muscle contraction of the

knee muscles, these displacements can be estimated

and minimalized to obtain an accurate gait

performance and ultimately bestows comfort and

safety for the user, by adjusting the lengths of link a,

b, g and h. These links remain a constant. The link g

will be used as a reference point of the system,

representing the thigh distal end. As well as link h,

representing the shank proximal end.

Mechanical Design of Knee and Ankle Exoskeleton to Help Patients with Lower Limb Disabilities

Figure 6: Four-Bar Linkage Properties 2 (Widjanarko,

2018).

The mechanical advantage delivered by this

mechanism may also be noted to produce the highest

efficiency, which primarily relates to the system

angular acceleration. As can be seen in Figure 6, the

output angle (φ) can be determined by two other

variables which are the input (θ) and follower (ψ)

angle using equation (1).

 



 

      

          





The shank angle with respect to the thigh will refer

to the output angle produced by this mechanism.

2.6 Mechanism Evaluation

A linear actuator is a term in which the output of a

motor is designed such that it is converted into linear

motion. Instead of producing a torque, a linear

actuator produces force to directly push an object. A

linear actuator working principle simply lies to

which type of mechanism it is applied (Budynas,

2008), the more common and less complex

mechanism is by using a leadscrew. The type of

motor used for a linear actuator is not critical as it is

only an output converting mechanism. The draw

back of a linear actuator would be the material used.

Table 5: Leadscrew Types.

There are four types of leadscrew as shown in Table

5. The most common lead screw type available in

the market is ACME thread, which has a distinct

trapezoid thread. The torque calculation needed to

rotate a lead screw mainly refers to these types. For

an ACME thread, the torque required to rotate this

lead screw is slightly larger than the other

counterparts, hence the larger contact area of an

ACME thread. The required torque is also based on

the orientation of the linear actuator.

2.6.1 Linear Actuator Torque

The amount of torque required to drive a leadscrew

to push a certain force is shown in equation (7).







  





  



 





   

 



 

F = Amount of force to be actuated





= The outer diameter of a leadscrew



= lead, equals to the number of start times pitch

length



= friction coefficient between leadscrew and

material



= ACME thread trapezoid angle

Equation (7) computes the amount of torque with a

direction of force against gravity, in other words the

required torque to raise the load. In the other hand,

the torque to lower the load is shown in equation (8).

A self-locking term could also occur when the

required torque is minus.







  





   



 





   

 







KONAS XI and PIT XVIII PERDOSRI 2019 - The 11th National Congress and The 18th Annual Scientiﬁc Meeting of Indonesian Physical

Medicine and Rehabilitation Association

Commonly, an extra component called as thrust

collars are used in a leadscrew mechanism which are

attach to the load end of the leadscrew to lift it.

Since this exoskeleton’s linear actuator design does

not apply this function,





is zero. The constant

values are listed below,





= 8 mm

 = 0.25



= 8 mm



= 29º

These constants are related to the dimension, type

and material of the leadscrew.

2.7 Frame Model

Figure 7: Exoskeleton Solidworks Model.

The frame model is shown in Figure 7. As

mentioned before, this exoskeleton is classified as an

orthosis which is externally equipped to the subject.

The frame would take an approximately 10 cm space

of the outer sides of the subject’s leg. To attach the

exoskeleton, the frame is equipped with a harness

fabricated through 3D Printing. For the foot model,

the frame was designed suitably for the subject

wearing a shoe. The four-bar linkage links was

designed to be assembled in two different planes to

avoid collision between the parts.

Figure 8: Linear Actuator Model.

The linear actuator design is shown in Figure 8,

equipped with a belt and pulley mechanism to

increase the output torque ratio. The rotary motion is

converted to linear due to the leadscrew rotation

causing the nut to drive back and forth. Both linear

actuator for knee and ankle joint are positioned for

each knee and ankle joint with a pivoting point and

custom displacement range. The pivoting point

allows an angular motion of the actuator during

extension and retraction. The displacement range for

each knee and ankle joint respectively are 8.9 cm

and 0.75 cm. The four-bar linkage mechanism is

only applied for the knee joint instead of both, the

reason being that the knee joint requires a wider

range of angular displacement whilst considering the

required angular velocity shown in Table 4.

Table 6. Four-Bar Linkage Properties.

The link sizes of the four-bar linkage are shown in

Table 6. The minimum angle for both input and

follower angle can be obtained from Solidworks. To

compute both angles with respect to the actuator

displacement is shown in equation (9) and (10).

    

    





  



Mechanical Design of Knee and Ankle Exoskeleton to Help Patients with Lower Limb Disabilities

  

  

   

   

  

   

 

The numbers are in accordance to the segment

sizes of the frame and solved using trigonometry. As

can be seen in equation (9). the follower link is

dependent to the input link. Using the equation of

output angle (10)., the output angle can be computed

using the variables in Table 6.

Represented as Φ, the output angle at maximum

is -81.53º deducted from the calculation as it

changes into a different quadrant, while the real

maximum value of the output angle is to be added

by 180º. Therefore, the angular movement range of

the knee joint is 93.56º by design.

As for the ankle joint, by using the same

trigonometry method, the relation between the ankle

angle and the ankle linear actuator displacement is

shown in equation (11).

   



    





 



The maximum dorsiflex angle is 8.18º and for the

plantar flexion is 21.71º (-).

2.8 Torque Calculation

Linear actuator converts rotary motion to linear

motion; therefore, the required torque of the motor is

based on the amount of force subjected to each

linear actuator.

2.8.1 Knee Joint

Figure 9: Knee Joint Frame Free Body Diagram.

The knee joint free body diagram is shown in Figure

9. The force that is going to be calculated is the

static force during a sitting position which is

presumed to have the highest amount of force. The

variables required to find F are listed below,





= Weight of LM inner shell





= Weight of Thigh Exoskeleton (Includes one

harness, MPU mount)





= Weight of Upper Body





= Weight of Thigh

The weight of the exoskeleton was estimated using

Solidworks, while the weight of body segments

refers to Table 3.

 



 

;











 



 



 



 

 





 

 

Through static force equilibrium (12) and

equation (13) the maximum amount of force acting

on the knee linear actuator is 351.454 N. The angle



is 19.95º obtained from Solidworks.

As the acting force has been computed, the

torque to raise and lower the load respectively are,

0.934 Nm and -0.042 Nm.

KONAS XI and PIT XVIII PERDOSRI 2019 - The 11th National Congress and The 18th Annual Scientiﬁc Meeting of Indonesian Physical

Medicine and Rehabilitation Association

2.8.2 Ankle Joint

Figure 10: Ankle Joint Frame Free Body Diagram.

The ankle joint free body diagram is shown in

Figure 10. The force is calculated during terminal

stance when the angle θ is at 8.18º. The variables

required to find F are listed below,

W = Weight of the upper body, one thigh, and one leg

in addition to the weight of the Knee LA, Thigh

Exoskeleton





= Weight of Shank in addition to shank

exoskeleton frame





= Amount of force the Ankle LM must endure





= Amount of force affecting the ankle joint





= W arm length









arm length





= 



arm length

 



= 0;

 























 









 

 



 

 





 



 



 

















 

 

Using the moment and force equilibrium method

(14) and (15), the force subjected to the ankle linear

actuator can be calculated with equation (16) that is

358.505 N with α (the angle between linear actuator

and shank) at 55.86º.

Therefore, the required torque to raise and lower

the load for ankle linear actuator respectively are

0.953 Nm and -0.043 Nm.

2.9 Inertia Calculation

During swing phase the actuators needs to withstand

the inertia of the bottom half of the leg, which is

mostly subjected to the knee joint actuator. To

calculate the inertia, the equation is based on a

simple pendulum inertia,

  









 

The





refers to the mass of the both shank and foot







, and





is the centre of gravity of the shank

and foot (





). The center of gravity can be

calculated using equation (18).











   



   



  



 

Substitute equation (17) in terms of total body mass

and height, the inertia equation (19) can be obtained.

  









  



 

2.10 Motor Type Selection

The type of motor chosen for the linear actuator is a

DC Motor. According to torque calculation in

subchapter 2.8, the torque required is not as

demanding as opposed to the speed requirement.

Considering the weight of the motor would affect

significantly to the exoskeleton load, a compact and

lightweight motor is preferable and should be taken

into account.

However, a motor with these specification,

especially with a compact size, is difficult to find in

the market and mostly come with a large dimension.

Therefore, one aspect should be sacrificed between

speed and torque and for this project, the torque is to

be prioritized.

Mechanical Design of Knee and Ankle Exoskeleton to Help Patients with Lower Limb Disabilities

Figure 11: Exoskeleton Final Product.

3 RESULT & DISCUSSION

The final product of the exoskeleton is shown in

Figure 11. It weighs 2.055 kg for each leg. The

material used for the frame is made of stainless steel.

However, the type may vary between 304 and 201

because of manufacturing miscommunication. The

pipe shaped stainless steel is also useful for cable

management. The foot harness is made from PETG

through 3D Printing, the same as the motor

mounting. The harnesses are equipped with Velcro

to fasten. The shank segment was made to be

adjustable, in the range of 5 cm.

Figure 12: Foot Frame.

As seen in Figure 12, the footing is also equipped

with Velcro fastener. To the right, the linear actuator

of the ankle joint was manufactured as it was

designed, connecting the foot segment to the shank

segment.

Figure 13: Knee Joint Actuator.

The knee linear actuator connects the thigh

segment with the triangle part which drives the four-

bar linkage mechanism to rotate the knee joint.

Apart from the whole exoskeleton, the knee linear

actuator weighs 0.520 kg each, while the ankle

linear actuator weighs 0.395 kg each.

3.1 Range of Motion

The range of motion was tested by powering each

linear actuator to reach the minimum and maximum

linear actuator displacement after product assembly.

The angle is the measured using a protractor.

Figure 14: Knee Actuator Movement Range.

Figure 14 shows the maximum extension and

flexion angle denoted as





and





. Respectively





and





are 98.5º and 5º, therefore the range of

angular motion of the knee joint is 94.5º.

KONAS XI and PIT XVIII PERDOSRI 2019 - The 11th National Congress and The 18th Annual Scientiﬁc Meeting of Indonesian Physical

Medicine and Rehabilitation Association

Figure 15: Ankle Actuator Movement Range.

Figure 15 shows the maximum dorsiflexion and

plantar flexion angle of the ankle joint, denoted as





and





. Respectively





and





are 15º

and 22º, therefore the range of angular motion of the

knee joint is 37.

The test result shows an accurate result in

comparison to the calculation, especially for knee

extension, flexion and plantar flexion of the ankle

with a slight error. However, the dorsiflexion angle

shows a larger value than expected.

It has been concluded that these errors were the

result of manufacturing imperfections in addition to

the linear actuator mountings which is made from

PETG that could deform far easily. Not to mention

that the range of motion was tested without any

additional load added to the exoskeleton which may

result in a larger error and even reach a breaking

point. The adjustment part of the shank segment

could also affect the test result due to the pipes

having different dimension that creates a gap that

causes the frame to tilt approximately up to 3º.

3.2 Load Test

Load testing was conducted to obtain the PWM

value when the linear actuator is subjected to a

certain load. The PWM value will then be

conditioned to the amount of load is being used

when conducting the gait analysis. The test resulting

the values below is obtained when the leg is in Mid-

swing stance. The tests are done using a 24V battery

cell.

Table 7: Knee Joint Load Test.

The PWM value reached its maximum value when

2.5 kg load is subjected to the linear actuator. This is

the point in which the end angle is starting to shorten,

as more load applied to the device. This value might

be affected by the slipping of the timing pulley and

the thrusting point shaft not being concentric with

the triangle. After certain cycles. The PETG

mounting shows a slight deformation, making the

inner shell and outer shell tilting.

Table 8: Ankle Joint Load Test.

The data collected when testing the ankle linear

actuator is not as complete as the knee linear

actuator. This is due to the slipping of the timing

pulley when higher load is added. The 0.6 kg is the

weight of the foot frame.

3.3 Actuation Speed Test

The linear actuator speed test was done by

incrementing PWM gradually and measure the speed

in which the linear actuator reach maximum its

maximum range. No additional load was added to

the system to confirm the calculation as well. The

test was done using a 24V power supply and a motor

driver to control the PWM.

Mechanical Design of Knee and Ankle Exoskeleton to Help Patients with Lower Limb Disabilities

Figure 12: Actuator Speed Progression

Figure 12 shows the progression of actuation

speed when given a certain amount of PWM.

According to the graph, the speed starts to slowly

increase after reaching 140 PWM, meaning the

motor is in the range of its rated output power.

Notice that there are two curves, this is to show the

difference when the linear actuator undergoes a

raising torque and lowering torque. The maximum

speed that the motor could produce is 1.738 cm/s.

Comparing to the required angular speed of each

joint in Table 4, the angular speed produced by the

linear actuator can be computed by inverting

equation (11). The ankle equation was chosen since

the required speed of the ankle joint is bigger than

the knee, in addition to the same motor used for all

linear actuators. The angular speed produced by the

linear actuator is 6.51 times slower than the

requirement.

4 CONCLUSIONS

Since the linear actuators are subjected to a large

amount of force, different material such as

aluminium is recommended to replace the motor

mountings. A tensioner would also be recommended

if the motor driving has fewer torque than needed

running in a belt mechanism.

The data shown in Table 6 nonetheless shows a

promising power produced by the linear actuator.

However, the speed is much less than the required

amount, meaning that the subject could not walk as

fast as the human average walking speed. Therefore,

this product is more recommended to be used as a

rehabilitation or slower activities.

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KONAS XI and PIT XVIII PERDOSRI 2019 - The 11th National Congress and The 18th Annual Scientiﬁc Meeting of Indonesian Physical

Medicine and Rehabilitation Association

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Mechanical Design of Knee and Ankle Exoskeleton to Help Patients with Lower Limb Disabilities