It is necessary for a robotic leg to behave like a spring to realize a periodic hopping, since it can be efficient and does not require complicated control algorithm. However, the impact force makes the realization of periodic hopping more challenging. In this letter, an optimal landing strategy for a hopping leg is proposed, which can realize continuous hopping motion only by natural dynamics. The proposed strategy can reduce the foot landing velocity to zero and thus minimize the impact force. The formulation to derive the optimal condition is derived theoretically based on two-mass leg model, and its effectiveness is verified through various simulations and experiments using a series elastic actuator-driven robot leg.
