  For humans, fast, efficient walking over flat ground represents the vast
majority of locomotion that an individual experiences on a daily basis, and for
an effective, real-world humanoid robot the same will likely be the case. In
this work, we propose a locomotion controller for efficient walking over
near-flat ground using a relatively simple, model-based controller that
utilizes a novel combination of several interesting design features including
an ALIP-based step adjustment strategy, stance leg length control as an
alternative to center of mass height control, and rolling contact for
heel-to-toe motion of the stance foot. We then present the results of this
controller on our robot Nadia, both in simulation and on hardware. These
results include validation of this controller's ability to perform fast,
reliable forward walking at 0.75 m/s along with backwards walking,
side-stepping, turning in place, and push recovery. We also present an
efficiency comparison between the proposed control strategy and our baseline
walking controller over three steady-state walking speeds. Lastly, we
demonstrate some of the benefits of utilizing rolling contact in the stance
foot, specifically the reduction of necessary positive and negative work
throughout the stride.
