Development of Wearable Sensor System for Measuring Body Joint Flexions

8/3/2019 Development of Wearable Sensor System for Measuring Body Joint Flexions

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DEVELOPMENT OFWEARABLE SENSOR SYSTEM

FOR MEASURING BODY

JOINT FLEXION

By,

S. Ashwath Narayan

1KS08EC080


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Abstract

This topic presents a novel approach for measuring

and monitoring human body joint angles using wearable

sensors. This type of monitoring is beneficial for therapists

and physicians as it allows them to assess patient's activitiesremotely. In our approach multiple flex-sensors are mounted

on supportive cloth to measure the flexion of a joint. The

changes in the resistivity of the flex-sensors are measured

using an electronic board.


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Introduction

For many medical and rehabilitation applications it is

desirable to continuously monitor patient's daily activities

at home without visiting hospital.

Wearable sensors (flex sensors) can be used to monitorthe patients without their physical presence.

The measured joint flexion angle can be transmitted over

phone or internet.


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Evolution ofWearable Sensors

Physiotherapists first used goniometers to measure joint

flexions.

Russell developed an internet based system to measure

the angle of knee joint remotely using a webcam. Another improvement was development of electronic

gloves for measuring the hand posture.

Then evolved the wearable sensors which used a single

sensor.


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Technologyused

The technology used behind the wearable sensors is the

motion sensing material called flex sensors.

A flex sensor is a thin strip which is flexible.

Flex sensors change the resistance depending upon the

amount of bend on the sensor.

The resistance is directly proportional to the deflection

caused by bending the sensor.


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Howdoflex sensorswork?

Flex sensors are analog resistors.

Flex sensors are made of carbon material and a flexible

substrate.

More the carbon less the resistance. When the sensor is bent, it produces a resistance value

which is relative to the bend radius.


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Flex sensorsinterfacing

We use multiple flex sensors mounted on a supportive

cloth to measure the range of knee motion.

The range of sensor resistivity is between 9k and 22k.

Based on the size of joint number of flex sensors can beemployed.

The resistance is converted to angle as shown further.


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The flex sensors give the corresponding resistance basedon the bend.

This resistance is converted into voltage using voltagedividers.

A voltage divider uses two resistors in series to divide theinput voltage by the ratio of the resistances.

To measure the voltage corresponding to the resistance,we need to keep a resistance value constant and the other

as variable resistance.

This variable resistance is obtained by connecting the flexsensor.


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The other method for converting the resistance variation

into voltage is by using BJTs.

Constant current sources are given to the sensors. Thechange in the collector current indicates resistance

variation and hence is converted to voltage.


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The output of the voltage divider or the BJT circuit is

given to ADC.

This voltage signal is amplified by using an non inverting

amplifier.


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This amplified signal is sent over the internet to thehospital.

The microcontroller interface in the hospital will convertthe received voltage signal into angles.

The formula used for this conversion is

(Vmax - Vmin)

(Amax - Amin)

where, Vmax and Vmin maximum and minimum voltages.

Amax and Amin maximum and minimum angles.

X is the voltage for one unit of degree.

X


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In order to find the dynamic model of the knee flexion, a

goniometer is used to measure the knee movement of a

subject versus time which is shown in Figure

The plot of angles vs the time obtained from the wearablesensors is as shown.


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The red line indicate the predicted value that is the value

recorded by using goniometer.

The blue line indicate the observed value that is the value

recorded by the wearable sensor.


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Advantages

Continuous monitoring.

Right treatment at the right time.

Easy to handle and wear.

Reducing the health care cost.


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Disadvantages

Flex sensors are expensive.

To maintain a good accuracy we need to use many flex

sensors which is not practical.

Flex sensors are very delicate which may lead to breakage

of the sensors due to repeated bending.


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Conclusion and Improvements

We studied about the wearable sensors to measure the bodyjoint flexion.

This system is economical and can be used for differentremote health monitoring applications.

We can make use of multiple sensors to improve the accuracy.We can use EK F (Extended Kalman Filter) to fuse the jointangle measured by multiple sensors and can also be used toremove noise.

In future we can make flex sensors which are of good qualityand small in size.

Moreover, we can utilize smart phones as a home monitoringsystem to perform the angle calculation and to transmit data toa remote server.


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Bibliography

http://sensorwiki.org/doku.php/isidm/introduction

http://sensorwiki.org/doku.php/tutorials/basic_sensor_inte

rfacing_techniques

http://www.youtube.com/watch?v=CkPekPiA-

sc&feature=results_main&playnext=1&list=PL

2346A71B114013CE

http://www.electro-tech-

online.com/microcontrollers/113836-convert-voltage-

angle-degree.html http://www.vti.fi/sites/default/files/documents/tn17_volt

ge_to_angle.pdf

http://www.edutalks.org/seminars/slides/wearable%20bio

sensors.pdf


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http://www.sparkfun.com/datasheets/Sensors/Flex/flex22.

pdf

T. G. Russell, G. A. Jull and R. Wootton. Can the Internet

be used as a medium to evaluate knee angle? Technical

and measurement report. University of Queensland,

Australia, 2003.

L. Dipietro, A. M. Sabatini and P. Dario. Evaluation of aninstrumented glove for hand-movement acquisition.

Journal of Rehabilitation. Advanced Robotics Technology

and Systems (ARTS) Laboratory, 2003.

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uinge ,P. H. Veltinkb and C.

T. M. Baten.

Ambulatory measurement of arm orientation. Journal of

Biomechanics. University ofTwente, 2005.

http://www.slideshare.net/raj7/wearable-biosensors


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THANK YOU

Documents

Development of Wearable Sensor System for Measuring Body Joint Flexions