Design and testing of a flexible exoskeleton-to-shoe interface

Abstract

A lightweight minimalist lower-limb exoskeleton has been designed that reduces the metabolic cost of walking. Currently, this exoskeleton must be permanently attached to a shoe; holes are drilled into each new shoe used, a practice that is neither flexible nor cost-effective. A new attachment system is proposed to temporarily but securely connect the exoskeleton to shoes of various sizes. This exoskeleton-to-shoe interface is lightweight, adjustable for various shoe sizes, and easy to attach and remove. This interface is meant to increase the testing flexibility and commercial potential of the exoskeleton. After the interface was designed and built, the stiffness of the interface was measured and compared to the stiffness of the original rigid attachment. The stiffness was calculated using exoskeleton torque and the corresponding angle of attachment. Torque was calculated based on force applied by the exoskeleton, and the time-varying angle was found using motion capture. The results of these measurements suggest that at the tested frequencies of 0.5, 1, and 2 Hz the stiffness of the exoskeleton-to-shoe interface, which ranged from 8.082 Nm/° to 16.94 Nm/°, is greater than the stiffness of the control, which ranged from 6.143 Nm/° to 6.957 Nm/°. At all tested frequencies, the interface stiffness remained equal to or greater than the natural ankle stiffness during level ground walking. Since the interface stiffness is greater than the natural ankle stiffness, this flexible interface has acceptable stiffness. A flexible, lightweight, and size-variable exoskeleton-to-shoe interface with higher than natural ankle stiffness has the potential to be useful in both future research and eventual commercialization of the exoskeleton.