Electrical Characterization Of Textile Sensors Computer Science Essay

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The purpose of this thesis is to calculate and find the conductivity of sensor. Sweat detection textile sensor is developed and the conductivity of the sensor is analyzed when there is a drop of saline water across the sensor. The detection is carried out with different frequencies and the conductivity is compared.

We take a new material and textile sensor to find impedance in different ways. First we take a special fabric containing conductive fiber of some length. Using multi-meter the resistance values can be found for each thread by placing one metal pointer to one side and other pointer at the end of it. The threads have the conductivity in alternative. After we get the total impedance, we can shorten the circuits with different lengths and get the impedance. The theoretical impedance value is also found by connecting the resistors to the breadboard. Now connecting to the sensor we compare the impedance with theoretical value of Fiber at different stages.

1.2 Motivation

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Sweat detector sensors is implemented to see the change in conductivity and analyze with the theoretical sample fabric. Textile sensors allow the implementation of non-invasive and comfortable measurement systems.

1.3 Goal

The main goal of this project is to develop and characterize the electrical impedance of the textile sensor and conductivity of the sensor that will detect sweating activity. A secondary goal is to develop and test a textile sensor design which could sense sweating correctly. The sensor would have to be non invasive and disposable.

1.4 Work done

The resistance of the sensor is prepared and developed to a good textile sensor. The sensor fabric was tested using saline water at different points and with different frequencies. The results are compared with theoretical impedance of the fabric.

1.5 Structure Of The Report

This Report is organized in 7 chapters and appendices. Chapter 1 gives a brief introduction of the project. Introduction of textile sensor and its applications is given in Chapter 2. Chapter 3 covers the introduction for Bio-impedance and the measuring instruments used for finding the impedance. Chapter 4 is written about the measurement of sample fabric through calculation and with connecting resistors in bread board. Chapter 5 is written about the hardware design of the project in which the textile based sweating sensor is discussed. Chapter 6 gives the results of the theoretical fabric and textile sensor. Conclusions and proposed future works are discussed in chapter 7. Appendices give some extra information to better understand.

Chapter 2.

Textile Sensor

2.1 Introduction

Textiles are inherent microstructures with fantastic properties: they are flexible and much more mechanically stable than foils. The term 'textronics' refers to interdisciplinary approaches in the processes of producing and designing textile materials. It is a synergic connection of textile industry, electronics and computer science with elements of automatics and metrology knowledge [K. Gniotek, Z. StempieÅ„, J. ZiÄ™ba: 'Textronics, a new field of knowledge' (in Polish), PrzeglÄ…d WÅ‚ókienniczy, no. 2, 2003. 2. K. Gniotek, I. KruciÅ„ska: 'The Basic Problem of Textronics', Fibres & Textiles in Eastern Europe January/March 2004. Vol. 12 No. 1(45)]. Textile sensors are becoming an emerging field in industry. The possibilities that this technology holds seem almost limitless. Currently, textiles are being developed for many applications and markets, including biomedical sensing, wearable computing, large area sensors and large area actuating devices [S. Jung, C. Lauterbach, M. Strasser, W. Weber, "Enabling technologies for disappearing electronics in smart textile", International Conference of SolidState Circuits, pp. 1-8, February, 2003.]. Additionally, clothing provides a large surface which can be used for sensing. The concept of textiles are developed and readily applied to many existing products. Textile sensors are becoming rapidly interconnected by technology, the addition of textile sensor components to everyday products, as well as specifically targeted designs will provide the ability to enhance product performance and provide new and unique services to customers

Conductivity over fabrics is one of the challenges in electro-textiles, different materials and ways are available: carbon black, some metals and recently conductive polymers are currently engineered in the market as fibers, yarns, pastes, etc [E. Pasquale, F. Lorussi, A. Mazzoldi, D. De Rossi, D., "Strain-sensing fabrics for wearable kinaesthetic-like system", Sensors Journal, IEEE , vol. 3, iss. 4, pp. 460-467, August 2003.]. that could be applied to fabrics by different standard techniques: weaving, knitting, coating, laminating, printing, etc. [Tünde Kirstein, Jose Bonan, Didier Cottet, Gerhard Tröster,. "Electronic Textiles for Weareable Computing Systems", Weareable Computing Lab, ETH Zürich, Switzerland. 2004.]. Some of these techniques are not versatile to achieve stable and homogeneous conductive tracks or surfaces with a predefined geometry. Mainly some attempts has been tried to trace conductive tracks with high conductivity by weaving monofilament conductive metal yarns [D. Cottet, J. Grzyb, T. Kirstein, G. Tröster, "Electrical Characterization of Textile Transmission Lines", IEEE Transactions on Advanced Packaging, Vol. 26, No. 2, May 2003, pp. 182-190] and recently other attempts involved techniques used in printed flexible electronics over fabrics by using conductive inks or pastes [Behnem Pourdeyhimi, Edward Grant H. Troy Nagle. "NTC Project: F04-NS17 Printing Electric Circuits Onto Non-Woven Conformal Fabrics Using Conductive Inks And Intelligent Control". National Textile Center Annual Report: November 2004.].

2.2 Types and Applications of Textile sensors

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Different types of textiles are being used for different biomedical applications. Some of them are described below.

2.2.1 Stretch Sensors

Stretch sensors are made of elastic fabrics. Their resistance changes on applying stress. Work is been done by several researchers in the field of stain sensors using conductive fibers, weaved with Lycra and other elastic materials, and coating elastic fibers with conductive materials such as carbon filled silicon [6-7]. These sensors are being used to measure breathing movement of lungs and joints movements.

2.2.2 Temperature sensors

These sensors are made of fabrics or polymers [8] whose resistivity changes with temperature. Such sensors are being used to measure body temperature. Temperature sensitive fabrics can be weaved on patients bed covers or within person's daily life clothing.

2.2.3 Pressure sensors

Pressure sensors also called capacitive sensors are used to measure pressure exerted by different body parts. In many cases their application is the same as the stretch sensors. These sensors are made by using three layers of two different textiles. A usual construction of such sensors is shown in the figure. They could be weaved with sports jackets, trousers and bed covers.

2.2.4 Textile Electrodes

Textile electrodes are made of different types of conductive yarns and polymers [9]. They could sense even very small voltages of different body parts e.g. heart. Textile electrodes are now becoming a popular choice because of their comfort of use and weave in cloths. Studies showed that measurements taken with textile electrodes are comparable with those of Ag/AgCl electrodes [10] for certain applications. Textile electrodes are used for measuring ECG, EMG and electrical bio impedance of different body parts. Chapter 4 / Textile Sensors - 23 - NuMetrex Heart Sensing Sports Bra is a garment that incorporates textile-sensing technology. Electronic sensing fibers are knitted directly into the fabric, where they pick up the wearer's heartbeat and radio it to a watch via a tiny transmitter in the front of the bra [11].

2.2.5 Moisture Sensors

Textile Moisture sensors are also made of conductive fibers or polymers, [12] and color changing textiles, whose electrical and chemical properties changes with appearance of moisture on them. These sensors are used to detect sweat on the body.

2.2.6 Chemical Sensors

Besides measuring physiological activities of the body, textile sensors are now developed to measure chemical characteristics of body fluids. One of the typical measurements is pH of sweat. These sensors are developed using special electrical circuitry and intelligent textiles which are small and capable of remote sensing [13].

Chapter 3.

Bio Impedance

Chapter 4.

measurement OF sample fabric

4.1 Introduction

A special fabric containing conductive fiber of some length with many threads is analyzed to get theoretical measurement of sample fabric.

4.2 Theoretical Measurement Of Sample Fabric Through Calculation

A special fabric containing conductive fiber contains the measure of 5.6cm measured by textile tape is taken as shown in Fig.4.1. The conductivity of the fabric is in alternative threads.

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Figure 4.1: Sample Fabric Consisting of 14threads

So the threads are taken into 2 parts A and B as shown in Fig.4.2. Using multi-meter the resistance values can be found for each thread by placing one metal pointer at the start of the thread and other pointer to the end of the thread (i.e) start of the horizontal thread. For instance as shown in Fig.4.2 the resistance is measured from A1 to A2 that belongs to A part. Similarly in B part the resistance is measured from B2 to B1. This is done for all the threads and the resistance for every thread is found.

Figure 4.2: Measuring Resistance for sample fabric

The threads from C to D have many series and parallel threads with the measure of 1.8cm. The resistance is calculated for every part of thread and average of it is taken. Thus gives the resistance value for A2 to B2. Thus the conductivity of the fabric is found from A1 to B1 as shown in the Fig.4.3.

Figure 4.3: Measure of First thread

For finding total impedance value we are going to reduce the circuit. Firstly we take the points B12, A13, A14, B13 and B14. Since the last two threads (A14-B13 and B13-B14) are in series we can add them so now the remaining points in the circuit looks like Y Transform. This is now changed to ∆ transform as follows

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This transformation is done by the formula

R∆ = RP / Ropposite

Where RP = R1R2 + R2R3+ R3R1 is the sum of the products of all pairs of impedances in the Y circuit and Ropposite is the impedance of the node in the Y circuit which is opposite the edge with R∆. The formulas for the individual edges are thus

Ra = R1R2 + R2R3+ R3R1 / R2

Rb = R1R2 + R2R3+ R3R1 / R3

Rc = R1R2 + R2R3+ R3R1 / R1

If we use the values in this formula we get

Figure 4.4: Converting Y to ∆ Transformation

Thus by getting the ∆ transforms the circuit looks like Fig.4.5. Now the resistance B11-B12 and B12-B14 are in parallel. So this can be eliminated by using the formula 1/RTotal = 1/R1 + 1/R2

Figure 4.5: ∆ Transformation

By the same way we take next Y transform as A12, B12, A13, and B14. As we continue doing the same, the circuit get reduces and finally we get everything in parallel. This parallel circuit is finally done by the formula

1/RTotal = 1/R1 + 1/R2 + . . . + 1/Rn

Using the above formula we can find the total impedance.

Impedance for each cm can be found by dividing the resistance with the total measure.

Resistance/cm=Total Resistance/3.8

4.2.1 Short Circuiting Different Threads

The change in impedance is found by short circuiting the threads with different lengths. Firstly 1cm from A1 and B1 are short circuited and now the resistance is taken from A1 to A2 and B1 to B2 as shown in Fig. 4.2.1.

Figure 4.2.1: Short circuiting the 1st thread for 1cm

Now the impedance is found by calculation done as before and now the change in impedance is noted. Short circuiting is then done for 2cm, 3cm and the change in impedance is found. To find more change in impedance we short circuit 2nd thread also for 1cm, 2cm and 3cm and this is also continued for 3rd thread and the impedance is found.

4.3 Theoretical Measurement Of Sample Fabric Using Bread Board

The resistance found for every thread is now checked connecting the resistors in breadboard. The resistors are connected in the sockets of the bread board in such a way that it is connected according to the diagram in chapter 3. After connecting the resistors the continuity between the sockets is checked in the multimeter. Once the continuity is checked till the last resistor the total impedance can be found by keeping the metal pointer in the starting of the resistor and keeping the another metal pointer to the end of the resistor as shown in Fig.4.3. The short circuited resistance value found in previous chapter is taken as resistors and the impedance for various changes in resistors is found.

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Figure 4.3: Impedance using Breadboard

Chapter 5.

Textile based sweating sensor

5.1 Introduction

Sweat detecting sensor is made by taking a new fabric similar to the sample fabric that is taken in Chapter 4. The sensor is designed to measure elongations in textiles. The sensor thread needs to be integrated or attached to a textile before characterization. The sample fabric is taken in long measurement Conductive fiber weaved as grid like structure in a cotton cloth.

5.2 Characterization Of Sweating Sensor

Sweat detecting sensor is made by taking a new fabric similar to the sample fabric that is taken in Chapter 4 but with long measurement textile. Conductive fiber weaved as grid like structure in a cotton cloth is taken in such a way that the threads are taken in the excess of 3.8cm. Now the sensor is designed by taking all the excess of threads from A part and B part. Both the parts of threads are twinned so that all threads from A and B parts are totally gathered and stripped to the buttons separately. Now the sweat detecting sensor is ready as shown in Fig. 5.2

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Figure 5.2: Sweat detecting sensor

The resistance between the two buttons gives the total impedance. This can be compared with the total impedance we got in chapter 3. The impedance is also compared by taking the 1st and 2nd button with all the threads.

Another multimeter is used for short circuiting in the way that the metal pointer is placed between the threads. So now we keep one multimeter in two buttons to find the total impedance and another multimeter keeping the readings in Amps and placing the metal pointer between the threads making a short circuit. When we keep the metal pointer in two threads, it means the thread between the two pointers is made short circuit and the impedance is found.

5.3 Working Principle Of Sweating Sensor

The sensor could be used in two ways. Firstly, the two electrode combination measuring the skin resistance between two electrodes. The resistance falls down when there is sweat in the skin. Instead of sweat the saline water (Nacl) is applied to the sensor to find the change in resistance. Saline water is applied in every part of the threads and the formation of the hole make short circuit and thus decrease in resistance is noted. As discussed earlier in chapter 3 Impemed is used to find the change in resistance for every second with the change in frequency. Resistance is measured for every second as saline water is dropped across the threads of sensor. The decrease in resistance is more when there is excess of saline drop across the threads. The resistance is compared with different frequencies.

Chapter 6.

Results

6.1 Resistance Characteristics Of Sample Fabric

The theoretical sensor is analyzed and characterized with taking the length as 3.8cm. The textile integrated sensor thread is characterized. The influences of temperature and humidity are not considered.

6.2 Resistance Characteristics Of Textile Sensor

As described, a textile sensor made of silver plated conductive fiber weaved in a cotton cloth is used in this project. The conductive fibers are weaved in a grid like structure. The sensors made are tested with the proposed design by making samples of same length and width that of sample theoretical sensor. For testing purposes a solution of saline water is used. The Resistance of the textile sensor with the design proposed in Chapter 5.3, is decreased due to two factors

* Due to short circuit, when a drop of saline water is poured on one of the open pore.

* Due to the fact that resistance of the conductive fiber weaved within the sensor decreases when it come in contact with saline water.

It is seen that after the test, on the sample, with saline water, resistance of the sensor does not change, no matter how much saline water we drop on it. Fig 6.1 shows the graph of resistance changes of the sensor of length 3.8cm for frequencies 1k and Fig.6.2 for 5k, 10k, 50k and 100k. This sensor does not have any holes between the conductive fibers and spaces between conductive fibers are very short. The resistance decreases in a very smooth way.

Figure 6.1 : Drop in Resistance for Frequency 1k

Figure 6.2 : Drop in Resistance for Different Frequencies