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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.
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.
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.
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
2.2.1 Stretch Sensors:
Stretch sensors have unique characteristics of changing resistance when stretched. They are made up of elastic fibers, which make the sensor very flexible. The resistance gradually increases when the sensor material is stretched. The sensor material has a nominal resistance of 1000 ohms per linear inch. These sensors are used to measure breathing movement of lungs and joints movements.
2.2.2 Temperature sensors:
Temperature sensor measures the hotness of any material might be textile or body. The sensors used in textile are made up of polymers that changes resistivity with temperature. The sensor is integrated into the fabric by weaving process. It has got many applications and the most interesting application is in measuring infant temperature.
22.2.3 Pressure sensors:
Pressure sensors measure the pressure information from the surface of fabrics under stress by means of capacitive sensing. The fabrics with these type of sensors composed of passive array of capacitors, whose capacitance depends on the exerted pressure on the textile surface.
2.2.4 Textile Electrodes:
In electronic textiles miniature electrical components are integrated into the fabrics in order to monitor different body movements and postures. These are very sensitive and thus have got much application, owing to their capability of measuring even the slightest change in body's temperature and voltage. Textile electrodes are being used in measuring ECG, EMG and other impedance produced by different body parts. In the case of skin-tight clothes Textile integrated EMG measurements are appropriate. These electrodes stayed fixed in position despite motion and sweating.
2.2.5 Moisture Sensors:
Moisture sensors are based on inter digital weave. These sensors measure the change in resistance brought in by the amount of water present, such as sweat. These sensors are made up of conductive polymers.
2.2.6 Chemical Sensors:
These are yet another component used in smart textiles. Chemical sensors are integrated directly on the inside elastic waistband of underwear, where they measure the biomarkers in sweat. These sensors give useful information about the wearer's health.
InÂ bio medical engineering,Â bio impedance is used to explain the response of a living organism to an externally applied electric current. It is a measure of the opposition to the flow of that electric current through the tissues, the opposite of electrical conductivity. The measurement of the bio impedance (or bioelectrical impedance) of the humans and animals helps us in measuring such things as blood flow and body composition known as EBI.
Bio impedance is used nowadays for theÂ developmentÂ of devices to measureÂ output and circulatingÂ blood volume. Electrical conductivity can vary as a result ofÂ breathing. The technique is used in both clinical medicine and research.
The Electrical Impedance of a material is the resistance a material offers to the flow of electrical charges through it. If the material is of biological nature, then such resistance is called Electrical Bio impedance.
To understand the electrical properties of biological tissues and formulating a comparison model with a textile fabric we will discuss briefly the effect of electricity on cellular level. A cell is defined as the fundamental unit of life as all living organisms are composed of cells. Most cells are joined to each other by means of an extracellular matrix or by direct adhesion of one cell to the other constituting different unities. The cell unities form a tissue ultimately. The main component of the cells is their cellular membrane, the structure of which is based on a lipid double layer in which proteins are distributed, allowing the formation of channels to exchange ions with the exterior. The cellular membrane is a dielectric interface. The electrically charged ions move and form accumulation on both sides of the membrane when constant electric field is applied. But due to an alternate current based electric field an alternating movement of electrons starts at both sides of the cellular wall, creating a relaxation phenomenon. The electrical behavior of biological tissues shows us the dielectric parameters dependence with the current frequency, due to the different relaxation phenomena that takes place when the current flows through the tissue. If frequency of the applied electric field is increased, the conductivity of most of the tissues is increased from a low value in direct current, to a constant level that keeps between 10 and 100 MHz This increase in conductivity is associated to a decrease in permittivity, from a high value at low frequency.
The impedance varies with the frequency of measuring signal depending upon the nature of tissues. The relationship between the impedance and frequency is nonlinear. The higher the frequency, the lower is the impedance
Previously the impedance measurements were expressed as resistivity, due to the small scale of the imaginary part, which is not considered. This is a correct approximation, especially at low frequencies (1 kHz or lower). This will become a complex number at higher frequencies as the imaginary part increases. But in case of our measurement we have considered lower frequency like 5 and 10 kHz as well as higher frequency like 50 and 100 kHz.
Measurement of Electrical Bio Impedance (EBI) can be classified into two categories. We have based our thesis experiments on the second category. Our category focuses on the calculations of characteristics of the body tissues, such as hydration, oedema, volume of body fluids, intra and extracellular volume, fat Percentage, and generally, the state of the tissues and the cells composing them. We have taken different values of Impedance with the help of multi-meter on a textile fabric, details of which are provided in this report.
measurement OF sample fabric
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 âˆ† transform as follows
This transformation is done by the formula
Râˆ† = 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 Râˆ†. 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 âˆ† Transformation
Thus by getting the âˆ† 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: âˆ† 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.
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
Textile based sweating sensor
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 multi-meter is used for short circuiting in the way that the metal pointer is placed between the threads. So now we keep one multi-meter in two buttons to find the total impedance and another multi-meter 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.
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.
Figure 6.1 : Simple Fabric NaCl 5k
Figure 6.2 : One Drop of NaCl 5k
Figure 6.3 : More Drop of NaCl 5k
Figure 6.4 : Dry Fabric of NaCl 5k
Figure 6.5 : More Dry Fabric of NaCl 5k
Figure 6.6 : Wet Fabric of NaCl 5k
Figure 6.7 : Wet Fabric (2mm) of NaCl 5k
6.2 Resistance Characteristics Of Multi-Frequency
Then we take resistance characteristics of Textile Sensor with multi-frequency.
The graphs of Min, Average and Max of Wet fabric 2mm
Figure 6.8 Wet Fabric 2mm NaCl multi-frequency
The graphs of 1 more drop of NaCl on 2mm wet fabric. Min, Average and Max.
Figure 6.9 One Drop of Water 2mm NaCl multi-frequency
Graphs of Min, Average and Max for an additional drop of NaCl on 2mm fabric, taking with multi-frequency
Figure 6.10 Additional Drop of Water 2mm NaCl multi-frequency
6.3 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 6.11 shows the graph of resistance changes of the sensor of length 3.8cm for frequencies 1k and Fig.6.12 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.