On the energetics of a switchable parallel elastic actuator design for monopedal running

Switchable Parallel Elastic Actuators (S-PEAs) represent a novel way for introducing compliant elements in legged robots. This paper investigates the energetic effects of a S-PEA design on running gaits in the context of the monopedal robot SPEAR, which has its knee driven by a S-PEA. In our implementation of the S-PEA concept, a mechanical switch located at the foot engages the spring during stance to store energy and disengages it during flight to allow for precise control of the leg´s configuration without interfering with the spring. Parameter optimization indicates that the S-PEA configuration outperforms common Parallel Elastic Actuator (PEA) designs by 10 - 20% in both hopping-in-place and hopping-forward gaits, and this advantage increases to about 40% at higher running velocities. It is deduced that the ability of SPEAR to adjust the effective stiffness of the leg without requiring additional energy, by simply changing the knee angle prior touchdown, explains part of this advantage.