Self-Contained 2-DOF Ankle-Foot Prosthesis With Low-Inertia Extremity for Agile Walking on Uneven Terrain

Accepting the heterogeneity of amputees and enabling them to walk comfortably using little effort with light weights is the main goal of prostheses. Moreover, providing the capability of walking on various ground conditions including uneven surfaces and obstacles is a challenging but important requirement. This letter presents a 2-DOF ankle-foot prosthesis having a compact size and low inertia. It has two active degrees of freedom in the sagittal and coronal plane using a parallel linkage mechanism. A unique spring mechanism strengthens the propulsion of walking without increasing the mass of the foot. In addition, heavy elements such as actuators, springs, batteries, and controller circuits are placed at the proximal part to reduce the inertia felt by patients and to minimize the metabolic cost. Capacitive type force sensors placed at the shank frame enable the precise measurement of force from the ground without increasing foot mass. The mass including the battery is 2.25 kg which is lightweight among 2-DOF ankle-foot prostheses and the low height of 204 mm allows a broad range of amputees with various physical conditions to use it. This letter describes a gait algorithm for uneven ground by using impedance control. The force sensor test and the preliminary walking test on uneven ground verify the effectiveness of the proposed prosthesis.