Aerodynamic Forces Impact on Vehicle Braking Longitudinal Dynamics with a Sliding Mode Controller

The control of automotive braking systems performance and wheel slip is a challenging problem due to the nonlinear of the braking process, vehicle body dynamics during braking and the tire-road interaction. When the wheel slip is not between the optimal limits during braking, the desired tire-friction force cannot be achieved, which influences the braking distance, the loss in steerability and maneuverability of the vehicle. A simple and at the same time realistic vehicle longitudinal braking model is essential for such challenging problem. In this paper, a new longitudinal rolling/braking lumped-vehicle model that takes vehicle aerodynamic forces in consideration is presented. The proposed model takes the rolling resistance force, the braking force and the aerodynamic lift and drag forces in consideration and investigates their impact on the vehicle longitudinal dynamics especially vehicle braking distance and time. An anti- lock sliding-mode controller is designed to provide wheel slip control during vehicle motion. This type of controller is chosen due to its expected robustness against varying road friction coefficient. The controllers provide an optimal braking torque control which minimize the braking distance by maintaining a desired slip ratio corresponding to the road condition. Based on this optimal braking control method, the impact of the vehicle aerodynamic forces (lift and drag) are investigated and presented.