DYNAMIC TERRAMECHANIC MODEL FOR LIGHTWEIGHT WHEELED MOBILE ROBOTS

Abstract

This doctoral thesis extends analytical terramechanic modelling for small lightweight mobile robots operating on sandy soil. Previous terramechanic models were designed to capture and predict the mean values of the forces and sinkage that a wheel may experience. However, these models do not capture the fluctuations in the forces and sinkage that were observed in experimental data.

The model developed through the course of this research enhances existing terramechanic models by proposing and validating a new pressure-sinkage relationship. The resulting two-dimensional model was validated with a unique high fidelity single-wheel testbed (SWTB) which was installed on a Blohm Planomat 408 computer-numerically controlled creepfeed grinding machine. The new SWTB translates the terrain in the horizontal direction while the drivetrain and wheel support systems are constrained in the horizontal direction but allowed to freely move in the vertical direction. The design of the SWTB allowed for a counterbalance to be installed and, as a result, low normal loads could be examined. The design also took advantage of the grinding machine's high load capacity and precise velocity control.

Experiments were carried out with the new SWTB and predictable repeating ridges were found in the track of a smooth rigid wheel operating in sandy soil. To ensure that these ridges were not an artifact of the new SWTB a mobile robot was used to validate the SWTB findings, which it did. The new SWTB is a viable method for investigating fundamental terramechanic issues.

A series of experiments at different slip ratios and normal loads were carried out on the SWTB to validate the new pressure-sinkage relationship which explicitly captures and predicts the oscillations about the mean values for the forces and sinkage values for both a smooth wheel and a wheel with grousers. The new pressure-sinkage relationship adds two new dimensionless empirical factors to the well known pressure-sinkage relationship for a rigid wheel. The first new factor accounts for changes in the local density of the terrain around the wheel and the second factor accounts for the effects grousers have on the forces and sinkage.