Study On Electric Power Steering System Control Algorithm Based On Vehicle Stability

Controller is a key technology in the research and development of Electric PowerSteering System(EPS), and control algorithm is the core of controller. At present, domesticand foreign EPS researcher focused on the study of power assistance control of EPS. Few ofthem took vehicle’s manoeuvre and stability into the EPS control algorithm. However,vehicle’s manoeuvre and stability is an important topic in today’s automotive research. Thereare many aspects that manoeuvrability and stability of vehicle at high speed. The forcereflected on driver from steering system is much influential on manoevrability and stability.The advantage of EPS is that assist torque, which could adjust the force feedback features onsteeringwheel, can be changed by software. Therefore, it is necessary to take themanoeuvrability and stability of vehicle into account in original EPS control algorithm.The steeringwheel torque conmpensation control algorithm is a supplement to the EPScontrol algorithm to improve vehicle stability. The steeringwheel torque compensation,desided by vehicle state variables, such as yaw rate, superposes with steeringwheel torquesensor input, and then motor assist torque are provided according to assistant control feature.Stability of vehicle in high speed could be improved because steeringwheel force feedbackhas been modified through changing assist torque. It makes driver’s manipulation morereasonable, then vehicle more stable.The study of this paper consists of following parts:1. The study on EPS assist control algorithm. Firstly, the definition and classification ofpower characteristic curve are discussed, and select the linear power curve. At base speed,three parameters of power characteristic are identified, and then by calculating the speedinductance at some typical speed, all of power characteristic curves at any kind of vehiclevelocity can be obtained through product of power characteristic at base speed and speed inductance. Secondly, return control algorithm is researched. Using steering wheel angle andsteering wheel angle’s rate, controller will determinate the status whether the vehicle is inreturn state. Based ideal reference returning model, slide mode algorithm is designed tocontrol the process of vehicle’s returning, which is better than PID algorithm that has toadjust the parameters when different driving cycles. Futhermore, the research of incrementalPID control algorithm ensures that EPS motor could provide assist torque in accordance withthe power demand.2. EPS steeringwheel torque compensation control algorithm that is related to vehiclestability factors are studied after the research of assist control algorithm in chapter2. Thecondition of entry or exit for EPS steeringwheel torque compensation control is determinatedby setting thresholds of yaw rate, reference vehicle speed, lateral acceleration etc. Twodegrees of freedom vehicle model, usually considerd as ideal driving state to maintainvehicle stability, and tire adhesion limit will calculate the desired yaw rate. The errorbetween the desired yaw rate and actual yaw rate from sensor, as control variable, passes intofuzzy controller, which outputs the torque compensation, which then is modified and limitedby som other driving state variables.3. Based on MATLAB/SIMULINK software, electric power steering control systemsimulation platform is established. The simulation platform includes vehicle dynamics model,EPS control system, driver model and graphic user interface(GUI). The vehicle dynamicsmodel consists of tyre model, wheel and body model, and EPS system dynamics model.Driver model consititutes the human-vehicle closed system. GUI makes the simulation moreconvenient.4. The evaluation methods and indicators of EPS performance and vehicle stability arediscussed. On the basis of simulation platform, according to requirement of evaluation, thecontrol algorithm are verified by simulation. The power characteristic curve studied couldensures steering lightly when in low speed driving cycle or pivot steering driving cycle.Slide mode algorithm enables vehicle to achieve a good return effect, and better that PIDcontrol. In the simulation, the motor current can follow the target current, that verifies theincremental PID control algorithm. Besides, simulation results show that EPS steeringwheel torque compensation control algorithm could improve the force feedback on driver, andvehicle steering system provides the driver with more “road feel”, so the stability of vehicleis effectively improved.