This paper deals with the analysis of planar bipedal robots, based on passive dynamic walkers, which are actuated only by actuation of the ankle joints. An overview of the major design characteristics of such robots and their influence on the feasibility of a stable limit cycle is presented. It is shown that robots which are fully powered by ankle actuation require a mass ratio of at least 10:1 between the upper and lower limb to obtain sufficient ground clearance during the swing phase at a wide range of walking speeds. The effect and necessity of the offset in the footshapes of many passive dynamic walkers is shown and the influence of the moment of push off on the required energy injection is treated. The results of the analysis are supported by simulations with a dynamic model of such a robot. The simulated model exhibits a very natural looking gait and walks with a wide range of velocities at low mechanical cost of transport. Simulation results are provided which confirm that pushing off before the swing leg collides with the floor is energetically more efficient than pushing off after the impact as also known from previous literature.
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