development of wearable sensor system for measuring body joint flexions
TRANSCRIPT
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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
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T. G. Russell, G. A. Jull and R. Wootton. Can the Internet
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http://www.slideshare.net/raj7/wearable-biosensors
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THANK YOU