Path tracking control of four wheel independently steered ground robotic vehicles

Because of their high degree of maneuverability, four wheel steered robotic vehicles have increasingly attracted interest in many applications. These robots are characterized by ability to easily maneuver tight turns. Two types of such vehicles have been identified: vehicles with independently steered wheels, and vehicles with mechanically coupled pairs of steered wheels. While the latter group has been easy to model and control, the former group still poses many control challenges. The common approaches employed in modeling and controlling vehicles with independently steered wheels either assume that the rear wheels will copy the front steering angles or allow wheel slippage to accommodate independent steering angles for all wheels. Both these approaches do not offer the sought maneuverability advantages. This paper revisits the problem of four independent wheel steering and proposes an approach that makes it possible to achieve maximum maneuverability while avoiding wheel slippage. It develops individual wheel constraints that enable the wheels to be independently controlled while satisfying the desired vehicle motion. Numerical simulation results have shown that this approach can indeed simplify the problem of controlling four steered wheel vehicles.