  A hopping leg, no matter in legged animals or humans, usually behaves like a
spring during the periodic hopping. Hopping like a spring is efficient and
without the requirement of complicated control algorithms. Position and force
control are two main methods to realize such a spring-like behaviour. The
position control usually consumes the torque resources to ensure the position
accuracy and compensate the tracking errors. In comparison, the force control
strategy is able to maintain a high elasticity. Currently, the position and
force control both leads to the discount of motor saturation ratio as well as
the bandwidth of the control system, and thus attenuates the performance of the
actuator. To augment the performance, this letter proposes a motor saturation
strategy based on the force control to maximize the output torque of the
actuator and realize the continuous hopping motion with natural dynamics. The
proposed strategy is able to maximize the saturation ratio of motor and thus
maximize the foot clearance of the single leg. The dynamics of the two-mass
model is utilized to increase the force bandwidth and the performance of the
actuator. A single leg with two degrees of freedom is designed as the
experiment platform. The actuator consists of a powerful electric motor, a
harmonic gear and encoder. The effectiveness of this method is verified through
simulations and experiments using a robotic leg actuated by powerful high
reduction ratio actuators.
