41. Vehicle Dynamics Control with Braking and Steering Intervention

From a driver’s perspective, controlling a vehicle means controlling the speed and the path curvature. In exceptional circumstances, e.g., in emergency evading situations, also the orientation of the vehicle has to be controlled. In a narrower sense, vehicle handling refers to vehicle dynamics like cornering and swerving and includes the vehicle stability. The advances in global chassis control technology have been used to further improve the vehicle safety and handling qualities. The effects of active systems are well understood in the context of how they contribute to the overall vehicle performance.

Altering the path curvature can easily be achieved by increasing the yaw gain such that the driver steering input is small. This strategy is only applicable up to a medium speed. The yaw rate’s normal driving range decreases significantly with vehicle speed, because the available tire–road friction is quickly saturated at high speed when the steering wheel angle input is too high. The strategy at high speed therefore must be to decrease steady-state yaw gain.

At the limit of friction, where safety becomes relevant, the handling controller determines how the vehicle remains stable. All available actuators are incorporated and coordinated to reach this goal. The active chassis gives the driver optimal support for avoiding accidents. In the region beyond the limit of friction, the main task of the control system is to prevent the car from skidding heavily so that the car remains on track.

During normal driving car drivers usually expect a linear yaw response of the vehicle with small phase lag. Most drivers have no experience of loss of linearity caused by saturation of tire forces. If saturation happens at the rear axle, the sideslip angle will increase quickly and therefore causes a hazardous driving problem for many drivers. The primary task of the control system should be to keep the vehicle sideslip angle small. An average driver feels uncomfortable when the magnitude of the sideslip angle exceeds a few degrees. State-of-the-art electronic stability control (ESC) systems limit the sideslip angle indirectly. ESC uses a reference yaw rate limited by the actual acceleration to account for the tire saturation. Additionally, the rate of change of sideslip angle is calculated and also limited.

Global chassis control delivers significant benefits in normal driving and particularly in emergency situations. The configuration and coordinated interaction of the active systems are the key success factors for enhancing the vehicle performance. International standards like ISO 26262 ensure quality and safety of the overall control system at the highest level.