Analysis of series elasticity in locomotion of a planar bipedal robot

The great promise of series elastic actuation as an effective mean to improve the efficiency of dynamic bipedal locomotion is challenged by the difficulties in properly exploiting the dynamics of the system, due to the mutual influence between inputs and stiffness of the elastic elements. Although numerical optimisation has proven to be a valid tool to approach this problem, the contexts in which the energy improvement justifies the greater design effort are not clear yet. To fill this gap, this work presents an extensive numerical study in which optimised walking and running gaits are compared for a planar bipedal robot, driven either by rigid or series elastic actuators, whose stiffness is selected concurrently with the input trajectories. The comparison shows that: i) the Cost of Transport relative to the soft robot with optimised stiffness is lower than that obtained for its rigid counterpart, especially in running; ii) the forward speed for which running is more efficient than walking is lower for the soft robot than for its rigid counterpart; iii) the soft robot with optimised stiffness can run significantly faster than its rigid counterpart.