Design and Characterization of a Nonlinear Stiffening Spring for a Series-Elastic Ankle-Foot Prosthesis

Abstract

Over 150,000 patients in the United States undergo a lower extremity amputation every year. Powered ankle-foot prostheses greatly improve amputees’ walking experiences by replicating the biological motion of human gait. Series elastic actuators improve the prostheses, simulating the function of a biological tendon by storing and releasing energy during gait, reducing the mechanical work required by the motor and transmission system. This thesis aims to describe the preliminary design and characterization of a nonlinear stiffening spring, which acts as the elastic element of a powered transtibial prosthesis with cycloidal drive transmission. A mathematical model for angular deflection of the spring wrapping around a cylindrical surface was created, and finite-element analyses were performed to analyze the impact of individual design parameters on the spring stiffness. An experimental setup was designed and built to measure the deflection angle of the spring as a vertical force is applied. Three springs made from nylon 6/6 were tested in this thesis at thicknesses of 26 mm, 27.5 mm, and 29 mm, which yielded a 931.4±3.7 Nm/rad, 1076±5 Nm/rad, and 1234±5 Nm/rad stiffness, respectively. This lays the groundwork for the development of a unidirectional carbon fiber foot spring whose high strength-to-weight ratio is able to achieve the target stiffness while minimizing weight.