Efficient Stabilization of Zero-Slope Walking for Bipedal Robots Following Their Passive Fixed-Point Trajectories

This paper presents an efficient method of stabilizing the gait of an underactuated biped with compliant legs and semicircular feet. First, the model is defined, incorporating elements that are often present in experimental biped robots. The biped's passive behavior is studied through numerical simulations that provide insight into the gravity's contribution as an energy input to the system. Based on this study, it is shown that an augmented biped -with the addition of a counterweight joint at the hip- is able to perform stable gaits with minimal input. This design is implemented easily as it does not require ankle torques; instead, both motors are mounted at the biped's hip. The control law used for the stabilization is the combination of virtual-gravity components with non-linear PD terms. The stable gaits performed by the augmented biped on level floor strongly resemble the passive gaits of the original biped walking on a slope, resulting in an efficient, natural-like motion of low transport cost.