Development of a Two-Axis Robotic Platform for the Characterization of Two-Dimensional Ankle Mechanics

Accurate characterization of ankle mechanics in lower extremity function is essential to advance the design and control of robots physically interacting with the human lower extremities, such as lower limb exoskeletons, active orthoses, and prostheses. This paper presents a two-axis robotic platform developed for the characterization of important neuromechanical properties of the human ankle, namely mechanical impedance and energetic passivity. This robotic platform is capable of simulating a wide range of mechanical (haptic) environments as well as applying precisely controlled perturbations to the ankle in 2 degrees-of-freedom (DOF) spanning both the sagittal and frontal planes. These features provide us with a unique way to characterize two-dimensional ankle mechanics while humans perform various lower extremity tasks in realistic physical environments. A series of validation experiments demonstrated that the platform can provide rapid perturbations up to an angular velocity of 100°/s with an error less than 0.1° even under excessive loading and simulate a wide range of haptic environments, from compliant to highly stiff environments, with an error less than 2.1% of the commanded values. A pilot human study demonstrated that the robotic platform could accurately quantify intrinsic ankle impedance in 2 DOFs with reliability higher than 97.5%. This study also confirmed that the platform could be utilized to quantify energetic passivity of the ankle in 2 DOFs. Implications for the design and control of lower extremity robots are discussed.