Hierarchical control in skid steer mobile robots with nonholonomics constraints

Skid-steered mobile robots are widely used in several applications due to their simple mechanical structure and their high maneuverability. This paper proposes a hierarchical control system for such robots that, at a high level, uses Nonlinear Model Predictive Control (NMPC) with a simplified prediction model, optimizing the longitudinal forces between the wheels and the ground to follow the desired trajectory. Using Pacekja’s formula, the reference slip for each wheel is obtained by interpolation, allowing the computation of the reference angular speeds of the wheels. Then, a proportional control is employed to find the required wheel torques, which are applied to a complete model of the skid-steer robot that takes into account the longitudinal slippage on the wheels. The proposed hierarchical controller is compared with a purely NMPC approach using the full model of the robot and a classical proportional control. Double-lane change and a circular trajectory tracking was performed. The resulting torque variation is compatible with values obtained in physical systems and the wheel skidding remained within the allowed limit.