Research On Road Feeling And Active Steering Control Strategy For Vehicle Steer-By-Wire System

Steering-By-Wire (SBW) system has advantages of advanced vehicle control system, which has no mechanical linkage to control the steering wheel and front wheels. It is possible to control the steering wheel actuator and front wheels actuator steering independently. Road wheel Motor steers the front wheels of the vehicle following the driver's command and steering wheel motor is committed to generate a reaction torque to the Steering wheel, as a virtual feedback of the road feeling. SBW system can increase the freedom to tune the steering feel and also could improve steering maneuverability. Furthermore, it suit to active steering control improving vehicle stability, maneuverability and active safety by driver-vehicle-road close loop system. The SBW system can enhance vehicle steering has already been undergoing worldwide development as the SBW system has many advantages. Therefore, the research on the theories and key technologies of SBW system in developing domestic automobile industry has important significance and application foreground.This Ph.D. dissertation is based on the project named"Research on Key Technology for Vehicle Steering-By-Wire system"(No. 50475009) and"Research on Control methods and Key Technology for X-By-Wire Vehicle"(No. 50775096) by National Natural Science Foundation of China. Based on the summary of the domestic and foreign research achievements,this dissertation aimed at studying the control strategy of the road feeling and active steering for vehicle. The main research work is summarized as follows: (1)Establishing a vehicle dynamic model for simulating research of SBW system.A domestic passenger car was chosen as benchmark and a vehicle dynamic model was established for simulating research of SBW system. The vehicle models included body model, wheel model, Electric Power Steering (EPS) model, wind force model, braking system model, powertrain model and tire model. In order to study on vehicle steering gain and stability control strategy,a driver model and a road friction model are also established. Some typical handling and stability simulations were performed to validate the vehicle dynamic model. The results showed that the vehicle model can describe the vehicle dynamic characteristic and can be used to research on SBW control strategy.(2)Establishing a SBW system model for structure and characteristic of SBW system.Based on an EPS system model, a SBW system model is established. The models included steering wheel model, road feeling system model, steering actuation system model, gear and rack model, front wheel model. Steering wheel angle and rack displacement were controlled by using PID control method to carry out close loop stability control. The results showed that the effectiveness of the control strategy. The model can be used to studying on road feeling and active steering control strategy.(3)Research on rack force estimation based on the Kalman Filter method for road feeling control strategy.Since SBW system removes mechanical linkages between steering system and front wheels, driver is unable to sense road information. Because of rack force is difficult to measure, an algorithm is proposed to the estimate rack force. The results showed that control strategy can effective feedback road information to driver. When high speed driving, the index values of road feeling for SBW and EPS system were similar,it verifies SBW system copy the road feeling properties as good as EPS system. When low speed driving,because of reduce friction moment of steering system, steering torque of SBW system was smaller than EPS system, could reduce driver burden.(4)Research on steering steady state gain constant and dynamic feedback correction control strategy. Based on variable steering ratio, a steady state control strategy for the ideal steering ratio was designed to keep the vehicle steering gain constant and make the steering characteristic of the SBW system unsensitive to the change of vehicle speed and steering wheel angle. With comprehensive evaluation of vehicle handling and stability and its optimization by genetic algorithm, the yaw rate gain was optimized and an ideal steering ratio was computed. The results showed that the introduced control strategy can keep the vehicle steering gain constant to reduce the driver burden, allowing the unskilled driver to steer the vehicle. Furthermore, considering that the steady state gain constant control strategy had some problems in vehicle high speed driving and low speed driving, a correct method was introduced by using fuzzy logic method. The results showed that steering ratio improved steer handling in low speed driving and steer stability in high speed driving. The proposed control strategy improved effectively the vehicle maneuverability.A dynamic feedback correction control strategy was proposed to improve vehicle maneuverability and stability based on steady state gain and the control strategy can improve vehicle handling behavior during normal driving. Compared active steering control system with active braking control system, the advantages and disadvantages of active steering control system were showed. Based on two DOF handling model, a yaw feedback stability control strategy was proposed which further integrates feedback control strategy that included vehicle speed, yaw rate and lateral acceleration. The results showed that the proposed control strategy can improve vehicle dynamics response and stability, also reduce driver burden under critical situations. Because of tire forces were limited on slippery road, active steering control system are normally not enough to keep vehicle stability, thus it needed to integrate with active braking control system to improve vehicle stability.(5)Hardware-in-the-Loop simulation test of SBW control strategy.A hardware-in-the-loop simulation test bench was built based on automobile driving simulator to be used to validate road feeling and active steering control strategy. The results showed that 1) road feeling control strategy fo the SBW system can provide the road feeling properties as good as EPS system in high speed driving but the steering torque of SBW system was smaller than EPS system in low speed driving; 2) the ideal steering ratio improved steer handling propecties in low speed driving and steer stability in high speed driving cases. The proposed control strategy effectively improved the vehicle maneuverability and reduced driver burden; 3) the control strategy improved vehicle maneuverability and stability based on yaw rate and integrated feedback methods.The innovative contributions of the dissertation were summarized as follows:(1)Since road feeling information include tire self-aligning moment and road friction force and was a function of caster angle, lateral force, mechanical trail, pneumatic trail many variables which are including difficult or very expensive to be measured. The dissertation proposed a rack-force estimation algrithem based on Kalman Filter for road feeling control strategy design. The simulation results validated the feasibility and efficientiveness of the designed control strategy.(2)A steering steady state gain constant and dynamic feedback correction control strategy was proposed based on variable steering ratio. The control strategy can mantain the vehicle steering gain constant to reduce the driver burden and allow unskilled drivers to steer the vehicle properly. The proposed dynamic feedback control strategy based on the vehicle state feedback improved effectivel the vehicle stability.(3)Based on the limilations of keeping steady state gain constant control strategy in high speed and low speed driving cases, a correct method was introduced based on fuzzy logic method. The results showed that steering ratio improved steer handling performance in low speed driving cases and steer stability in high speed driving cases. The proposed control strategy effectively improved the vehicle maneuverability.