Velocity disturbance rejection for planar bipeds walking with HZD-based control

Robots walking under hybrid zero dynamics (HZD)-based control are susceptible to velocity disturbances because the controller is typically designed for one speed. LQR-based orbital stabilization control is one means to address this issue using feedback on the unactuated velocity. The approach, though, is difficult to implement in real time experimentally even on planar bipeds with relatively few links. This work extracts simple heuristics from simulated planar bipeds rejecting velocity disturbances under orbital stabilization control to approximate that functionality. The heuristics are layered on top of traditional HZD-based control of a five-link planar biped robot for experimental validation. Results show that the heuristically modified controller yields more efficient and more stable walking for the biped than does HZD-based control alone. It also enables rejection of larger decelerating disturbances and more rapid return to the desired walking cycle.