Energy-Efficient Locomotion Generation and Theoretical Analysis of a Quasi-Passive Dynamic Walker

This Letter presents a robot walking control method, which we call “quasi-passive dynamic walking.” The method is targeted at underactuated legged robots and applied to obtain energy-efficient limit cycle gait on level ground. To achieve efficient locomotion, as well as to overcome the underactuation of the system, two key points are implemented into this method to positively utilize the passive dynamics of the system. The first one is to initialize the walker at the fixed Poincaré section obtained from passive dynamic walking on a gentle downhill. The second one is to indirectly excite the hip angle by periodically oscillating a wobbling mass, which is attached to the body frame. The walker is, therefore, able to step forward on level ground without any torque actuation. Moreover, the phase diagram of the generated gait is entrained to limit cycle by the periodical oscillation of the wobbling mass. Numerical simulations and theoretical analysis are conducted to evaluate the efficiency as well as the local stability of the gait. Our control method enables underactuated legged robots to walk extremely efficiently on level ground with only one actuator, which provides the easiness for implementation on real machines.