Abstract:

Objective Knitted flexible capacitive sensors have been widely studied for their inherent softness and comfort, structural simplicity and low loss. However, most of the sensor electrode materials are exposed to the external environment or in direct contact with the human body, prone to oxidization with long-term use. Moreover, the traditional encapsulation materials used for sensors have poor air permeability, affecting the wearing comfort of human body. Therefore, a knitted composite structure capacitive sensor was proposed to place the electrodes inside the sensor to improve the sensing efficiency of the sensor and the human body experience.

Method The silver-plated conductive yarn and cotton yarn were used to prepare the double-side-effect electrode using the computerized flat knitting machine with different thickness intervals in the fabric to form the "sandwich structure" capacitive sensor. According to the formula C=(ε₀ε_rS)/d, the change in the distance between the electrodes causes change in capacitance value so as to achieve sensing. The assembled sensor was placed on an electronic universal testing machine for compression experiments, and leads from both ends of the electrodes were connected to the TH2830 LCR digital bridge, and the values were recorded simultaneously.

Results The electro-mechanical properties, sensitivity, linearity, compression reversibility, hysteresis and repeatability of the sensor were studied and analyzed. The strain-capacitance curve of the sensor showed that with increasing compressive strain the capacitance value increased gradually, which is consistent with the mechanism elaborated in Equation (1) (Fig. 5). As important index for sensing performance, which is usually expressed as the ratio of the relative rate of change of capacitance to the stress, sensitivity of the sensor was characterized. With the gradual increase of the strain value, the sensitivity of the three different thickness sensors demonstrated a gradual increase with two different output characteristics (Fig. 6). The sensor with a thickness of 20 mm achieved a sensitivity of 0.091 kPa^-1 within 15 to 50 kPa. The linearity of the sensor at each stage was tested, and it was found that the linearity became worsened as the compression distance was increased (Tab.1). It was also found that although the three thickness sensors produced the maximum height difference at the maximum compression distance, the difference was within the error range and produced useful value (Fig. 7). Hysteresis and repeatability were used to characterize the dynamic performance index of the sensor, and a 20 mm thickness sensor was selected for hysteresis study. Repetitive compression tests with different strains were performed for all thickness sensors (Fig. 8), and the capacitance curves of the output during continuous cyclic stress loading and unloading at the same strain value were consistent overall, and the capacitance output curves of the sensor with 20 mm thickness at different strain values produced less fluctuation and smoother curves. The loading and unloading curves of the sensors had a certain height difference and the maximum hysteresis error of 16.2% took place at the stress value of 8.79 kPa (Fig. 9).

Conclusion Flexible capacitive sensors of knitted composite structure were prepared by attaching double-side-effect flexible electrodes to three different thicknesses of warp knitted spacer fabrics. The sensors show two different compression output characteristics, with good linearity but lower sensitivity when the strain is in the range of 0-30%, and reduced linearity when the strain range is 30%-60%. With the increase of compression distance, the sensitivity of the sensor is improved. Among them, the larger the thickness of the dielectric layer, the greater the rate of sensitivity increase, which is expected to be applied in the field of large strain limb motion monitoring. Continuous repetitive compression electrical curves of the three different thickness capacitive sensors at different strain values show good repeatability, which also indicates that the designed sensors have good and stable sensing performance. However, the increase in thickness makes the spacer filaments less stiff against bending and increases the sensing hysteresis. This provides some suggestions for the selection of the fabric dielectric layer in the future. To further improve the durability in practical applications, it would be an interesting research to explore the one-piece double-sided effect knitted capacitive sensor in the future.

Key words: knitting, double-sided effect, flexible electrode, spacer fabric, sensing performance