Adaptive control of four-wheel-steering off-road mobile robots: Application to path tracking and heading control in presence of sliding

In this paper, automatic path tracking of a four-wheel-steering vehicle in presence of sliding is addressed. The attractive feature of such a steering system is that, despite of sliding phenomena, both lateral and angular deviations can be explicitly controlled. Indeed, previous research has demonstrated that high-precision path tracking on a low grip terrain can be achieved with two-wheel-steering vehicles. However, in this case, only the lateral deviation is kept satisfactorily close to zero, the angular deviation is non null in order to compensate for sliding effects. In this paper, previous adaptive control laws are extended to the case of four-wheel-steering mobile robots with the aim to servo both lateral and angular deviations. Relying on an extended kinematic model, a backstepping control approach, which considers successively front and rear steering control, has been designed. Real world experiments have been carried out on a low adherent terrain with a four-wheel-steering vehicle equipped with a single RTK-GPS. This demonstrates the capabilities of the proposed control law and its robustness in real all-terrain conditions.