Cooperative Control of Autonomous Mobile Robots in Unknown Terrain

This paper presents a distributed control framework for groups of wheeled mobile robots with significant (nonnegligible) vehicle dynamics driving on terrain with variable performance characteristics. A dynamic model of a high-speed robot is developed with attention to representation of wheelterrain performance characteristics. Using this model, aspects of distributed, cooperative control on unknown terrain are investigated. A potential function path planning and cooperative control algorithm is combined with a local slip controller on each robot to provide high-speed control of vehicle formation. Local slip control is shown to reduce sensitivity of the distributed path planning and control method to tire-terrain performance variation and its resulting effect on dynamic behavior of the robots. Computationally efficient methods for real-time assessment of force-slip characteristics are presented to provide slip setpoints for this control architecture. INTRODUCTION The problem of cooperative control of multi-robot systems has resulted in a multitude of control schemes to produce various behaviors. In general, control laws and strategies have been developed using point-mass vehicles, or vehicles operating at speeds in which a point-mass or linear model holds. The exception to this is the control of vehicle platoons on a highway, or that of airborne vehicles, whose nonlinear dynamics cannot be neglected [1-3]. This paper incorporates a dynamic model of a four-wheel drive (4WD), differential steered, high speed mobile robot within an existing cooperative control method and summarizes control issues encountered during formation control from arbitrary initial conditions. The model assumes that tires exhibit nonlinear behavior at high slip or slip angle. Thus, the vehicle dynamics are described by the rigid-body equations of motion, while the external tire forces and moments are analytic or semi-empirical functions of sliding velocities and terrain parameters. Issues encountered when introducing a dynamic vehicle within a cooperative control scheme are illustrated using a potential function control method studied extensively for pointmass robots. Because existing methods address the general problem of cooperative control and path planning well, it is useful to modify these methods for dynamic robots in a manner that allows for robust or adaptive operation for a range of terrains, operating conditions, speeds, and initial conditions. We introduce a local wheel slip controller for each robot that serves this purpose. In order to implement slip control, wheel slip estimates and an appropriate slip setpoint are required. We develop a computationally efficient method for estimating force-slip characteristics at the terrain-tire interface. Slip estimates can then be used with the potential function path planning and control approach to provide robust, high speed operation during cooperative formation control. The paper summarizes the mathematical model of the robot, describes the control framework, presents the force-slip estimation methodology and results, and demonstrates the performance of the cooperative control framework by computer simulation.