Investigation To Control Of An Active Front Steering System

Steering system is an important component for lane changing control of wheeled vehicles. Its performance influences vehicle steerability and stability directly. Active front steering (AFS) varies the steering ratio electronically in direct relation to the speed and road conditions. Under normal road conditions at low and medium speeds, the steering becomes more direct, requiring less steering effort of the driver, increasing the car's agility and drivability. At high speed, the steering becomes less direct offering improved directional stability. In addition, road information can be fed by the mechanical link maintained between the front wheels and the steering wheel. As a result, the AFS system provides the vehicle with a new standard of driving agility, amenity and safety. It is the trend in development of the steering system.Most of the existing AFS systems are developed in association with hydraulic power units and a planetary gear set. Therefore, some disadvantages appear inevitably due to the servo system in the hydraulic power steering and its cooling.The AFS system is designed based on a commercial electric power steering (EPS) system. Active steering is realized with a planetary gear set and an angle assist motor. The angle assist motor (AFS actuator) is used to impose variable steering ratio (VSR) control and to improve vehicle stability. The original torque assist motor (EPS actuator) is applied to modify the driver steering torque according to the speed and driving conditions. Therefore, the AFS system can regulate the steering torque and steering angle by the EPS actuator and AFS actuator, respectively. The motor used to regulate the steering torque can avoid the disadvantages of using hydraulic system in the existing AFS system.As a core part in the AFS system, the controller plays an important role in the AFS'performance. For investigation to the AFS control, a simplified model and the vehicle model are developed. Then, the control logic and the strategy of the AFS system are designed based on steering performance and vehicle stability. In treatment of the AFS system, the controls of EPS actuator and AFS actuator are decoupled. The EPS actuator is controlled to modify the steering effort, while the AFS actuator is controlled to impose VSR for the vehicle stability enhancement. In control of the vehicle dynamic stability, both the yaw rate and estimated sideslip angle are applied. The sideslip angle control is utilized to compensate the tracking error caused by single yaw rate control.During the driving, the vehicle is subjected to internal and external disturbance of the system. In addition, uncertainties caused due to the nonlinear friction, sensor noise and load varieties also exist in the system. The theory of robust control is introduced to the AFS control. According to working conditions, a standard H∞control is applied in the EPS actuator control for the purpose of prevention from external disturbance and sensor noise, while a mixed LQR/H∞optimal control is applied to the vehicle stability control of AFS actuator to deal with the crosswind disturbance and perturbation of cornering stiffness. Therefore, the stability and robustness of the AFS system control to all kinds of disturbance can be ensured.The AFS controller is developed according to the control strategy and also tested by means of the hardware-in-the-loop (HIL) simulation method. For the HIL simulation, a nonlinear vehicle model is provided to be a simulation platform for the AFS system. Effectiveness of the present AFS controller is verified by the HIL simulation procedure. By test results, it is indicated that the present AFS system appears to be effective in modifying the steering effort and improving the vehicle stability.