Auton Robot DOI 10.1007/s10514-015-9530-4 A sliding mode controller with a nonlinear disturbance observer for a farm vehicle operating in the presence of wheel slip Javad Taghia 1 · Xu Wang 1 · Stanley Lam 1 · Jay Katupitiya 1 Received: 26 January 2015 / Accepted: 7 December 2015 © Springer Science+Business Media New York 2015 Abstract A sliding mode controller with a nonlinear dis- turbance observer is proposed and developed to control a farm vehicle to accurately track a specified path. The vehi- cle is subjected to lateral and longitudinal slips at front and rear wheels. The unpredictability of ground contact forces which occur at the wheels while traversing undulating, rough and sloping terrains require the controllers to be sufficiently robust to ensure stability. The work presented in this paper is directed at the practicality of its application with both matched and unmatched uncertainties considered in the con- troller design. The controller is designed using an offset model derived from the kinematic model and its operation is verified by simulation and field experiments. In the sim- ulations, the kinematic model based controller is used to control both a kinematic model and a dynamic model of a tractor to verify the performance of the kinematic model based controller. The proposed controller is compared with two other nonlinear controllers, namely, back stepping con- trol and model predictive control. In the field experiments, the three controller were used to control the physical tractor to Electronic supplementary material The online version of this article (doi:10.1007/s10514-015-9530-4) contains supplementary material, which is available to authorized users. B Javad Taghia taghia.javad@gmail.com; j.taghia@unsw.edu.au Xu Wang xu.wang3@student.unsw.edu.au Stanley Lam s.lam@unsw.edu.au Jay Katupitiya j.katupitiya@unsw.edu.au 1 School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, NSW 2052, Australia follow a specified path. Simulation and experimental results are presented to show that the proposed controller demon- strated the required robustness and accuracy at all times. Keywords Sliding mode control · Path tracking · Autonomous ground vehicles · Autonomous farming 1 Introduction The use of autonomous systems in many industries is rapidly advancing. In the agricultural industry, the application of smaller modular autonomous vehicles is promising in deliv- ering productivity advantages while they are operating with high accuracy on agricultural terrain. Among these are the ability to lay the crop highly accurately according to a pre- specified crop layout plan and to use the knowledge of crop location to manage the crop right throughout the cropping season. Similarly, knowing the crop locations also means knowing where crop has not been planted and hence her- bicides can be accurately applied to where they are needed such as the inter-row space. The high accuracy autonomous path tracking capabilities not only bring in the cost sav- ings, but also considerable ecological and environmental advantages. In general, large-scale agricultural activities are time-consuming, tedious, hazardous and the proposed path tracking accuracy is unable to be achieved by human opera- tors. Without a sophisticated crop localization system, high precision path-tracking is essential for reliable maneuvering of an autonomous system in the field among the growing crop to ensure that the crop is not damaged. The aim of this study is to develop controllers that can deliver such accuracy. Although the focus of this paper is on farming vehicles, more specifically tractors, the presented approach is applicable to 123