Testing

To understand sensor performance, the electrical properties of a number of sample sensors were explored in dynamic and static tensile load conditions (A. Atalay et al. 2017). Tensile force was applied in the course direction of the knit structure for characterization of the hybrid sensor and the fabric electrodes. 

Sensors were dynamically tested on a commercial electromechanical tester (Instron 5544A) . Capacitance was measured with a capacitance meter (Model 3000, GLK Instruments) connected to the integrated sensor leads with probes. Via a common I/O interface, (BNC-2111, National Instruments Corp. USA), the load, extension, and capacitance data were synchronously obtained and logged. All sensors characterized were produced in a standardized size (cut to 80 mm x 10 mm with integrated connectors) and have an active, stretchable area of 65 mm x 10 mm, due to the attachment of handles. Sensors were preconditioned by stretching to 100% applied strain for at minimum 10 cycles. 

Verification

Changes in dielecric thickness and electrode area play an important role in capacitance change as the sensor is strained. Figure 1 shows area change measurements of a rectangle of just conductive fabric, a rectangle of just silicone and the textile silicone hybrid sensor corresponding to applied strain. Figure 1b. shows a higher area increase for the hybrid sensor under applied strain due to the penetration of silicone through the mesh structure of the fabric, which fills the inherent air gaps within the fabric structure and resists perpendicular shrinkage.

Figure 1c. shows a 16% plastic deformation for the fabric only sample under repeated cycles of 120% strain. Introducing silicone into the fabric network reduced the plastic deformation to 7% under identical conditions. The silicone also prevents rolling of the fabric from its edges after the stretching and relaxation cycles creating a more dimensionally stable structure.

Figure a.	Figure b.
Figure c.	Figure d.
Figure 1. a) Representative sketch of area change for the sensor, the conductive fabric and the silicone. b) Area change as a function of strain from sensor, conductive fabric, and silicone. A second degree polynomial fit is applied to the data. c) Representative sketch of recovery after 150% strain for the sensor, the conductive fabric and the silicone. d) Plastic deformation percent for the sensor, conductive fabric, and silicone. A logarithmic fit is applied to the data.

 

Testing Data

The influence of dynamic strain on the electrical performance of the sensor was investigated by stretching and releasing the sensor at 100% strain and a speed of 24 mm/s (Figure 1). The relative change in capacitance  as a function of strain is highly linear based on a simple linear fit (R² = 0.999) and corresponding gauge factor is 1.23 (Figure 2).

Figure 1.	Figure 2.
Figure 1. Relative change in sensor capacitance upon triangular cyclic straining to 100% at 0.11Hz. Figure 2. Relative capacitance change as a function of applied strain for 20 cycles. A linear fit to part of the cycles with increasing strain is presented with a red dotted line, yielding a gauge factor of 1.23

 

Drifting characteristics of the sensor under static loading was also measured. Drift error was calculated as the change in the sensor capacitance response to a constant strain value. The drift values of the strain sensor found were 0.3%, 0.7%, 0.6%, and 0.5% for the strain levels of ε = 0.25, 0.50, 0.75, 1.0 respectively. 

Figure 3.

Static drift of the sensor under constant strain levels at ɛ=0.25, 0.5, 0.75, 1.0 holding for 20 seconds.

 

Other sensor performance experiments and results are listed in the table below. These experiments includes fatique performance at 1000 cycle to see long term usage performance of the sensor, sensor bandwith to see the covarage of human body activities which can reach upto 10 Hz. In addition, response time, hysteresis, resolution, and electromechanical failure characteristics also were investigated.

Applied test for characterization	Sensor results
Fatigue performance at 1000 cycle dynamic strain	5% and 12 % decrease on signal amplitude at 25% and 100% strain respectively
The temporal response of the sensor	<30ms (including any delays related to instrumentation)
Sensor frequency bandwidth at 10% applied strain	Decrease on signal amplitude starting from 27Hz
Max. Hysteresis	0%, 0.2%, 0.7%, 1.5%, 2.5% for the applied strains at 25%, 50%, 75%, 100% and 125% respectively
Resolution	0.54% and 1.24% at 25% and 100% strain respectively
Linearity	R2 = 0.999
Gauge Factor	1.23
Drift of the stain sensor under static loading for 20s	0.3%, 0.5% drift for 25% and 100% respectively
Electromechanical failure of the sensor	Signal loss at 170%, permanent deformation on sensor elasticity and handle joints at 220%