3D Kinematic Modeling and Evaluation of Rough-Terrain Locomotion Modes for an ExoMars-like Mobility Subsystem

Abstract

One of the main challenges of planetary rover missions is the traversability of rough terrain areas, steep slopes and very soft soil fields. In the past, rovers repeatedly struggled while driving especially in these soft soils, with the Spirit rover even having to be abandoned. With ESA's upcoming ExoMars mission, a rover with an articulated suspension will land on Mars in 2021 that offers additional degrees of freedom through its actuated deployment joints. A full-body kinematic model allows to take full advantage of such a locomotion system. In this thesis, such a model was derived for EXM-BB2, a mobility subsystem prototype for the ExoMars rover. The model allows to specify desired body velocities and additional constraints on joint rates to achieve desired motions and includes a slip formulation on wheel level. Additionally, kinematic redundancies can be exploited by the introduction of secondary sub-tasks through null-space projection. Based on this model, two locomotion modes were implemented for increased traversability performance in rough terrain: Balancing, a mode for body pose adjustment that allows to increase the rover's tip-over stability and decrease its body tilt, and Wheel Walking, a mode for increasing gradeability on slopes and traction in soft soils. The effectiveness of the kinematic model and the locomotion modes were shown in a series of tests in the Planetary Exploration Laboratory at DLR. Balancing was able to reduce the rover body pitch and yaw angle by 5 degrees in uneven terrain and decrease side-slip while driving sideways along a slope by more than 50 percent. Wheel Walking managed to outperform Normal Driving (which only actuates the rover wheels) significantly in terms of traction on a slope. Using Wheel Walking, the rover managed to traverse a 30 degree slope with 70 percent of slip while Normal Driving failed at 21 degrees. Additionally, using Wheel Walking, energy consumption for slope traversal could be reduced by 50 percent.