DYNAMIC TERRAMECHANIC MODEL FOR LIGHTWEIGHT WHEELED MOBILE ROBOTS

摘要

This doctoral thesis extends analytical terramechanic modelling for small\nlightweight mobile robots operating on sandy soil. Previous terramechanic\nmodels were designed to capture and predict the mean values of the forces\nand sinkage that a wheel may experience. However, these models do not\ncapture the fluctuations in the forces and sinkage that were observed in\nexperimental data.\n\nThe model developed through the course of this research enhances existing\nterramechanic models by proposing and validating a new pressure-sinkage\nrelationship. The resulting two-dimensional model was validated with a\nunique high fidelity single-wheel testbed (SWTB) which was installed on a\nBlohm Planomat 408 computer-numerically controlled creepfeed grinding\nmachine. The new SWTB translates the terrain in the horizontal direction\nwhile the drivetrain and wheel support systems are constrained in the\nhorizontal direction but allowed to freely move in the vertical direction.\nThe design of the SWTB allowed for a counterbalance to be installed and, as\na result, low normal loads could be examined. The design also took advantage\nof the grinding machine's high load capacity and precise velocity control.\n\nExperiments were carried out with the new SWTB and predictable repeating\nridges were found in the track of a smooth rigid wheel operating in sandy\nsoil. To ensure that these ridges were not an artifact of the new SWTB a\nmobile robot was used to validate the SWTB findings, which it did. The new\nSWTB is a viable method for investigating fundamental terramechanic issues.\n\nA series of experiments at different slip ratios and normal loads were\ncarried out on the SWTB to validate the new pressure-sinkage relationship\nwhich explicitly captures and predicts the oscillations about the mean\nvalues for the forces and sinkage values for both a smooth wheel and a wheel\nwith grousers. The new pressure-sinkage relationship adds two new\ndimensionless empirical factors to the well known pressure-sinkage\nrelationship for a rigid wheel. The first new factor accounts for changes in\nthe local density of the terrain around the wheel and the second factor\naccounts for the effects grousers have on the forces and sinkage.