Passive dynamic walking has attracted attention in last decades, since it solves about energy efficiency problem of biped robots. Many latest biped robots have arc-shaped feet without the ankle to achieve energy-efficient and high-speed walking, from results of passive dynamic walking researches. However, these robots have difficulty to keep static states. Moreover, these robots have problems for motions such as a push-off, running, and jumping, since they cannot use ankle torque. As robot feet to solve these problems, flat feet with ankle elasticity and viscosity at ankles have attracted attention. Walking performance of the biped robots depends on ankle elasticity and viscosity. Hence, we are interested in ankle inertia from point of view of mechanical impedance. By optimization of ankle elasticity, viscosity, and inertia for biped walking, we consider that we can realize biped robots achieving energy-efficient and high-speed walking. However, the conventional flat feet cannot easily change ankle inertia since they have restrictions of design due to the foot size and weight. Therefore, we propose novel flat feet which has an elastic element, a viscous element, and an inerter of inertia elements at ankles. To achieve energy-efficient and high-speed walking, we can easily design ankle elasticity, viscosity, and inertia by proposed flat feet. In this paper, we show that biped robots with the proposed flat feet achieve energy-efficient and high-speed walking.

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