Walking planetary rovers – Experimental analysis and modelling of leg thrust in loose granular soils

One of the principal differences between locomotion in granular soils using legs when compared with wheels is that the drag between the leg assembly and the regolith material provides additional thrust. Experimental work is presented which demonstrates that this additional force is substantial, and can significantly augment legged vehicle Drawbar Pull. The paper also demonstrates that the drag force depends in a highly non-linear manner on sinkage depth, and linearly on leg cross section yet is only weakly dependent on leg cross-sectional shape, leg material frictional properties, or leg velocity. Comparison with modelled forces using established wedge theory techniques demonstrates poor correlation between predicted and actual results; in contrast, a modelling approach based on an analysis of the dynamics of granular materials produces an excellent correlation with experimental results and enables the drag force to be accurately characterised by deriving a constant coefficient which is characteristic of the soil material. Future work will investigate the relationship between this characteristic coefficient and the physical properties of the soil material to develop a robust method of predicting the coefficient for any soil.