Implementation of a Natural Dynamic Controller on an Underactuated CompassBiped Robot Natural dynamic controllers aim to perform the desired task by exploiting the natural dynamics of the system. This can be accomplished by generating torque patterns to actuate the system rather than accurately following a predefined trajectory. We have previously demonstrated natural dynamic control of compassbiped in simulations. Here we demonstrate successful implementation of this dynamic controller on an underactuated compassbiped robot. The parameters of the controller, and in particular the magnitude and timing of torque primitives, were optimized using multiobjective optimization via genetic algorithm, accounting for speed and energy efficiency. While the current implementation is in openloop, this strategy can be extended to include feedback to enhance walking over a wide range of terrains. This proofof concept provides the basis for future extensions to more complex robots.

Local stability of PD controlled bipedal walking robots Abstract   We establish stability results for PD tracking control laws in bipedal walking robots. Stability of PD control laws for continuous robotic systems is an established result, and we extend this for hybrid robotic systems, an alternating sequence of continuous and discrete events. Bipedal robots have the legswing as the continuous event, and the footstrike as the discrete event. In addition, bipeds largely have underactuations due to the interactions between feet and ground. For each continuous event, we establish that the convergence rate of the tracking error can be regulated via appropriate tuning of the PD gains; and for each discrete event, we establish that this convergence rate sufficiently overcomes the nonlinear impacts by assumptions on the hybrid zero dynamics. The main contributions are (1) Extension of the stability results of PD control laws for underactuated robotic systems, and (2) Exponential ultimate boundedness of hybrid periodic orbits under the assumption of exponential stability of their projections to the hybrid zero dynamics. Towards the end, we will validate these results in a 2link bipedal walker in simulation.