A comprehensive wheel-terrain contact model for planetary exploration rover design optimization

Abstract

In the context of the European Space Agency's (ESA) Aurora Programme there are two near future robotic missions which will provide survice mobility with wheeled rovers: ExoMars and Next Lunar Lander - NLL. These vehicles are the main motivation of this work; thery are supposed to drive with high performance over rough terrains containing basically rocks, sand and inclined slopes. Our contact model aims to describe the interaction between each rigid wheel of a rover and three different terrain types: 1) rigid surface, 2) complex-shaped rigid objects (representing rocks), and 3) soft soil. Transition among these three "meta-states" is permitted for each wheel of a multi-wheeled rover. The uneven terrain is smothed on the space domain giving a continuous three-dimensional surface; the discontinuities are depicted by the rigid objects, and its collision is detected by the proper computation of the contact plane and the contact forces. The uneven terrain can be as rigid as the rocks (modeled by Coulomb's friction law) or as soft as sand (modeled by Bekker's equations). Optimization of the mechanical structure is a crucial task in the design phase to achieve high performance. However, there are three essential features which are required to perform batch simulations with the optimization process: stability, robustness and speed of the simulations. These required features have guided the selection of the used impact/rolling modes. The internal states and its transitions are thoroughly explained to clarify the main difficulties in the batch simulations. The contact model is partially validated by drawbar-pull experiments in soft soil testbed with a breadboard model of the ExoMars rover. Future advanced versions of the contact model are focusing on flexible wheel modeling, and first attempts in that direction are also provided and commented.