The ocean world Europa is a prime target for exploration given its potential habitability [1]. We propose a mobile robotic system that is capable of autonomously traversing tens of meters to visit multiple sites of interest on a Europan analogue surface. Due to the topology of Europan terrain being largely unknown, it is desired that this mobility system traverse a large variety of terrain types. The mobility system should also be capable of crossing unstructured terrain in an autonomous manner given the communications limitations between Earth and Europa.A wheel-on-limb robotic rover is presented that may actively conform to terrain features up to 1.5 wheel diameters tall while driving. The robot uses a sampling-based motion planner to generate paths that leverage its unique locomotive capabilities. The planner assesses terrain hazards and wheel workspace limits as obstacles. It may also select a mobility mode based on predicted energy usage and the need for limb articulation on the terrain being traversed. This autonomous mobility was evaluated on chaotic salt-evaporite terrain found in Death Valley, CA, an analogue to the Europan surface. Over the course of 38 trials, the rover autonomously traversed 435m of extreme terrain while maintaining a rate of 0.64 traverse ending failures for every 10m driven.

This paper details a novel pressure-sinkage model for small-diameter, rigid wheels on dilative soils. Pressure-sinkage models are fundamental to the prediction of UGV mobility on deformable terrains. The proposed model builds on previous work in which the flat-plate pressure-sinkage assumption of classical terramechanics was shown to yield diminished accuracy for UGVs with wheels less than 50 cm in diameter. It has been shown that classical pressure-sinkage models can be modified by a diameter dependent term, yielding greatly increased accuracy. Here, an investigation into the effect of wheel width on the diameter-dependent model is detailed. Results from over 250 pressure-sinkage tests on three soils using 85 wheel geometries are summarized. The results of this investigation are used to create a comprehensive pressure-sinkage model for dilative soils that includes wheel width and diameter parameters. The physics of the model are visually validated with X-ray images of sub-surface soil deformation during the wheel indentation process. A comparison between the dilative soil pressure-sinkage model and a previously obtained model for compactive soils is also presented. The pressure-sinkage model presented here can be used to improve the accuracy of the terramechanics framework and UGV mobility predictions.

Applications ranging from planetary exploration to military operations require small unmanned ground vehicles to traverse deformable terrains such as sand or moist earth. Crossing such terrains can be difficult because failure to generate enough traction can result in immobilization and mission failure. Thus, the ability to accurately predict a vehicle's traction on deformable terrain is critical. Traditionally this has been accomplished via terramechanics based on Bekker theory. However, it has been shown that classical terramechanics loses considerable accuracy when applied to vehicles with wheels less than 50cm in diameter. This paper details the development of a modification to the pressure-sinkage relationship used in Bekker theory that explicitly includes a dependence on wheel diameter. The new model is integrated into a numerical simulation that predicts the tractive performance of an experimental unmanned ground vehicle (UGV). Field tests are performed on sandy terrain, and the results validate the simulated predictions. The new model is found to be significantly more accurate than previous models.