Energetics of passivity-based running with high-compliance series elastic actuation

Abstract

The efficiency of running gaits in nature results in large part from passive elastic oscillations on springy legs. In this paper, this principle is applied to robotic systems by endowing them with high compliance series elastic actuators in which the electric motors are decoupled from the joints by elastic elements. Periodic motor inputs excite the natural dynamic motion of the robot and create a passivity-based running motion. An optimisation algorithm minimised energy expenditure and estimated the necessary initial model states and the coefficients of a parameterised excitation function for the simulations of a two-dimensional hopping monopod and a planar bounding quadruped. Gait synthesis within this framework was analysed with respect to energy consumption, particularly as a function of running speed. Different solution groups were found, each of them corresponding to a characteristically different movement which proved to be most efficient for the corresponding speed range. This shines a different light on the meaning of ‘gait’ in the context of robotics, and directly contributes to a better understanding of the creation and exploitation of different modes of locomotion in legged robotics.