Traction Processes of Wheels in Loose, Granular Soil

This dissertation presents analyses of subsurface motions of soil beneath different traction devices and develops new explanations of traction processes of wheels operating in loose granular soil based on these observations. This dissertation shows how these findings are useful for the development of planetary rover mobility systems.

Shear Interface Imaging Analysis (SIIA), is a new technique, developed as part of this thesis research. SIIA is employed for visualizing the effects of wheel operation on the soil beneath a rim, in richer detail than before possible. SIIA relies on high-speed imaging of sub-surface soil and on computer vision software to produce soil displacement fields, of high fidelity. The resulting data provides new insight and can reveal misconceptions about how wheels generate traction.

Two comprehensive studies relying on SIIA are undertaken: the investigation of wheel grouser mechanics and the investigation of push-roll locomotion. Soil forward motion, at a wheel leading edge, is identified as a key behavior for the grousered wheels. As a result, an equation for grouser height/spacing relationship to achieve a higher performance grouser configuration is developed and validated. This expression relates grouser configuration to wheel parameters (wheel radius) and operational parameters (sinkage and slip).

The soil mechanics behind Push-roll locomotion for high net traction and soft ground applications are presented. SIIA reveals that high thrust generated by push-roll locomotion is due to ground failure of the soil. Confirmation of the type of soil failure and of the application of operation in soft ground (where most vehicles would be embedded), brings forward the mobility gains of this non-typical locomotion mode and as a possible use for future planetary missions.

Additionally, insight into fundamental traction processes such as thrust, sinkage and motion resistance, are discussed with experimental evidence from soil displacement fields. This research proves that accounting for soil motion is of the utmost importance for the understanding of traction in loose, granular soils.

As a result of the specific technique utilized for directly studying soil motion, this research enables improved analysis and new design relevant to planetary rover mobility.