Accurate GPS-based guidance of agricultural vehicles operating on slippery grounds

Chapter 6 ISBN: 1-59454-594-4The development of automatic guidance systems for agricultural vehicles is receiving a large attention from both researchers and manufacturers. The motivations in such automated devices are, on one hand, to reduce considerably the arduous driving task, and on the other hand, to improve the efficiency and the quality of the agronomic work carried out. Such guidance devices require realtime vehicle localization on an unstructured area, such as agricultural fields. Nowadays, RTK GPS sensor appears as a very suitable sensor for these applications, since it can supply this information with a satisfactory centimeter accuracy at a high 10 Hz frequency, without requiring any preliminary equipment of the field. In this chapter, it is demonstrated that very accurate curved path following can actually be achieved by agricultural vehicles, even on slippery ground, relying on a single RTK GPS sensor. In a first step, in order to benefit from recent advances in Control Theory, sliding effects have been omitted, and therefore guidance laws have first been designed relying on a vehicle kinematic model. More precisely, taking advantage from structural properties of these models (they can be converted into a so-called chained form), curved path following has been achieved by designing a non-linear control law. Full-scale experiments reveal a very satisfactory guidance accuracy, except when the vehicle enters into sharp curves or when it moves on sloping fields. In these two latter situations, guidance accuracy is damaged since the vehicle undergoes sliding effects. In agricultural applications, vehicle dynamic models appear untractable from the control design point of view. Therefore, it is here proposed to describe sliding effects as a structured perturbation acting on vehicle kinematic model. Adaptive Control framework, and more precisely Internal Model techniques, can then be used jointly with the above mentioned non-linear control law to reject sliding effects, when still preserving all the advantages of the previously designed non-linear guidance law. Experiments demonstrate satisfactory guidance accuracy when the vehicle moves along a slope or when it executes sharp curves, excepted at their beginning or end. These transient guidance errors mainly ensue from delays introduced by the actuation device. Since the shape of the path to be followed is known, beginning/end of curves or slopes can be anticipated. Model Predictive Control framework is here used to provide such an anticipation. Satisfactory experimental results display the performances of the overall control scheme