Automatic guidance of a four-wheel-steering mobile robot for accurate field operations
Guidage autonome d'un robot mobile à quatre roues directrices pour les travaux agricoles de précision
Résumé
As the world population growth requires an increasing level of farm production while the environment preservation constitutes nowadays a priority, the development of new agricultural tools and methods is required. In this framework, the development of robotic devices can constitute an attractive solution, particularly in the field of autonomous vehicles. Accurate automatic guidance of mobile robots in such an environment constitutes a challenging problem for researchers, mainly due to the low grip conditions usually met in such a context. From assisted steering systems to agricultural robotics, numerous control algorithms have been studied to achieve high-precision path tracking and reach an accuracy within +/-10cm, whatever the ground configuration and the path to be followed. However, most existing approaches consider classical two-wheel steering (2WS) vehicles. Unfortunately, by using such a steering system, only the lateral deviation with respect to the path to be followed can be satisfactorily controlled. Indeed, the heading of the vehicle remains dependent on the grip conditions, and crabway motions are for example systematically observed on a slippery slope, leading to inaccurate field operations. To tackle this drawback, a four-wheel steering (4WS) mobile robot is considered, enabling to servo both lateral and angular deviations with respect to a desired trajectory. The path tracking control is designed thanks to an extended kinematic representation, allowing to account on-line for wheel skidding, while a backstepping approach permits to manage the 4WS structure. It results an approach taking part of both rear and front steering actuations to fully compensate for sliding effects during path tracking. Moreover, a predictive algorithm is developed in order to address delays induced by steering actuators, compensating for transient overshoots in curves. Experimental results demonstrate that, despite of sliding phenomena, the mobile robot is able to automatically achieve accurately a desired path, with respectively lateral and angular errors within +/-10cm and +/-2°, whatever its shape and whatever the terrain conditions. This constitutes a promising result in order to define efficient tools to the tomorrow's agriculture challenge.