Abstract:

Objective The fabric strain sensor made of silver-coated nylon yarn has the problem of coating oxidation corrosion, which affects the measurement accuracy and can cause the sensor failure. In order to improve the environmental stability of fabric strain sensor, a new design concept is proposed in this paper, where TPU/CB/PA conductive yarns with polyamide(PA) yarn as core layer and polyurethane (TPU) and carbon black (CB) composite material as core sheath structure were prepared based on coaxial wet spinning technology. Then the conductive yarns were knitted together with finished spandex yarns to form a knitted fabric strain sensor. Due to the absence of active chemical elements, this type of sensor theoretically has the advantages of oxidation resistance, corrosion resistance and sweat resistance.

Method TPU/CB/PA conductive yarns with polyamide (PA) yarn as core layer and polyurethane (TPU) and carbon black (CB) composite material as core sheath structure were prepared based on coaxial wet spinning technology. The conductive yarn was knitted together with spandex yarn as an inlaid part for fabric strain sensor. The sensor samples were prepared with four types of knitted structures, namely plain grain, 1×1 rib, double rib, double reverse, four different Mosaic schemes and different densities. When the external strain was applied to the sensor, the conductive part of the coil was deformed, so that the sensor resistance changed, and the received mechanical signal was converted into an electrical signal output. Different strains made the degree of deformation of the coil different. In this paper, the resistance change of the sensor under 5%-50% tensile strain is measured to study the performance of the sensor and its influence parameters.

Results The prepared conductive yarn skin and core layer were tightly bonded. The resistance value of the 5 cm length of the yarn was within 27-31 kΩ, indicating relatively stable resistance, and the resistance value met the requirements of the preparation of the sensor. The sensor properties of the four types of structures prepared by inlay scheme 2 were better than those prepared by other inlay schemes. The relation between the resistance change rate and strain of the four types of structures sensor samples prepared under the strain of 5%~50%. The resistance change rate of the ribbed sample began to decrease after 45% strain, the resistance change rate of plain sample began to decrease after 40% strain, the resistance change rate of double-reverse sample fluctuated after 25% strain, and the resistance change rate of double-ribbed sample fluctuated after 35% strain. The repeatability range of the sensor stable resistance of the four types of structures of the embedded scheme 2 was 35%-50% for the rib sample, 30%-50% for the plain sample, and the double reflex 2 and double rib samples were stable under the strain of 45%-50%. The ribbed sensor sample passed 500 fatigue tests. On the basis of the ribbed sensor sample, the density of the fabric was reduced, and the effective strain range of the sensor was expanded to 20%-50%, and the life fatigue reached at least 500 times. The ribbed sample was sewn on the elastic belt, which initially achieved the monitoring of human respiration and joint movement.

Conclusion A conductive yarn with a skin core structure is prepared by using wet spinning technology to wrap a conductive layer of polyurethane/carbon black on nylon yarn, and the prepared conductive yarn is further used for the preparation of sensors. Based on the characterization of the sensor properties, It can be concluded that the sensor properties of the four types of structures prepared by inlay scheme 2 are better than those prepared by other inlay schemes. In the four types of knitted fabric, plain, 1×1 rib, double reverse, double rib, the effective strain range of the sensor sample of plain and 1×1 rib is 30%-45%, and the fatigue life is at least 500 times. Plain grain and 1×1 rib are more suitable as knitted fabric for preparing strain sensors. The density of the fabric sensor coil is found to affect the stability of the sensor. When the density of ribbed sensor samples is reduced, the effective strain range of the sensor is expanded from 35%-45% to 20%-50%. However, when the density decreases, the relative resistance of the sensor also decreases. It is a problem to balance the effective strain range and the relative resistance change. The sensor can effectively monitor breathing, joint movement, and other signals, and has a good development prospect in the field of flexible intelligent wearable.

Key words: fabric-based strain sensor, knitted fabric, wet spinning, breathing monitoring, motion monitoring, conductive yarn