  We present a hierarchical whole-body controller leveraging the full rigid
body dynamics of the wheeled bipedal robot Ascento. We derive closed-form
expressions for the dynamics of its kinematic loops in a way that readily
generalizes to more complex systems. The rolling constraint is incorporated
using a compact analytic solution based on rotation matrices. The non-minimum
phase balancing dynamics are accounted for by including a linear-quadratic
regulator as a motion task. Robustness when driving curves is increased by
regulating the lean angle as a function of the zero-moment point. The proposed
controller is computationally lightweight and significantly extends the
rough-terrain capabilities and robustness of the system, as we demonstrate in
several experiments.
