This paper focuses on the use of passive dynamics to achieve efficient walking with simple mechanisms. A torso is added to a biped walker; and hip actuators, instead of ankle actuators, are used. A numerical approach is used to find the optimal control trajectories. A comparison between the cost functions of simple feedback control and optimal control is presented. Next, springs are added to the biped walking model at the hip joints to demonstrate the advantage of hip springs in terms of energy cost and ground conditions. The comparison between the torque costs with and without hip springs indicates that hip springs reduce the torque cost, particularly at a high walking speed.

To date, most passive walkers have been designed with arc-shaped feet rigidly attached to the legs. However, the friction torque against yaw is often insufficient because of their contact conditions with the ground. We developed a simple 3D straight-legged passive walker with flat feet and ankle springs. Flat feet were used to gain enough scrubbing friction to oppose unstable yaw motion. Springs were attached to the ankle to produce torque resulting in roll and pitch motions that mimic those of 3D passive walkers with arc-shaped feet, while the friction torque against yaw should be sufficient. The spring constant for the roll motion at the stance ankle is obviously an important factor in enabling the straight-legged robot to rock adequately from side to side to avoid problematic scuffing of the swing leg so it can swing forward. We used numerical simulations to determine the value of the spring constant. Experimental results indicated that our 3D straight-legged passive walker with a 0.77-m leg can walk more than 2 m at about 0.46 m/s.

This study aims at a design technique of energy-efficient biped walking robots on level ground with simple mechanisms. To do this, we focus on the passive dynamic walkers which can walk stably down a shallow slope without actuators and controllers. On level ground, active walking should be studied because the mechanical energy is mainly lost through the swing-leg impacts with the ground. In this paper, numerical simulations show that planar biped robots with torso can walk efficiently on level ground over a wide range of speed by only using hip actuators. The hip actuators are used for a torso and swing-leg control based on passive-dynamic walking. The torso is used to generate active power replacing gravity used in the case of the passive walk. The swing-leg control is introduced to walk stably over a wide range of speed.

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