Study On Integrated AFS And EPS For Steering Control System

Steering system is of vital importance to control driving direction and it exerts considerable influence on handling stability. Active Front Steering (AFS) is a device that can change steering ratio according to vehicle speed and driving condition through a DC motor. In terms of low and medium driving speed, AFS reduces the steering ratio to decrease the steering angle of steering wheel, thus ensuring steering agility. With regard to high vehicle speed, AFS increases the steering ratio to enhance steering stability and driving safety. Meanwhile, stability control is another advantage of AFS system. Electric Power Steering (EPS), on the other hand, can provide torque assistance through another DC motor. We can control the output torque of motor to coordinate the steering agility in low vehicle speed and appropriate steering feel in high vehicle speed. Additionally, returning control can be adopted to improve return-to-center performance.Currently, most AFS systems adopt planetary gear set on the basis of hydraulic power steering to realize the functionality. However, the overall system is too complex and indispensably has drawbacks due to inherent hydraulic system. Moreover, EPS alone can neither realize variable steering ratio control nor vehicle stability control. Thus we combine AFS and EPS to establish a new steering system. It can avoid the disadvantages of hydraulic system and ameliorate steering stability tremendously. In this view, integrated system of AFS and EPS is of great importance theoretically and practically.Secondly, AFS variable steering ratio control strategy is designed based on constant yaw rate gain and corresponding simulation is carried out. The stability control strategy of AFS is designed based on fuzzy-PID control, and relevant simulation is investigated with US (under steer) and OS (over steer) characteristics cars to obtain the application range of stability function of AFS.Additionally, mathematical model of steering reaction torque is built. A brand-new EPS assistant curve is designed based on steering reaction torque and basic principles of assistant curve. Comparative analysis shows the superiority of steering agility to piecewise assistant curve and curving assistant curve in low vehicle speed and better steering feel compared with piecewise assistant curve. Simulation results validate the analysis results. Finally, we conclude the control strategy of integrated AFS and EPS system. Compensation control of EPS with the intervention of AFS is analyzed and return-to-center control is researched. Corresponding control strategy is proposed. Simulation verifies the effectiveness of control strategy.