Time-evolution of electrical resistance-strain hysteresis curve of embroidered stretch sensors and their application in reliable human motion tracking

In this study, resistive-type stretch sensors based on embroidered conductive threads were investigated to determine the best combinations of various conductive threads and fabrics, as well as the ideal embroidery patterns and their optimal geometric parameters. Among various conductive threads, a silver-plated nylon fiber of 280 denier was selected as the best conductive thread, because it featured the highest gauge factor and excellent stretchability and reliability. Among the various tested fabrics, the poly fine brushed knit fabric could endure high repetitive stretching without any permanent deformation, thus guaranteeing an apparent electrical resistance-strain curve. Zigzag, couching, overcasting, and decorative patterns were embroidered on this poly fine brushed knit fabric using the 280 denier silver-plated nylon fiber. The electrical resistance curves of the embroidered conductive threads were measured as functions of the applied strain and frequency. The zigzag pattern exhibited the best resistancestrain response and the highest gauge factor. The electrical resistance-strain curve was also measured through a cyclic loading test involving 100000 cycles. Results indicated that the hysteresis behavior and drift in electrical resistance may have hindered accurate measurements of the strain during the stretching and releasing process, which needs to be corrected by using a compensation algorithm. To demonstrate the proposed stretch sensor, knee motions were monitored using a resistive-type stretch sensor implemented on athletic cloth.