Abstract (from IROS 2023 paper)

A new control paradigm that uses angular momentum as a key state variable in the linear inverted pendulum model has opened a plethora of possibilities for the control of bipedal systems. This new paradigm, known as the ALIP model, has been validated in cases where the robot’s center of mass evolves in a plane. Walking up or down stairs or stepping onto or off an object may violate this assumption. In this extended abstract, we explore a variation on the ALIP model that allows the length of the virtual pendulum formed by the robot’s center of mass and stance foot to follow smooth trajectories during a step. This model is coupled with a control strategy constructed from a novel combination of virtual constraint-based control and an MPC algorithm to stabilize a stair climbing gait. Simulations on a 20-degree of freedom model of the Cassie biped show that the controller is able to achieve a periodic gait.