Swing leg retraction using virtual apex method for the ParkourBot climbing robot

This paper describes a simple control scheme for the two-legged dynamic climbing robot, the ParkourBot. Inspired by the swing leg retraction (SLR) method used on running robots, we propose to implement an SLR controller that does not require an actual apex crossing event as an initiator. The familiar SLR control for running is initiated when the center of mass (CoM) of the robot reaches the apex of its trajectory. Often, dynamic climbing gaits do not reach an apex. We therefore define an alternative initiator named, the virtual apex. We evaluate two simple models, the conservative spring loaded inverted pendulum (SLIP) model, and the more realistic non-conservative SLIP model. We show, in simulation, that using SLR we are able to stabilize and control the otherwise unstable conservative SLIP model. We also show, that using the SLR significantly increases the robustness of the non-conservative SLIP model. Finally, we validate our results through experiments on the ParkourBot climbing robot.