Lateral-Directional Stability and Manual Control of Understeering and Oversteering Vehicles in Off-Road Conditions Based on a 2-DOF Cornering Compliance Vehicle Dynamics Model

Abstract

The lateral-directional stability of four-wheeled ground vehicles with understeer and oversteer characteristics in off-road conditions were investigated using a locally linearized vehicle dynamics model with two degrees-of-freedom. Analytical results confirm the widely accepted understanding that the open-loop response for an understeering vehicle with the steered wheels fixed parallel to the vehicle centerline is stable for all vehicle speeds, while an oversteering vehicle is unstable above a certain ‘critical speed’ for the same fixed steering condition. However, results for both understeering and oversteering vehicles using a path-centered steer angle input, defined relative to the path tangent, were found to be stable for all forward speed conditions. These results also indicated that the path-centered steering can be used for lateral-directional control with suitable gain. Therefore these results indicate that a manual control strategy based on path-centered steer angle inputs can be used to stably control a vehicle for all forward speeds, in both understeering and oversteering conditions, provided the tire cornering force continues to increase with slip angle, which is generally the case for vehicle off-road or snow conditions. These results are also consistent with ground vehicle operator acceptance and preference of vehicles with oversteering characteristics in off-road conditions.