Fig. 6: Trapezoidal wave input versus time (upper) and corresponding electrical response of knitted piezoresistive fabric sensor versus time (lower)

Results have shown that applying a trapezoidal wave as mechanical input (see Fig. 6), the electrical resistance value increases up to a certain value, and then it decreases to a steady-state value. This relaxing time is too long to make fabric sensors suitable for monitoring real time human body movements. When a decreasing input is applied, the electrical resistance shows a little positive peak before decreasing to the settling value.

In order to face the problem related to the non-linearity showed in Fig. 6, with the aim of using the fabric sensors in realm time, that requires a correction of sensor delay, a model have been formulated in [6].

In Fig. 7 the knitted piezoresistive sensor response has been compared to the signal obtained with Biopac pneumograph in order to monitor the respiratory activity, both the signals have been acquired simultaneously.

Fig. 7: Signal derived by KPF sensor (upper) and Biopac pneumograph (lower) during normal respiration.