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Development of a Series Elastic Actuator for Active

Knee Exoskeleton

Oishee Mazumder,1 Ananda Sankar Kundu

1, ,Karan Gupta

2, Subhasis Bhaumik

1

1School of Mechatronics & RoboticsBengal Engineering & Science University, Shibpur

Kolkata, India2Vertex Robotics, Mumbai, India

[email protected], [email protected], [email protected], [email protected]

Abstract—this paper describes the design and development of

a series elastic actuator for active knee exoskeleton which can beused for rehabilitation and load augmentation purposes. The

device consists of a motorized ball screw mechanism along with aspring placed between the motor and load. The spring strain is

measured to get accurate estimate of force which can be

controlled for amplifying force according to user’s intention. This

device can also be used to mimic human knee motion during

walking by controlling the impedance of the device duringvarious phases of gait accordingly. Modeling, specifications and

preliminary test result of series elastic actuator based exoskeleton

knee has been provided in this paper.

This device has huge application in the field of rehabilitation

robotics.

Keywords—exoskeleton; series elastic actuator; gait cycle; load augmentation;

I.

INTRODUCTION

The basic concept for exoskeleton systems have been

suggested and developed for over a century with applications

ranging from construction, manufacturing and mining to

rescue and emergency services. In recent years, research hasbeen driven by possible uses in medical rehabilitation and

military applications. The importance of research and

development in assistance technologies to compensate

pathological gait have been recognized since the beginning of

the twentieth century and numerous challenges still lie ahead

to make their clinical application a reality. Within the last

decade, research in the area of exoskeletons and activeorthoses has experienced a revival. These efforts are split

between developing technologies to augment the abilities of

able bodied humans, often for military purposes and

developing assistive technologies for handicapped persons.

Despite the differences in intended use, these two fields face

many of the same challenges and constraints, particularly

related to portability and interfacing closely to a humanoperator. In any exoskeleton design a high level of human

machine interaction is required.

The interface between an actuator and its load is commonly

designed to be as rigid as possible, (Pratt & Williamson 1995).

[1, 2] Increasing stiffness improves precision, stability and

position control bandwidth. However, the use of such interface

may incur some problems like friction, torque oscillations and

noise. According to Pratt et.al [2], an impedance control

interface is generally required when there is human-machine

contact. Series elastic actuators present ideal characteristics

for use in human-machine interaction like force control,

impedance control (possibility of low impedance), impactabsorption, low friction and bandwidth.

The idea behind the series elastic actuator is the inclusion of

an elastic component between the motor’s output and the load.

The measurement of the elastic deformation is related to the

applied load force, trough the dynamic characteristic of the

spring. The series compliance increases force fidelity, aproperty especially useful in the impedance controlled

applications in fields such as wearable robotics and human-

machine interfacing [3]-[8].

SEAs have been implemented successfully in lower extremityprosthetic and exoskeleton devices for medical purpose as

well as load augmentation. Au et al. used series and parallelelasticity in the design of a powered ankle prosthesis [13],

which was shown to lower the metabolic cost of walking in

transtibial amputees [11]. Additionally, Veneman et al. [7]

designed a lower extremity exoskeleton using Bowden cable

driven SEAs. BLEEX [9] and Roboknee[16] are two very

popular series elastic actuator based exoskeleton design which

focuses on load augmentation and force amplification.

In this paper, we describe the design specification and

modeling of a series elastic actuator that we have developed

for an active knee exoskeleton. Preliminary test results are

provided for design validation. A force measurement scheme

for control application has been implemented. The concept of

controlling the impedance for normal walking and forceamplification modes are outlined.

II.

SERIES ELASTIC ACTUATOR

Series elastic actuators are devices where elastic components

are introduced between the motor’s output and the load. From

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the deflection of these components, it is possible to measure

the force applied to the load and to control it. Also, the

mechanical impedance of the actuator/load interface can be

regulated to the typical values of joint’s stiffness and damping

presented by humans during the walking.

The SEA consists of a DC motor fixed to a ball screw

followed by an elastic coupling. The platform motion is driven

by the nut, which converts the rotational ball screw movement

into linear movement of the platform. To obtain force andimpedance control of the actuator a set of springs are

introduced between the platform and the end effector. When

the DC motor is driven, the nut moves forward or backward,

compressing the pair of springs. The springs apply force to the

load through the end effector.

The force and impedance control is done by measuring the

spring deflection and by the Hooke’s law, F=k.x the force

applied to the load is calculated.

A. Modelling

Series elastic actuators can be modeled as a second order

spring mass damper system along with driving force on themass and position input from spring.

Fig. 1. Mass spring model for the Series Elastic Actuator.

System equation with equivalent motor mass mm, damper

coefficient bm, and elastic constant k given by

.. .

mm xm +bm xm = F m-F l (1)

F l= k (xm-xl) (2)

Where xm is the linear position of the lead-screw nut, xl is the

load position, F m is the force generated by the motor and

output force F l . The damper coefficient bm is found from the

force and velocity constraint of the DC motor

bm= Fmax/Vmax (3)

where Fmax and Vmax are maximum force and velocity the

DC motor can reach, respectively. Force F l , which drives the

load, is therefore function of F m and xl ,

Fl(s) = Fm(s)-(mms2+bms)xl(s)

mms2 /k+ bms/k+1

The force applied to the load thus can be controlled by a

closed loop system with feedback by measuring the deflection

of the springs.

III. DEVICE DESIGN

A solid model of the actuator module is shown in Fig. 2. A 200W

brushed DC motor is used as the main actuator. The motion is

then transferred to a ball – screw system through a belt pulley

system. The reduction ratio of the belt-pulley is kept as 3:1. Beltpulley system ensures minimum play compared to spur gear type

reduction. Also the belt acts as a safety link between the main

actuator and the DC motor. The linear guide assembly for the ball

screw is supported by two sets of stainless steel rod and linear

bearings. Finally the driven end of the linear actuator is again

connected to another platform through two sets of springs.

Fig. 2. Model of the developed Series Elastic Actuator

TABLE I. SPECIFICATION OF THE SERIES ELASTIC ACTUATOR

Total length of actuator 0.3 m

Travel length of actuator 0.12m

Carriage Speed 0.2 m/sec

Compressive force on actuator 750NWeight 2.5 kg

Gear reduction by timing belt 3:1

Damping spring stiffness 100KN-M

Ball screw pitch 4mm

Motor operating voltage 24 V

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The module will be assembled with a knee brace mechanism. For

normal activities, range of knee motion varies from zero to sixty

degree during walking and at most 110 degrees for squatting or

kicking and torque requirement can be as high as 130Nm. Design

has been made based on knee biomechanics with respect to angle,

velocity and torque requirement. Standard biomechanical curve

are provided in Fig.3. Fig 4a and 4b shows the actuator operation

during different knee movements.

Fig.3. Biomechanics curve for knee motion

A. Control

The exoskeleton will be controlled in two modes. First mode is

for estimating the gait phase and knee angle to reproduce knee

action during walking. For this we need to mimic the knee

behavior in terms of trajectory and torque response. This mode

will be implemented by developing a closed loop position servo

for tracing knee trajectory at swing phase and impedance control

by controlling spring stiffness in the stance phase. Optical

encoder, potentiometer or special infra red sensor like

phototransistor reflective object sensor will be used to implement

position control of the spring carriage.

Fig.4. SEA motion with knee bending

Second mode is force amplification mode for load augmentation

purpose. The device will be programmed to perform force

amplification in such a way that the force required by the

quadriceps muscle is significantly less than what would be

required without the device. For this we first need to estimate the

intent of the user and then amplify the force. User intention can

be tuned with respect to load as per user’s comfort. On

application of load, spring will be compressed and the

compression will be measured by an IR position sensor. An

embedded program will operate and control the actuator once a

specified load threshold is crossed.

Along with user’s comfort load tuning, EMG of the user from

quadriceps muscle will also be extracted and processed to get the

user intent. Fusing EMG data along with load data for particular

user will ease the controllability of the system. Force control will

be implemented using a closed loop PD control in embedded

platform.

Fig.5. Photo of the manufactured prototype

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IV EVALUATION

The prototype developed (Fig.5) was tested for evaluatingits basic specification and performances. Performance of themotor was tested via plotting load vs. power curve of themotor. Table II shows the result of the experiment and Fig.7shows the experimental set up. Fig.8 shows the load vs. powercurve which gives us an idea about the power requirementduring activities.

TABLE II POWER LOAD CHARECTERISTICS OF SEA

SL.NO VOLTAGE CURRENT POWER LOAD

1 2.3 6.57 15.11 9.6

2 2.6 7.6 19.76 13.85

3 2.9 7.68 22.272 15.10

4 3.2 8.44 27.008 18.54

5 3.6 11.01 39.363 22.80

6 4 11.87 47.48 26.12

7 4.6 14.38 66.148 32.24

8 4.7 14.89 69.983 34.30

9 5.2 14.88 77.363 38.36

10 5.4 16.74 90.396 40.5

11 6.0 18.06 108.36 43.12

Fig.6. experimental set up for measuring load vs. power

Fig.7. Load vs. power characteristic curve for the developedSEA

Another experiment was conducted to measure the springstiffness and variation of spring compression with respect toload. A phototransistor reflective object sensor was placedbetween the moving plates of the device. The sensor isinterfaced with a 10 bit ADC. On application of load, thesensor reading and corresponding spring compression wasmeasured simultaneously. Experiment set up is shown in Fig.8.From the readings obtained we can map user’s comfort indexor intention with load and spring movement to control the

device in force amplification mode. Response between appliedload and spring compression is plotted in Fig.9.

TABLE III LOAD VS. SPRING COMPRESSION AND CARRIAGEDISPLACEMENT

SL.N0 LOAD(Kg) VERNIER

READING(mm)

CHANGEINREADING

SENSORREADING

1 NO LOAD 75.95 0 660

2 1 75.70 0.25 606

3 2 75.66 0.29 503

4 3 75.55 0.40 440

5 4 75.45 0.50 363

6 5 75.34 0.61 343

7 6 75.20 0.75 281

8 7 75.09 0.86 266

9 8 74.99 0.96 246

10 9 74.89 1.06 218

11 10 74.65 1.30 201

Fig.8. Experimental set up to measure carriage displacementwith load

V.DISCUSSIONS

In this paper, we have presented the design development and

initial evaluation result related to a series elastic actuatorworking as a knee joint exoskeleton. We have manufactured

the prototype and work is going on to provide the actuator

with some position command related to knee movement and

move the actuator as a position servo to mimic the action of

human knee. Separate controller for position servoing and

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force feedback will be developed. Our next prototype will

include controllers and human intention detection sensors.

Fig 9. Load vs. spring compression.

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