The Fluid Field SLIP Model: Terrestrial-Aquatic Dynamic Legged Locomotion

This paper describes the development of a single reduced-order dynamic model that captures running on land, running while submerged, and for the first time swimming on the surface of water. By capturing the effect of fluid forces on both the body and the leg, the Spring-Loaded Inverted Pendulum (SLIP) model is extended to operate in amphibious and aquatic regimes. Three distinct stable motion patterns, or dog-paddle type gaits are identified when swimming at the air-water interface. The model shows that, for surfaces swimming, alteration of the leg stroke frequency and length produces gaits that are either smooth and efficient or are vertically oscillatory and exhibit rapid disturbance rejection. Furthermore, when the model is examined at the physical parameter values corresponding to dogs (specifically Labrador Retrievers), the animal-based control parameters demonstrate nearly optimal performance.