Research On Integrated Control Strategy Of Electric Power Steering And Active Front Steering Based On The Estimation Of Tire Forces

The steering system is an important part of automotives. The performance ofsteering system directly influences road feel as well as the handling stability anddriving safety. Addressing the drawbacks of traditional hydraulic system, the ElectricPower Steering (EPS) system can provide available road feel, which makes it verysaleable in small-and-medium-sized car market. The Active Front Steering (AFS)system can effectively improve vehicle handling stability and driving safety. AlthoughAFS is still a component in premium cars currently, it will be a necessary part of non-luxury cars in the future. Thus, integrating EPS and AFS system to pursue furthersteering performance could be the inevitable trend of automotive industry.This paper conducts a research about the integrated control of EPS and AFS underthis background. Primarily, the two subsystems of EPS and AFS are analyzed andresearched respectively. Subsequently, the integrated system is studied. The maincontent of this paper is listed below.(1) The structure of integrated system is formulated and the integrated systemmodels including steering system model, nonlinear vehicle model, tire model andresistance torque model are built in MATLAB/Simulink. Vehicle tests in Carsim, whichrepresent real car tests, validate the accuracy of simulation model.(2) Three kinds of assist curve are introduced and the linear type assist curve ischose in the design of EPS assistant control. The EPS control strategy is researched indetail including assist control, return-to-center control, damping control andcompensation control. Moreover, the method of switching aforementioned controlmodes is introduced. The simulation results show the effectiveness of EPS controlstrategy.(3) A novel modified variable steering ratio curve for AFS is designed. It canminimize the torque ripple of AFS motor on the basis of keeping stable vehicle systemgain being a constant. Thus, it can improve vehicle handling performance as well asmotor performance. Furthermore, optimal LQR control strategy is used to design theAFS controller. The simulation results show that the proposed AFS controller caneffectively improve vehicle handling performance and driving safety.(4) The functionalities of integrated system of EPS and AFS are formulated indetail, and the inner influences of two subsystems are also analyzed. Finally, an EPS correction control strategy is proposed based on the estimation of front axle lateralforce. This correction control strategy can effectively address the interference of AFS toEPS and coordinate the operation of two subsystems.