Research Into Electro-mechanical Properties Of Knitted Sensor Under Strip Biaxial Elongation And Application In Monitoring Body Movements

The development of technology attributes to a smart and multi-functional garment. Connecting wearable strain sensors with garments can take good advantage of the conductive materials and the lightness, flexibility and comfort of textile fabric and can also give much convenience to our daily life. Smart garments can be used in health care field to monitor vital sign parameters of human body. For example, it will transfer alarm signal to terminal PC for first aid when the old fall down and have abnormal vital sign; it can also be utilised in sports and athletic area to detect the body movements and gestures in order to support standard training and avoid mistakes from referees.Some researchers have investigated the electro-mechanical performance of strain sensors, of which sensing mechanism is based on the relationship between mechanical deformation and electrical resistance change. However, these studies are about the electro-mechanical behaviours of strain sensor under relaxed state and unidirectional tensile. In fact, fabric are loaded by plain force such as shear and tensile and three dimensional force. Hence, it is necessary to analyse this electro-mechanical property from a perspective of two dimensional elongations and then explore the sensitivity and stability of strain sensor during monitoring body movements.The aim of this paper is to fabricate a flexible knitted sensor with plating knitting technique, to explore its electro-mechanical performance under strip biaxial elongation and to build relevant models, and then to apply it to monitoring garments for detecting body movements. After that, this paper will make a new strain sensor based on cotton/stainless steel fibre blended yarn and analyse its sensing behaviour.The contents of this paper are described as follows.(1) This paper studied the theoretical relation between force and strain of knitted fabric under strip biaxial direction and the macro electro-mechanical model of knitted strain sensor, built an equivalent resistance formula based on strain and tensile load. The high fitting correlation coefficient demonstrated a good agreement between experimental and theoretical results. The effect of contact force on contact resistance has also been investigated to prove that there is little influence of contact resistance between silver-coated conductive loops on the knitted fabric. In the structure model of knitted loop, the results showed that yarn segments transferred from legs to head loop and sinker loop under strip biaxial elongation in course direction while reverse transfer was found under strip biaxial elongation in wale direction. An equivalent resistance network was established and accorded well with the experimental results.(2)The effect of various tensile conditions on the resistance of knitted fabric has been analysed. The results showed the maximum value of resistance variation increased linearly with the maximum strain and there was small hysteresis loop between load curve and unload curve. The maximum tensile force increased with the rise of stretching speed, where resistance variation showed a slightly upward trend. Besides, small elongation speed attributed to an obvious hysteresis loop. The dwell time at maximum and minimum strains lead to a downward trend of resistance. In anisotropy of tensile angles measurements, significant resistance variation and hysteresis were caused by the growth of angles.(3)This investigation also explored the relation between structure parameters and sensing behaviour of knitted strain sensors. Strain sensor had different resistance change degrees and repeatability when the arrangements between conductive loops and non-conductive loops were different. Moreover, the density of fabric and the size of conductive area made little influence on the stability of resistance. The resistance variation ascended drastically with the rise of the number of conductive courses while decreased with the rise of the number of conductive wales.(4) A smart shirt and a knee cap were designed to detect body movements such as elbow, shoulder and knee joints. The results showed that the elbow sensor responded well to the stretching and bending of elbow, the sensor at knee joint part had good sensibility and repeatability during different knee movements. There was a little difference between shoulder movements and resistance due to the loose contact between sensor and shoulder. In the breathing test, sensor displayed well the sleeping process of the object.(5) A series of cotton/stainless steel fibre blended yarns have been fabricated, where the fibres were mixed evenly. An equivalent linear density formula has been built to determine the twist of blended yarns. Its validity has been proved by the blended yarns with different stainless steel fibre contents but similar twist angles. The electrical percolations based on both weight and volume fractions of metallic fibres were explored by measuring the resistance of all the yarns with various fixed length. The results showed there was a great dependence of both electrical percolations on the fixed length. Besides, the resistance of blended yarns showed a downward trend and then an upward trend with the increase of strain. The contact resistance between blended yarns dropped drastically due to the rise of contact force. Fabric sensors were made from these blended yarns and experienced a downward trend of resistance under unidirectional elongation and showed a higher sensibility than that of knitted fabric from silvercoated yarns.The electro-mechanical behaviour of knitted strain sensor has been systematically investigated in this paper. For example, the macro equivalent resistance model and resistance network only based on length-related resistance, the influence of tensile conditions and fabric structure parameters on the fabric resistance, smart shirt and knee cap to detect body movements. Besides, a new strain sensor based on cotton/stainless steel fibre has been made and its sensing performance has been explored. This investigation can serve theoretical proof for analysing electromechanical behaviour of knitted strain sensors, provide design method for promoting smart garments and indicate the direction of new strain flexible sensors.