Hybrid toe and heel joints for biped/humanoid robots for natural gait

A biped/humanoid robot with toe and heel joints is capable of more natural locomotion due to the additional degrees of freedom available. Though passive spring loaded joints can help to some extent, they introduce constraints between the joint torque and the joint angular displacement. In this paper, we propose to use hybrid active/passive toe and heel joints in order to accommodate a wide range of step lengths and speeds as well as uneven terrain. The toe joints are spring loaded to minimize the maximum torque required and thus the amount of energy consumed when the toe actuator is maintaining the desired Zero Moment Point (ZMP). By controlling the torque produced at the toe joints, the ZMP of the biped robot is allowed to move smoothly from heel to toe during Single Support Phase (SSP) and from the toe of the trailing foot to the leading foot during Double Support Phase (DSP). Two different ZMP trajectories during toe support phase are compared. The effectiveness of the proposed strategy is demonstrated through numerical simulations on a biped robot model. Finally, the use of heel joint is discussed in relation to providing additional support during double support phase, allowing larger step lengths, as well as absorbing impact during heel strike.