Abstract:

Objective Due to the limitation of the Young's modulus of the dielectric layer of the current flexible capacitive sensor, the performance of the capacitor, such as sensitivity, cannot meet the requirements, so material selection and structural design of the electrodes and dielectric layers of fabric sensor are required to improve the sensing performance. In this paper, a series of polypyrrole composited silk fabrics were used as fabric electrodes, and wool fiber aggregates were used as dielectric layers to construct an all-fiber capacitive pressure sensor.

Methods Based on the calculation formula of effective dielectric constant, the pores in the fabric electrode and the air dielectric layer have a positive impact on the sensitivity of the sensor. Therefore, the polypyrrole composite silk fabrics were used as the electrode, and fiber and air aggregates were used as dielectric layers to study the effects of different fabric structures, different types and contents of fibers in dielectric layers on the performance of capacitive sensors. Application explorations of human motion and safety detection were also done.

Results The square resistance of polypyrrole composited crepe satin fabric (69 g/m²) was the smallest, which was 42 Ω/□. This is because the density of crepe satin silk is the largest, the diameters of yarns are also larger, the warp yarn is twisted, and the weft yarn is not twisted. The sensitivities of cotton and wool fiber capacitors were better than that of acrylic fiber. Because wool fiber has better elasticity and uses less, so wool fiber is finally selected as the dielectric layer. When the height of the dielectric layer is higher, the wool content is larger, the dielectric constant is larger, and the capacitance value is larger, but when the height is too high, the air content decreases, and the deformation ability of the overall dielectric layer decreases, thereby reducing the capacitance change rate, so he height of 1.4 cm as the dielectric layer had the best performance. The fabric 5^# (69 g/m² plain crepe satin) has the highest capacitance of 66 pF when it is used as the electrode. This is probably because the porosity of the fabric 5^# is the smallest, the effective area of the electrodes is the largest, and the capacitance is the highest. With the increase of applied pressure, the capacitance increases and the capacitance change rate also increases. The highest sensitivity was 1.08 N^-1 at 0.098 N. In the process of applying pressure, the structure of the fabric electrode will change, which will cause the change of the effective relative area and the air content in the fabric electrode, which will also affect the dielectric constant. When the pressure is greater than 1.96 N, the capacitance changes rate of the fabric 5^# is the largest, and its sensitivity is the best, so the fabric 5^# is used as the capacitance sensor electrode. The capacitive sensor has good stability, and is expected to be used in limb movement monitoring and safety monitoring in public places.

Conclusion 1) The influence of fabric structure on electrical properties can be concluded as: the greater the fabric density, the denser the yarn arrangement, the more conductive paths, and the smaller the resistance; the fabric electrode not only affects the effective relative area, but also affect the dielectric constant, which in turn affects the overall capacitance. The effect pattern needs further study. 2) The optimized assembly conditions of the capacitive sensor are the wool fiber and air aggregates with a height of 1.4 cm as the dielectric layer, and the electrode is the polypyrrole composite fabric of crepe satin (69 g/m²). The existence of air in the dielectric layer has a great influence on the height of the dielectric layer and the change of the dielectric constant during the compression process; the structure of the electrode fabric will affect the dielectric constant and effective relative area during the compression process. The above factors will ultimately affect the sensitivity of the sensor. Therefore, the next step will be to further optimize the structure of the dielectric layer and fiber composition to find a quantitative relationship, thereby improving the sensitivity of the sensor. 3) Application studies have shown that the capacitive sensor has the ability to sense the bending changes of fingers and the proximity of metal objects and fingers within 10cm, which is expected to apply this multifunctional, low-cost electronic fabric sensor to artificial skin, wearable health detection and contactless detection equipment superior.

Key words: conductivity, fabric sensor, polypyrrole, fiber dielectric layer, fabric electrode