Research On Active Front Steering System Of Vehicle

Active Front Steering System (AFS) can provide a superposition angle to the front wheel, which is independent of the driver but the velocity and automobile steering state parameters, realizing the variable steering ratio function. 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 drive ability. At high speed, the steering becomes less direct, offering improved directional stability. In addition, road condition 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 great performance of driving agility, amenity and safety. The research on AFS is currently a major trend all over the world because of the safety and road sense.The new AFS system is based on a commercial electric power steering (EPS) system. The original torque assist motor (EPS motor) is applied to modify the driver steering torque according to the speed and driving conditions. Active steering is realized with a double planetary gear set and an active steering motor (AFS motor). The active steering motor is used to impose variable steering ratio control on the steering system, improving the vehicle stability. Therefore, the AFS system can regulate the steering torque and steering angle by the EPS and AFS motor respectively. It is proved that the new AFS gearing has great advantage over the BMW gearing by comparing the two gearings, which is the vital basis of the AFS control system.A statics and kinesiology model of the AFS is built on the epicyclical gear trains, which is validated by the simulation model. On the basis of these models, the relationship between the angle of the steering wheel, velocity of vehicle and superposition angle were analyzed. The curve of perfect variable steering ratio was designed,it was confirmed by the simulation result.The AFS controller plays an important role in the AFS'performance. A correct and precise model is the key to the design of AFS control strategy. A model of AFS motor and operating load is built to complete the control method. On the basis of this model, the paper designed cascade control integrated PID and fuzzy logic control, including three circles. The simulation model was built on the platform of MATLAB/SIMULINK and the results show that it can run with good performance. The test system of AFS was studied, including hardware system and software design.