A 6 DoF, Wearable, Compliant Shoe Sensor for Total Ground Reaction Measurement

In this paper, we present a compliant six-axis sensor for total ground reaction measurement, the calibration methodology, and preliminary data. The sensor is intended for gait analysis and is designed to have minimal effect on natural gait. A triaxial optical force sensor is combined with an array of pressure sensing films to form a wearable compliant six-axis force/moment sensor. Two sensor units were developed for the toe and heel and used in two types of experiments: stepping on the sensors that are mounted on the ground and attaching the sensors under the shoe while walking. The data from the sensors are compared with measurements obtained from a standard force plate. The deflections induced by the sensor compliance exhibit a slight nonlinear force-deflection relation. Regardless of the nonlinear effects, the sensor is accurately calibrated with a linear least squares method. To see how well these nonlinearities could be calibrated for, a nonlinear calibration with a neural network was used. For the sensors attached to the floor, a linear calibration achieved an RMSE of 4.49% while the neural network achieves 2.68%. For the wearable sensor, the linear calibration RMSE was 9.39% and the neural network RMSE was 5.21%. When the orientation data of the sensors (measured by a motion capture system) were added as an input to the neural network calibration, the RMSE was reduced to 3.25%.