Robust sliding mode control of an agricultural tractor under the influence of slip

Precise guidance of agricultural vehicles is an area which is beginning to benefit from an increased awareness of its importance and its challenge. It is important as farming is becoming increasingly and justifiably automated, and because precision and autonomous farming is more necessary now, in order for the industry and individual farms to be competitive. It is a challenge because agricultural vehicles operate in quite uncertain and unpredictable environments. It is important that vehicles are guided with precision in a lateral as well as longitudinal direction, while being subjected to often significant disturbance forces, or slip, due to uncertain and sloping terrain. Little work has been carried out thus far in the guidance of such vehicles where vehicle slip is significant. In this paper, the authors provide an alternative approach to the robust trajectory tracking of an agricultural tractor under the influence of lateral and longitudinal velocity slip. This approach takes into account the dynamics of the steering system, and caters for slip velocities which are bounded but time varying. This is achieved via the use a robust combination of sliding mode control and integrator backstepping. Robust stability is achieved, ensuring the trajectory error is bounded to an adjustable region around the origin. Simulation results show that the alternative approach proposed yields suitable robust trajectory tracking.