Discrete Ground Contact Events as a Gait Synthesis Mechanism in Legged Robots*

Periodic legged locomotion tasks for robotic walkers require persistent interaction with the ground.
Despite this, many models employed to generate gaits treat contact with the ground as an unpredictable disturbance that interferes with reference tracking.
Event-selected systems, non-smooth dynamical systems recently formalized by Burden and Revzen to describe legged robots with simultaneous contacts, explicitly allows inclusion of ground contact at irregular times.
It anticipates a control scheme to generate a stable periodic solution that is structurally stable despite poor knowledge of the underlying dynamics of the robots, or the unknown features of the ground.
Using control, discontinuous changes between smooth dynamics occur only finitely often per period of a desired solution, taken in this instance to be a polygonal curve through state space; with only low bandwidth and designed event surfaces, a desired curve is stabilized.
In order to show that the regime is sufficient to generate a stride, it was implemented on a RHex-class robot to generate an alternating sequence of foot falls using only event selected vector fields, creating a useful behavior exclusively through contacts.