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.

Keywords: conductive yarns; knitted sensor; performance evaluation; stitched sensor; textile strain sensors.

Figure 1

Single jersey knit stitch showing (a) course and (b) wale direction and their density (number per unit length).

Figure 2

Inlaid yarns (gray) embedded in a double jersey structure [32].

Figure 3

(a) Double lockstitch embroidery. The upper thread can be conductive; (b) Tailored fiber placement (TFP) embroidery; useful for fixating of functional fibers [34].

Figure 4

Schematic response of sensor after applied strain steps (black pulses).

Figure 5

Non-linearity and hysteresis examples.

Figure 6

Definitions of: (a) Working range; (b) Horizontal and vertical hysteresis.

Figure 7

(a) Knitted sensor [35]; (b) Knitted coil sensors in arm sleeve [46].

Figure 8

(a) Knitted single warp steel and carbon structures; (b,c) tubular structures [53].

Figure 9

(a) 1 × 1 rib knitted structure; and (b) its elongation behaviour [55].

Figure 10

(a) Loop-wise embedded conductive yarn in knitted interlock structure; (b) electromechanical response [38].

Figure 11

Effect of stitch size on electromechanical behaviour of full cardigan fabrics. (a) With respect to strain and (b) with respect to time [62].

Figure 12

Effect of yarn materials [62].

Figure 13

(a,b) Alternating courses of conductive and non-conductive yarns; (c) resistance change [64].

Figure 14

Course-wise response of (a) Cotton-steel knitted sensor and (b) nylon plated material; Wale-wise (transverse) responses of (c) cotton-steel and (d) nylon plated material [65].

Figure 15

Images of knitted rib structures. (a) 1 × 1, (b) 1 × 3, (c) 1 × 2 and (d) 2 × 2 structure [66].

Figure 16

(a) Resistance and (b) gauge factor rectangular knitted sensors of different length and 6width; Based on [67].

Figure 17

(a,b) Unit cell of knit [68]; (c) Resistor network approach [53].

Figure 18

Simplified equivalent network model [68].

Figure 19

(a) Coverstitch structure and (b) resistance versus displacement curve [37].

Figure 20

(a) Zigzag stitch and (b) corresponding resistance plot; (c) Coverstitch and (d) corresponding resistance plot [42].