A Novel Model of Interaction Dynamics between Legged Robots and Deformable Terrain

Navigating natural environments with deformable terrain is a difficult challenge in robotics. Understanding the interaction dynamics between robots and such terrain is an important first step in enabling them to explore these environments. Terramechanics models are largely developed and tested on wheeled and tracked platforms, but with the advent of readily available lightweight legged robots, developing an understanding of how robot feet interact with the terrain becomes increasingly important. Works on estimating terramechanical properties of deformable sands and soils use an underlying assumption that translation of the robot foot along the surface of the terrain is due to internal shear deformation of the soil. We show that for lightweight legged robots, this is not the case. Shear forces acting on the foot of a robot during a stride are not accurately predicted by the widely-used Janosi-Hanamoto formula. We propose a new model in which two forces acting on the foot dominate the foot-terrain interaction - gross sliding friction and bulldozing resistance - and propose a model of how these forces act on the foot. We test this model on multiple soil types with different foot materials. Experimental data, collected on a testbench equipped with actuators and sensors identical to those deployed on a robot in the field, is used to validate our proposed model.