Performance Evaluation of Knitted and Stitched Textile Strain Sensors

Abstract

By embedding conductive yarns in, or onto, knitted textile fabrics, simple but robust stretch sensor garments can be manufactured. In that way resistance based sensors can be fully integrated in textiles without compromising wearing comfort, stretchiness, washability, and ease of use in daily life. The many studies on such textile strain sensors that have been published in recent years show that these sensors work in principle, but closer inspection reveals that many of them still have severe practical limitations like a too narrow working range, lack of sensitivity, and undesired time-dependent and hysteresis effects. For those that intend to use this technology it is difficult to determine which manufacturing parameters, shape, stitch type, and materials to apply to realize a functional sensor for a given application. This paper therefore aims to serve as a guideline for the fashion designers, electronic engineers, textile researchers, movement scientists, and human–computer interaction specialists planning to create stretch sensor garments. The paper is limited to textile based sensors that can be constructed using commercially available conductive yarns and existing knitting and embroidery equipment. Within this subtopic, relevant literature is discussed, and a detailed quantitative comparison is provided focusing on sensor characteristics like the gauge factor, working range, and hysteresis.

Development of Low Hysteresis, Linear Weft-Knitted Strain Sensors for Smart Textile Applications

“…Hysteresis values can be calculated along the resistance axis (vertical) or along the strain axis (horizontal), and these values will be referred to as resistance hysteresis and strain hysteresis , respectively [ 18 ]. For a strain gauge in an application, the expected errors in the measured strain are of primary concern; thus, as a performance indicator, we prefer to use the strain hysteresis values and not the more commonly reported resistance hysteresis [ 22 , 23 ].…”

“…Liang et al also remark that a promising way to reduce hysteresis is to use Eeontex in the sensor base structure [ 14 ]. The resistivities of the currently available conductive yarns vary widely, and the study of Raji et al [ 17 ] seems to suggest that sensors with conductive yarn with a resistance of 77 ohm/cm have an optimum GF in the resistance range of 40 to 120 ohms, but the effect was not studied in detail [ 18 ]. The type of knit design is another essential parameter that affects the sensor performance, and many different studies to investigate the effect on sensor performance were reported.…”

Development of hysteresis-free and linear knitted strain sensors for smart textile applications

“…Hysteresis values were calculated as the maximum strain difference between stretched and unstretched cycles, scaled to the working range. The working range defines the maximum and minimum strain values that can be measured by the sensor [5].…”

“…Strain sensors commonly used for mechanical engineering applications are typically limited to strain not larger than 1 %. For the on-body applications, we need to be able to measure strains up to 30-40% [5]. In addition, for unobtrusive monitoring of the sensors need to be breathable, washable, and stretchable.…”

“…Nevertheless, due to the unstable characteristics of knitted structures, which result in high hysteresis values, poor sensing performance, and narrow working range, they are currently not commonly integrated into practical applications and mostly remain as a patch form [8]. To be able to apply the knitted strain sensors in practical applications, they must fulfill several requirements such as low hysteresis, linear resistance vs. strain relationship over a certain working range up to 40%, and stable sensor properties with high sensitivity [5].…”