A Novel Sensorised Insole for Sensing Feet Pressure Distributions

Abstract

Wearable sensors are gaining in popularity because they enable outdoor experimental monitoring. This paper presents a cost-effective sensorised insole based on a mesh of tactile capacitive sensors. Each sensor's spatial resolution is about 4 taxels/cm 2 in order to have an accurate reconstruction of the contact pressure distribution. As a consequence, the insole provides information such as contact forces, moments, and centre of pressure. To retrieve this information, a calibration technique that fuses measurements from a vacuum chamber and shoes equipped with force/torque sensors is proposed. The validation analysis shows that the best performance achieved a root mean square error (RMSE) of about 7 N for the contact forces and 2 N m for the contact moments when using the force/torque shoe data as ground truth. Thus, the insole may be an alternative to force/torque sensors for certain applications, with a considerably more cost-effective and less invasive hardware.

Keywords: capacitive sensors; pressure distribution; sensorised insole; tactile sensors array; wearable sensors.

Figure 1

(a) Main components of the iCub skin. (b) Array of capacitive sensors. Each sensor implements 12 taxels and hosts the capacitive transduction electronics. (c) Deformable dielectric with conductive top layer. (d) Micro-controller tactile board (MTB).

Figure 2

Printed circuit boards (PCBs) covered by the skin, deformed under contact pressure.

Figure 3

(a) Calibration setup based on vacuum bags. (b) Calibration setup based on isolation chamber.

Figure 4

(a) Insoles prototype. (b) Inner part of the insole containing the gaps that accommodate the digital converter integrated circuits. (c) Computer-Aided Drafting (CAD) design of the support for the electronics. (d) Micro-controller tactile boards (MTBs) consistency: one for each patch of the insoles.

Figure 5

(a) Vacuum bag experiment. The pressure inside the bag was reduced by using a vacuum pump. (b) Sensorised shoes equipped with force/torque (FT) sensors and insoles.

Figure 6

Capacitances measured by one triangle (i.e., 10 taxels) and pressures measured by the pressure sensor during the vacuum bag experiment that consisted of three cycles of pressure reduction and increase.

Figure 7

Capacitances measured by one triangle (i.e., 10 taxels), vertical force and horizontal moments measured by the sensorised shoes.

Figure 8

Taxel activation of the left insole in the experiment with sensorised shoes.

Figure 9

Vertical force $f_z$ and horizontal moments $m_x$ and $m_y$ tracking. (a) Task T3. (b) Task T4.

Figure 10

Zoomed view of the tracking error to highlight the delay between FT sensors and insole estimations.

Figure 11

Center of pressure (CoP) tracking. (a) Task T3. (b) Task T4.

Figure 12

Online visualisation tool for the insoles. On the left-hand side, the comparison between centres of pressure estimated by force/torque (FT) sensors and insole. In the middle, estimation of vertical contact forces $f_z \left(\mathrm{N}\right)$ and moments $m_x$, $m_y$$\left(\mathrm{N}\mathrm{m}\right)$ using the calibrated insole. On the right-hand side, the pressure distribution.