E-Knitted Textile with Polymer Optical Fibers for Friction and Pressure Monitoring in Socks

Abstract

The objective of this paper is to study the ability of polymer optical fiber (POF) to be inserted in a knitted fabric and to measure both pressure and friction when walking. Firstly, POF, marketed and in development, have been compared in terms of the required mechanical properties for the insertion of the fiber directly into a knitted fabric on an industrial scale, i.e. elongation, bending rigidity, and minimum bending radius before plastic deformation. Secondly, the chosen optical fiber was inserted inside several types of knitted fabric and was shown to be sensitive to friction and compression. The knitted structure with the highest sensitivity has been chosen for sock prototype manufacturing. Finally, a feasibility study with an instrumented sock showed that it is possible to detect the different phases of walking in terms of compression and friction.

Keywords: friction sensor; gait analysis; plantar pressure; polymer optical fiber; pressure sensor; smart textiles.

Figure 1

(a) Example of a knitted structure with loops (single jersey); and (b) the same knitted structure with an inlay yarn (the ground yarn making loops is shown in black and the inlay yarn in grey).

Figure 2

Principle of the bending behavior test.

Figure 3

(a) Picture of the measurement system and schematization of the testing configuration for (b) compression tests and (c) friction tests.

Figure 4

Load-strain curve for the three studied optical fibers: Empa bi-component (black), Empa Geniomer^® (dark grey) and GigaPOF 50-SR (soft grey).

Figure 5

Evaluation of the plastic behavior of optical fibers under bending, with a radius of 1 mm: Empa Geniomer^® (a) before testing and (d) 2 min after testing; Empa bi-component (b) before testing and (e) 2 min after testing; GigaPOF 50-SR (c) before testing and (f) 2 min after testing.

Figure 6

Pictures of the knitted fabrics: (a) single jersey; (b) 1 × 1 rib; and (c) 1 × 1 interlock fabrics. The scale bar indicates 10 mm.

Figure 7

Repeatability of the measurement: evolution of the compression load (a) and irradiance loss (b) for a compression test (example with single jersey under a nominal load of 3 N). The black line represents the average curve and the gray area the standard deviation.

Figure 8

Reproducibility of the measurement: evolution of the compression load (a) and irradiance (b) for a compression test (example with single jersey under a nominal load of 3 N). The black line represents the average curve and the gray area the standard deviation.

Figure 9

Evolution of the loss of irradiance relative to the applied compression load for the three knitted structures with the error bars representing the standard deviation. The dotted lines correspond to the second order fitting curves.

Figure 10

Repeatability of the measurements: (a) evolution of the normal force and (b) evolution of the loss of irradiance relative to the displacement of the slider for five repetitions of the measurement on the same single jersey sample under a nominal load of 10 N. The black line represents the average curve and the gray area the standard deviation.

Figure 11

Reproducibility of measurements: (a) evolution of the normal force and (b) evolution of the loss of irradiance relative to the displacement of the slider for five samples of single jersey under a nominal load of 10 N. The black line represents the average curve and the gray area the standard deviation.

Figure 12

Evolution of (a) the normal load, (b) tangential load, (c) coefficient of friction, and (d) the loss of irradiance relative to displacement for the single jersey knit under a nominal load of 10 N. The grey line shows the forward and the black the backward cycle of the slider. Each curve corresponds to the average of 25 measurements.

Figure 13

Evolution of the irradiance loss relative to (a) the normal load applied and (b) the tangential load for the three knitted fabrics, with the error bars representing the standard deviation.

Figure 14

Deflection of the POF at the back of the slider in the single jersey fabric. The direction of movement is indicated by the arrow.

Figure 15

Deflection values of the POF at the back of the slider for the three fabrics.

Figure 16

Comparison between the evolution of the irradiance loss as a function of the normal load during friction (F_norm) or compression tests (F_comp) for (a) single jersey; (b) 1 × 1 rib; (c) 1 × 1 interlock, with the error bars representing the standard deviation.

Figure 17

(a) Picture of the knitted sock with the three optical fibers; (b) magnification of the optical fiber at the heel position; (c) setting up the accelerometer.

Figure 18

Evolution of the irradiance loss during walking at 2 km/h for three optical fibers placed at three different positions under the feet, for three walking steps. Each curve is the average of five measurements.

Figure 19

Evolution as a function of time of (a) the acceleration signals and (b) the light signals during walking at 2 km/h for three optical fibers placed at different positions under the foot and the corresponding step of the walking gait. Each curve is the average of five measurements. The plain lines represent the average curves and the gray areas the standard deviation.