Design and Evaluation of a 2-DOF Instrumented Platform for Estimation of the Ankle Mechanical Impedance in the Sagittal and Frontal Planes

This paper describes the fabrication and initial evaluation of a vibrating platform with two degrees of freedom (DOF) to estimate the human ankle's mechanical impedance in two DOFs; namely dorsiflexion-plantarflexion (DP) and inversion-eversion (IE). The device consists of an actuation and a force plate module. The actuation module generates torque perturbations up to 168 N·m in DP and 26 N·m in IE in the force plate module using Bowden cables. This provides a low-profile system that can be installed in a walkway. The frame of the force plate module rotates in two DOFs, applying torque perturbations to the human ankle in DP and IE. The ankle's rotations are measured using a motion capture camera system. The analytical and numerical approaches for estimation of the ankle's torques, rotations, and impedances are presented. A system validation using a mockup was conducted to verify the system's ability to estimate the impedance of a physical system in two DOFs. The developed system was capable of identifying the mockup's physical properties up to 15 Hz. The mockup's impedance magnitude at 0.9 Hz using a stochastic identification method was shown to be within 1.68% and 0.54% of the mockup's stiffness in DP and IE, respectively.