Humanlike ankle-foot complex for a biped robot

In this paper, we propose a design of a robotic ankle-foot complex based on the human functional-anatomic ankle-foot structure. The proposed foot consists of three links, two joints, and four plantar muscles, whose mechanical stiffness can be controlled by utilizing McKibben pneumatic actuators. With this structure, a deformable medial longitudinal arch in a human foot can be emulated. We developed a musculoskeletal biped robot to which the proposed feet are implemented and measured its walking motion, especially the deformation of the robot foot. It is found that the foot generates a truss mechanism and a windlass mechanism, which are important functions of a human foot for shock absorption and energy storage and reuse. We also conducted a walking experiment with various parameters of a plantar muscle's tonus to see how the tonus affects to ground reaction forces (GRFs) and its walking behavior. It is found that the GRF had two peaks as well as human walking and the shape of the GRF curve changes according to the tonus of the plantar muscle. We analyzed the impulse of GRF, finding out that a truss mechanism and a windlass mechanism works effectively with appropriate tonus of the plantar aponeurosis.