Torso inclination enables faster walking in a planar biped robot with passive ankles

There is a category of biped robots that are equipped with unactuated or passive ankles. We call them passive ankle walkers (PAWs). Because the unactuated ankle cannot provide the push-off at the end of stance phase as human ankles do, fast walking in PAWs is more challenging. In this paper, in order to realize fast walking in PAWs, we propose a simple strategy—torso inclination. To test this strategy, we studied a PAW model with simulation and prototype experiments. The simulation has shown how the torso inclination affects the walking speed and the energy efficiency of the PAW. Considering the “reality gap” problem of simulation, we have also experimentally tested this strategy with a real robot. By analyzing both the simulated model and the experimental results of the real robot, we identified the mechanism that accounts for fast walking in torso-inclined PAW