In this paper, a sliding mode controller with disturbance observer (DOB-SMC) is developed for a tractor as a farm vehicle operating in the presence of longitudinal and lateral slips at front and rear wheels. Farm vehicles intended for autonomous operations usually traverse at low speeds along straight lines. As very low accelerations are involved kinematic models seem sufficiently accurate for developing controllers. However undulating, sloping and very uncertain terrain, besides rough environment, introduces significant disturbances which makes the control design challenging. The proposed DOB-SMC is able to guarantee precision as well as robustness for a typical tractor. In the modelling, lateral and longitudinal sliding are incorporated in the otherwise ideal kinematic model and an error model is developed for the kinematic model. Then a DOB-SMC is developed to cope with disturbances. We prove that the proposed controller is stable in the presence of significant slip. Simulation results prove acceptable performance of the proposed controller in comparison to two other controllers in the literature.
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