Turnability of a Small UGV With Individual Wheel Skid-Steering on Soft Terrain

Conventional turning mechanisms of wheeled skid-steer vehicles provide different velocities of the wheels at the left and right side while the wheels of each side are rigidly coupled and, thus, rotate with the same angular velocity. This turning principle has been employed for decades and provided high turnability of small unmanned ground vehicles (UGV) in both indoor and outdoor conditions with a high grip between pneumatic tires and surface of motion. However, as an analysis revealed, small UGVs habitually demonstrate non-sufficient turnability on deformable, extremely moisturized and heterogeneous terrains. This paper proposes and develops new skid-steering inputs that are associated with de-coupled wheels, which are individually driven by either mechanical drivelines or electric motors. The skid-steering inputs are introduced as four kinematic discrepancy factors (KDF), which individually and, the same time in coordination with each other characterize kinematic and force variances of the UGV four drive wheels. In the paper, the KDF are explicitly determined as skid-steer variables through tire characteristics and parameters of individual mechanical and electric drivelines. The KDF are suitable for a lower vehicle sub-system control to individually manage the wheel torques. The effectiveness of the skid-steering inputs is illustrated by analyzing vehicle understeer and oversteer maneuvers that are caused by various combinations of terrain conditions. A smooth and continuous transition from one to another KDF combination converts vehicle maneuvers and improves UGV turnability.