Active front wheel steering control based on ideal transmission ratio

Abstract: Active front steering system with variable transmission ratio control can solve the contradiction of steering portability at low speed and steering stability at high speed, and coordinate the vehicle handling stability and road feeling of the driver. The study of the active front steering system usually assumes that the steering system is able to generate the needed additional angle of front wheel ideally, and it is used for vehicle motion control. However, this method is lack of the study of the ideal transmission ratio law that the additional angle should follow, and the corresponding control strategy of the additional angle. In order to solve these problems, the linear vehicle model with two degrees of freedom, tire model, and active front steering system model involving steering wheel, steering shaft and double row planetary gear mechanism, the lower planetary gear sun wheel mechanism, rotation angle motor and gear rack mechanism are established. Then, the ideal transmission ratio law with fixed yaw rate gain is designed for active front steering system, based on which the additional angle control strategy of active front wheel steering is studied. According to these, the simulation model with Matlab/Simulink software is established. Selecting the speed at 20 and 100 km/h, simulation analyses of angle step input and sinusoidal input for the proposed control strategy are conducted. The simulation shows that at the speed of 20 km/h, the amplitude of yaw rate with the ideal transmission ratio for active front steering becomes larger, compared with the fixed transmission ratio. At the same time, the vehicle has better steering sensitivity, which is conducive to achieve good steering performance at low speed. At the speed of 100 km/h, the amplitude of yaw rate and sideslip angle becomes smaller with the ideal transmission ratio, in contrast to the fixed transmission ratio. Thus, the vehicle has better steering stability, which is conducive to the stability of vehicle at high speed. In order to verify the force characteristics improvement of the active front steering system based on the designed power assisted controller with the proposed control strategy, the original position and low-speed steering test and the middle position test of steering wheel are carried out. In original position and low-speed steering test, the steering wheel torque of the vehicle is small under the designed assisted controller, whose maximum torque is only 4 N·m. The operating torque of the driver is small, which realizes the steering portability at low speed. The results of simulation analysis and experimental verification show that, the designed control strategy can ensure the vehicle has good sensitivity at low speed and good stability at high speed, which can achieve perfect steering quality. The results of this study can provide the theoretical basis for the design and development of active front steering system.