Force control for planar spring-mass running

In this paper, we present a novel control strategy for spring-mass running gaits which is robust to disturbances, while still utilizing the passive dynamic behavior of the mechanical model for energy economy. Our strategy combines two ideas: a flight phase strategy, which commands a hip angle trajectory prior to touchdown, and a stance phase strategy, which treats the spring-mass system as a force-controlled actuator and commands forces according to an ideal model of the passive dynamics. This combined strategy is self-stable for changes in ground height or ground impedance, and thus does not require an accurate ground model. Our strategy is promising for robotics applications, because there is a clear distinction between the passive dynamic behavior and the active controller, it does not require sensing of the environment, and it is based on a sound theoretical background that is compatible with existing high-level controllers.