Actively Articulated Wheel-on-Limb Mobility for Traversing Europa Analogue Terrain

Exploring the water-rich surface of Europa is a desirable planetary exploration objective for the coming decades. Knowledge of the Europan terrain is relatively uncertain and therefore to perform mobile surface operations a versatile mobility system capable of adapting its locomotion is desirable. Our work summarizes preliminary efforts to evaluate a variety of wheel-on-limb mobility modes on Europan analogue terrain populated with unstructured salt-evaporite formations. A redundant wheel-on-limb quadruped, RoboSimian, has been adapted to perform traverses over unstructured terrain with obstacles up to 50 cm tall (1.6 times its wheel diameter). Multiple mobility modes that are combinations of wheeled and limbed motion primitives are proposed and evaluated over varying roughness terrain as a function of energy usage. In combination with these mobility modes, a mode-agnostic actively articulated suspension control scheme that maintains ground traction, regulates chassis orientation and prevents suspension saturation is presented. The controller uses an inverse kinematics solver that leverages pre-computed lookup-tables for each limb. This is shown to be simple yet effective in the field and is able to scale to a robot with 32 degrees-of-freedom.