Wireless Ground Reaction Force Sensing System Using a Mechanically Decoupled Two-Dimensional Force Sensor

We present the design process of a mechanically decoupled two-dimensional force sensor (M2D) for the implementation of a wearable ground reaction force (GRF) sensing system. The proposed sensor is capable of sensing up to 1000 and
$ \pm \text{300 N}$
in the vertical GRF (vGRF) direction and the anterior–posterior GRF (apGRF) direction, respectively, with accuracies of
$\text{2.81} \pm \text{0.29}\%$
and
$\text{1.67} \pm \text{0.15}\%$
, respectively. With only two reflective optical sensors for each M2D, one for each axis, the mechanical structure of the M2D allows two-dimensional forces to be measured with an axial interference of approximately 1.37% and 2.39% and a repeatability of 0.95% and 1.35% for vGRFs and apGRFs, respectively. A wireless, wearable GRF sensing system was implemented using two M2Ds attached to the outsole of a shoe: one at the heel and the other at the forefoot. The normalized root mean square error value between a conventional stationary force plate and the proposed system for 30 consecutive steps of five subjects was approximately
$\text{4.93} \pm \text{2.56}\% $
and
$\text{4.22} \pm \text{2.12}\% $
for the vGRF and the apGRF, respectively.