Brake Control for Lateral Vehicle Dynamics
Examensarbete för masterexamen
Automotive engineering (MPAUT), MSc
This thesis describes the evaluation of the potential in utilizing the brakes on a vehicle to perform a small-overlap avoidance maneuver as a potential future active safety system and is done through both experimental and simulated testing. A strong reason for using brakes instead of steering is the availability of a powerful actuator that can be controlled by-wire while being less intrusive than a mechanically connected steering-actuated system. A 2008 SAAB 9-3 test vehicle is equipped with a separate electronic hydraulic brake system, enabling differential braking that can be implemented in a way to generate a yawing motion of the vehicle similar to turning the steering wheel. Simulink was used to develop a controller to autonomously perform the avoidance maneuver, requiring only an input describing the desired lateral displacement. In order to verify the functionality of the controller without experimental testing the vehicle dynamic simulation software, CarSim, was used. The developed avoidance maneuver controller was identical in both simulation and in the test vehicle to ensure proper correlation between the two. In order to evaluate and quantify the differential braking potential, the experimental tests were conducted in different conditions and levels of intervention. Parameters such as velocity, vehicle load, road friction, and lateral displacement were varied to achieve a good performance estimation. To minimize driver intrusiveness during a differential braking avoidance maneuver tests were also conducted with spacers installed between the front wheels and hubs to remove the lateral component of the scrub radius and thereby decrease the torque steer effect during braking. However the results proved that self-aligning moment is more dominant than torque steer and the installed spacers had a negligible effect on both driver intrusiveness and avoidance maneuver performance. To evaluate the potential of differential braking, the maneuvers were compared to corresponding simulated and experimental front wheel steered maneuvers to generate a reliable and quantifiable assessment of the differential braking potential. The experimental steered maneuvers were performed with a steering robot to ensure high accuracy and consistency. The results show that there is indeed potential in utilizing differential braking to perform an avoidance maneuver. At higher velocities, differential braking has the potential to perform avoidance maneuvers both at a shorter distance as well as duration compared to steering, especially for smaller lateral displacements. Differential braking decelerates the vehicle, shortening the traveled distance while at the same time increasing the duration to perform the maneuver. At lower velocities, a steered maneuver is performed at both a shorter distance as well as duration, compared to differential braking since the vehicle is not as sensitive to steering inputs and larger steering amplitudes can be applied, resulting in more effective lateral movement.
Farkostteknik , Transport , Vehicle Engineering , Transport