444 J SCI IND RES VOL 69 JUNE 2010Journal of Scientific & Industrial Research

Vol. 69, June 2010, pp. 444-448

*Author for correspondence

E-mail: neel5278@rediffmail.com

Low cost prototype development of electronic knee

Neelesh Kumar1*, Sanjeev Soni1, Amod Kumar1 and B S Sohi2

1Central Scientific Instruments Organisation (CSIO), Chandigarh 160 030, India

2University Institute of Engineering & Technology (UIET), Chandigarh 160 014, India

Received 20 January 2010; revised 16 March 2010; accepted 17 March 2010

This study presents design and development of an electronic knee, which uses attached sensors, microcontrollers and

electro pneumatic valve arrangement to emulate near natural walk. Being all sensors and mechanical assembly developed

indigenously, prototype would be a low cost electronic knee.

Keywords: Embedded system, Microcontroller, Prosthetic

Introduction

With increase in road accidents, machine calamities

etc, 5.5 lakhs people in India have lower limb

amputations every year and need some kind of

prosthesis1. Low cost prosthetic products are not able

to perform coordinated and complex movements of a

knee joint2,3. Sophisticated electronic knee are very

costly (OttoBock costs Rs 18-20 lakhs). With the

advent of integration of technology, numerous sensors,

microcontrollers and accurate feedback controls,

orthotic and prosthetic devices will no longer be

separate, lifeless mechanisms, but intimate extensions

of human body structurally, neurologically and

dynamically4. Advances in biomedical engineering have

led to introduction of computer-controlled, variable-

damping prosthetic knees5,6. With advent in complex

feedback sensor and control system, device designs

are equipped with high degree of adaptability and

reliability7.

This paper describes indigenously designed and

developed a low cost electro-pneumatic knee by

integration of indigenously developed electro-

goniometer and force resistive sensor (FSR) in order

to control swing phase dynamically.

Experimental Section

Electronic knee consists of electro-goniometer and

FSR as sensor mechanisms8,9. Knee design is based

on controlling a swing phase using a pneumatic cylinder

mechanism attached with drive (motor) control mechanism

to its flow control valve. Required energy to extend knee

into new gait cycle is provided by a spring mechanism.

Electro-Goniometer

An electro-goniometer that measures angles between

body segments (Fig. 1) was developed. Material used for

fabrication was aluminum (thick, 2 mm). A single turn

potentiometer of 1 K was used. For testing of developed

electro-goniometer, experiments on individuals were

performed to achieve a suitable calibration factor. It was

observed that there is considerable change in knee flexion

angle range with walking speed.

Sensor Modules

Sensors are employed to measure walking phase and

walking speed of patient wearing prosthetic. To record

knee flexion angle, an electro-goniometer was designed

and developed. In order to determine heel strike and toe

off, which gives information about walking phase (swing

or stance), FSR sensor was fabricated and tested. FSR is

a polymer film device, which exhibits decrease in

resistance with an increase in force applied to active

surface. Its force sensitivity is optimized for use in human

touch control of electronic device. FSR was developed

using carbon impregnated conductive polyether

polyurethane foam (surface resistivity, 104-105 /square;

density, 45-55 kg/m3). It is sandwiched in two pieces of

general purpose PCBs (Fig. 2a). For a simple force to

voltage conversion, FSR device is tied to a measuring

KUMAR et al: LOW COST PROTOTYPE DEVELOPMENT OF ELECTRONIC KNEE 445

resistor in a voltage divider configuration. Developed FSR

accurately gives heel strike and toe contact time and

phase information (Fig. 2b). FSR gave accurate and

repeatable results (Table 1). Developed force-sensitive

resistors were able to sustain large forces exerted while

using as foot sensors.

Electro-pneumatic Control

In present arrangement, a pneumatic cylinder at knee

joint is used to adjust swing phase resistance. Developed

electro-pneumatic cylinder (Fig. 3) contains a needle

valve to adjust resistance in knee joint. This control valve

is driven by a stepper motor. Valve adjusts flow air in

cylinder, which in turn builds required resistance.

Controller Unit

ATMEGA32L microcontroller (8-bit) was used for

processing information and controlling electronic knee.

Sensors for sensing angle and walking phase are

embedded in knee. Microcontroller in real time scans

information from various sensors for knee flexion angle

and walking phase. It combines both inputs and matches

for corresponding walking speed. This knowledgebase

was developed by extensive trial on variety of individuals.

Based on walking speed detected, it adjusted flexion

Fig. 2FSR sensor: a) Developed sensor; b) Placement of FSR sensor

a)

Fig. 1Electronic Goniometer: a) Mechanical layout of goniometer fixing clamp and potentiometer holder

assembly; b) Placement on human body

b)

446 J SCI IND RES VOL 69 JUNE 2010

resistance of cylinder by giving control pattern to stepper

motor, which controls opening and closing of flow control

valve.

Control Algorithm

In complete system, critical part is control algorithm,

which processes information from sensors and then

operates valve. Control of electronic knee joint consists

of different modes of operation in control algorithm as

Table 1FSR results for two different subjects

Subject weight Walking speed Toe contact Heel strike

kg

Time, s Amplitude, V Time, s Amplitude, V

48 Slow 1 3.90 0.95 3.78

Fast 0.7 4.20 0.64 3.90

72 Slow 0.65 4.01 0.85 4.03

Fast 0.62 4.05 0.59 4.01

follows: i) calibration mode; ii) training mode; ii) execution

mode; iv) backup mode (safe mode); and v) fail safe

mode. Software codes for each mode are written and

tested. Artificial knee, powered with ON, operates in

execution mode (Fig. 4), which consists of slow mode,

normal mode and fast mode. In slow mode, knee detects

a particular range of sensor inputs and adjusts valve

position to offer damping required in slow walking.

Power Requirements

Device is powered with ENIX Li-Ion rechargeable

battery (15 V, 2.2 Ah). Control algorithms are designed

Fig. 3Mechanical layout of electropneumatic cylinder Fig. 4Execution mode flow chart

KUMAR et al: LOW COST PROTOTYPE DEVELOPMENT OF ELECTRONIC KNEE 447

in such a way so that control valve will operate only

when patient changes his walking mode from slow to

normal and normal to fast. Total current drawn by

electronic knee while operating control valve is less than

200 mAmp. In all other conditions, when only sensors

give signals to controller, system draws 65 mAmp current

at 5V level.

In order to provide accurate control and save power

saving, mode change is decided only when change

conditions are met for more than two times

consecutively. As prosthetic gait is slow varying activity,

this approach would save power effectively in comparison

with normal operation of electronic knee. This means

knee can easily operate for full day walking activity of

amputee without any need of recharging. Battery status

of knee is checked by controller and it generates audio

alarm for low battery. In low battery condition, knee will

run in slow speed walking mode and provide maximum

stability to patient. After low battery, device needs to be

plug-in for charging. It will take roughly 2 h for full

charging.

Results

This study presented indigenously designed and

developed a low cost electro-pneumatic knee (length, 40

cm; wt, 1600 g; flexion angle, 60; weight-bearing

capacity, 100 kg). In prototype of knee joint (Fig. 5),

socket and sache foot was procured from a rehabilitation

center. Socket was fabricated according to residual

stump of patient. For every patient, due to difference in

knee to toe length, some minor adjustments are required.

Fig. 5Prototype of developed knee joint

448 J SCI IND RES VOL 69 JUNE 2010

Experimental analysis of indigenously developed and

tested electro-goniometer and FSR showed similar results

as compared to standard available sensors. Component

and fabrication cost (Rs 50,000/-) of developed prototype

is very low as compared to available electronic knee

from Otto Bock or Endolite (Rs 3.5-20 lakhs). It can be

lower when it goes for production.

Conclusions

A prototype with patient selected walking speed

through switch is tested and fabricated. Performance

quantification of knee will be done by questionnaire

methods. By incorporating more number of different

sensor mechanism for measuring kinematics and kinetics,

a near natural knee can be designed but this will raise

cost of prosthetic.

References

1 Hatwell M S, Oderkerk B J, Ari S C & Inbar G F, The

development of a model reference adaptive controller to control

the knee joint of paraplegics, IEEE Trans Automat Cont, 39

(1991) 683-691.

2 Irby S E, Kaufman K R & Sutherland D H, Electronically

controlled long leg brace, in Biomed Engg Conf, vol 29 1996,

427-430.

3 Zlatnik D, Steiner B & Schweitzer G, Finite-state control of a

trans-femoral (TF) prosthesis, IEEE Trans Cont Syst Technol,

10 (2002) 408-421.

4 Previdi F & Carpanzano E, Design of a gain scheduling controller

for knee-joint angle control by using functional electrical

stimulation, IEEE Trans Cont Syst Technol, 11 (2003) 310-325.

5 Dollar A M & Herr H, Active orthoses for the lower-limbs:

challenges and state of the art, in Proc 2007 IEEE 10th Int Conf

on Rehabilitation Robotics (The Netherlands) 12-15 June 2007.

6 Cheoltaek K, Ju-Jang L & Xinhe X, Design of biped robot with

heterogeneous legs for advanced prosthetic knee application,

in SICE-ICASE Int Joint Conf, (Bexco, Busan, Korea) 18-21

Oct 2006.

7 Andrysek J & Chau G, An electromechanical swing-phase

controlled prosthetic knee joint for conversion of physiological

energy to electrical energy: Feasibility study, TBME-00465-

2006.R1, 2006.

8 Lee J-W & Lee G-K, Gait angle prediction for lower limb

orthotics and prostheses using an EMG signal and neural

networks, Int J Cont Automat Syst, 3 (2005) 152-158.

9 Berry D, Microprocessor prosthetic knees, Physical Med

Rehabilitat Clinics of North America, 17 (2006) 91-113.