Research On Electro-Mechanical Properties Of Conductive Knitted Flexible Sensors And Measurement Of Underwear Pressure

With the development of science and technology, especially with the rapid development of information technology, intelligent textiles have been become the cutting-edge technology in the field of textile research focus. In the ordinary way, it compromises the Textiles, Garment, Information Technology, Computer and Microelectronics Technology, which often includes sensors, information transmission processing system and information storage systems. It can collect physiological parameters and comfort signals timely in accordance with the different needs, and feedback and deal with the signal correspondingly. And the sensor is key factor of the smart textiles, which can make the testing processing intelligently and comfortably.Now, the research of wearable flexible sensor is still in the exploration initial stage, measuring accuracy, sensitivity and reproducibility has not been reached commercial requirements yet, so it need further research and exploration. And so far, fabric structure resistance-strain sensors in domestic and international research are basically based on the knitted structure. But there is no systematically theoretical study of conductive plain knitted fabric in electro-mechanical properties. And as a stretch-to-fit garment, in particular as a knitted seamless underwear, it contain spandex or other elastic yarn gennerally. So it need design different structure and processing technology in accordance with clothing comfortable pressure in different parts of the human. But no matter what kind of instrument is used to test clothing pressure on the body, there is always some interval between the clothing and skin due to the thickness of sensor. So it reduces the testing accuracy and comfortability.Ultimate goal of this thesis is to product a conductive knitted structure flexible sensor, do electro-mechanical analysis and characterization and test the knitted seamless underwear pressure with the flexible sensor. Build a conductive plain knitted flexible sensor resistance model and analyze factors of the equivalent resistance when it is suffered to tension; choose the appropriate flexible sensor structure for the optimal design of parameters; analyze the clothing pressure model to determine the relationship between clothing pressure and the sensor resistance; design a knitted seamless underwear with some flexible sensor and test the change of the sensors resistance with different ease garment to a standard female body to determine the underwear pressure distribution and the garment ease on the influence of garment pressure. This thesis consists of three parts:1. The Electro-mechanical properties of conductive plain knitted flexible sensorBased on the loop structure of a plain weft knitted fabric, the hexagonal resistance model of the conductive plain knitted fabric is established. And Kirchhoffs law and loop current method are employed to calculate the circuit network and obtain equivalent resistance. To verify the feasibility of the theoretical model, some conductive plain knitted flexible sensors are preparated and measured their resistances. It is found that the conductive fabric circuit network is an integrated series and parallel complex network; it is a multiple circuit parallel to the wale direction whereas it is series along the course direction. In order to simplify the calculation process, only m courses by1wales unit loops are needed to calculate the equivalent resistance. This theory study can instruct the design of wearable sensors.Three factors to influence the effect of the conductive plain knitted fabric resistance are analyzed, which contact the yarn segment resistance, yarn segments transfer the loop resistance and contact resistance of the overlapped yarns in a loop. Based on the conventional resistance law, the relationship between yarn segment resistance and tensile strain is analyzed; analysis the knitted fabric geometry changes on the the yarn segment transfer in the stretching process, based on the conductive elastic plain knitted fabric hexagonal resistance model, Kirchhoffs law and loop current method are employed to calculate the circuit network and obtain the effect of the yarn segment transfer the loop resistance; the contact load and deformation between two knitting loops are calculated using the classical knitted fabric mechanics model of Kawabata and Popper; Holm’s electrical contact theory is used to calculate the relationship between the contact resistance and the contact force in an elastic fabric sensor. It is found that the yarn resistance increases with the increase of tensile strain; the yarn segment transfer the loop resistance affects the conductive elastic plain knitted fabric resistance, it makes the resistance linearly increase with the tensile strain parallel to the course direction; the contact resistance of the overlapped yarns in the fabric decreases as a power-law function with the tensile strain.Two computer program can give the effect of yarn segments transfer the loop resistance and contact resistance to the fabric equivalent resistance, based on the hexagonal resistance model of the conductive plain knitted fabric. They can predict the relationship between the fabric resistance and the tensile strain. And the relationship between sensor resistance and strain is tested to verfy the analytical calculation.2. The selection of conductive knitted flexible sensors and optimization designSilver plated yarn is used to knitted5kinds of weft knitted structure fabric. Appropriate flexible garment pressure sensor is selscted by their conductive properties with uniaxial tensile testing and comparing the sensitivity coefficient and resistance repeatability.We can optimize the strain gage by studying the influence of the resistance of the yarn per unit length ρt, the width of a unit structure w, the loop lengthl, number of wales n, and number of courses m. An Optimization design simulating the electro-mechanical property of the conductive elastic knitted flexible sensor is built based on loops structure under biaxial extensions. To systematically examine the effects of parameters on sensor strain gage, model experiments were designed by response surface methodology (RSM). It is found that:to design a flexible sensor, the width of a unit structure w and the loop lengthl are significant model terms, whithin a reasonable range, lower unit width while higher loop length gives a higher strain gage. However, the higher resistance of the yarn per unit length ρl, the larger number of wales n and smaller number of courses m can supply a larger resistance, which make the measurement easier and more convenient. So to design a higher strain gage and convenient measurement sensor, lower unit width, higher loop length, higher resistance of the yarn per unit length ρt, larger number of wales n and smaller number of courses m can be choosed. The parameter studying can guide the knitted flexible sensors design.3. The Electro-mechanical properties of knitted flexible sensors and underwear pressure measurementBecause it is difficult to test the garment pressure influence effecting, clothing pressure is built according to the Laplace Equation. An oval mechanical model at circumference of weft seamless underwear is built, based on the analysis of body shape, to analyze the curvature radius at arbitrary circumference and part. In order to quantify the static and dynamic behavior of the compression fabric in the wearing seamless underwear process, we use the LLY-56B pressure measuring system to simulate the garment pressure changing with the three-dimensional. The flexible sensor directly facing the sensor distributing on surface of hemisphere are used to explore the influencing factors of the static and dynamic behavior, such as press-depth, press-velocity, standing time and so on. Also two another indexes (static pressure fatigue and dynamic pressure fatigue) are analyzed. In the test pressure processing, the equivalent resistance of the flexible sensor is tested to determine relationship of flexible sensor equivalent resistance and garment pressure contemporary. The research achievement is referential for further study on comfort pressure of seamless underwear and ease of clothing.Design a knitted seamless underwear with some flexible sensor and test the change of the sensors resistance with different ease garment to a standard female body to determine the underwear pressure distribution and the garment ease on the influence of garment pressure. Based on the relationship between the flexible sensor equivalent resistance and strain, and synthesizing the relationship between the garment pressure and the strain, the flexible sensor equivalent resistance VS the garment pressure is anyalyzed. The relationship expression fits the experiment result.The results systematically and clearly show the model resistance of the conductive plain knitted fabric; three factors to influence the effect of the conductive plain knitted fabric resistance to explore their conductive properties with uniaxial tensile testing. Clothing pressure on human arbitrarily girth is built according to the analysis of body shape, the relationship between the sensor resistance and garment pressure is analyzed. Based on the change of the sensors resistance with different ease garment to a standard female body, determine the underwear pressure distribution and the garment ease on the influence of garment pressure. The study provides a theoretical basis for the design of the knitting flexible senso. And it make simplified flexible sensor possible because of the flexible sensor and clothing in one.