Figure 2:

Dynamic walking model with controlled lateral foot placement and external foot rotation. (A) The model has three unactuated degrees of freedom, roll motion about the stance ankle, and one for each leg rotation in the sagittal plane. (B) The model is passively stable in the forward direction, but has one unstable mode in the lateral direction, shown with pole locations of the linearized step-to-step dynamics, relative to the unit circle. (C) The model also has active control over external rotation of the leg to alter heading, and splaying of the legs for lateral foot placement, both applied symmetrically to both legs. Linear controllers can stabilize the system, using only lateral foot placement, only external rotation, or both methods together. When perturbed to the right, the model may be stabilized by stepping out to the right, or externally rotating toward the right, or both.