Design and Experimental Validation of a Velocity Decomposition-Based Controller for Underactuated Planar Bipeds

This letter extends the use of the velocity decomposition metric for underactuated mechanical systems to the design of an enhanced hybrid zero dynamics (HZD)-based controller for biped robots. The metric has been used to quantify the control authority of underactuated planar bipeds by measuring the amount of nonlinear coupling between the actuated and unactuated degrees of freedom. Here, an offset to the biped's desired torso angle is applied in proportion to the error in the unactuated velocity, as determined through the decomposition. This offset aids in rejecting both acceleration and deceleration disturbances to the unactuated velocity. Simulation results with a point-foot, three-link planar biped show that the velocity decomposition-enhanced controller has identical performance to transverse linearization feedback control and outperforms both the unmodified HZD-based controller and an alternative hip velocity-enhanced controller in rejecting disturbances. Experimental results with the planar, point-foot, five-link biped ERNIE confirm these same trends. The velocity decomposition-enhanced controller enables return to steady-state walking in fewer steps following a disturbance and with lower average and peak torques.