First of all, based on the detailed analysis of the principle of vehicle stability control, the problem of over steering or insufficient steering easily induced by vehicle steering is studied. Based on the main parameters of vehicle running state – sideslip angle of mass center and yaw angular velocity as the main control variables, the control strategy is designed, and an effective PID controller adapted to different vehicle speed, different road surface and different steering wheel angle is given. In recent decades, the global automobile industry has been developed on a large scale, the number of automobiles has risen sharply, the pressure of road traffic is increasing day by day, and traffic accidents occur frequently, which makes the traffic safety problem become a social problem, which has caused people to attach great importance to the safety of automobiles, especially to the active safety. Vehicle safety is mainly divided into passive safety and active safety according to the different action time before and after traffic accidents. Among them, the active safety of automobile is a safety system with prevention as the core. It effectively protects the safety of drivers and pedestrians in road environment through prevention in advance. This has become the focus of automobile safety research in this century. It is found that vehicle handling and stability is an important performance affecting the safe driving of vehicles at high speed. The research of stability control system aiming at improving the active safety of vehicles has become the focus of vehicle dynamics research. Vehicle dynamic stability control is an active safety technology to control all aspects of vehicle dynamics, which should include the control of vehicle longitudinal dynamics, lateral dynamics and vertical dynamics. It is a new type of vehicle stability and handling performance improvement by adjusting the size and matching of wheel longitudinal braking force after anti-lock braking system and traction control system. Active safety system is also a new type of active safety control technology, which enables the vehicle to achieve good handling stability and direction under various road conditions. With the development of Expressway and the progress of automobile technology, the speed of automobile is increasing. The design maximum speed of modern cars is generally more than 200 km/h, and the speed of cars on expressways is usually more than 100 km/h. In addition, the driver’s non professional development trend also requires better controllability and higher safety. Therefore, thermostatic element the safety of automobiles, especially the active safety, has been paid more and more attention, and has become the focus and hotspot of technology development of major automobile companies. ESP consists of traditional braking system (vacuum booster, pipeline and brake), sensors (four wheel speed sensors, steering wheel angle sensor, lateral acceleration sensor, yaw angular velocity sensor, brake cylinder pressure sensor), hydraulic regulator, electronic control unit (ECU) of vehicle stability control and auxiliary system (engine management system). The basic idea of ESP control is to prevent the vehicle from entering an uncontrollable and unstable state by controlling the quasi-steady state. For the control of vehicle yaw and lateral stability, the most authoritative methods in domestic and foreign automotive theorists are to control wheel steering angle, control the vertical load distribution on the wheel, and control the driving force and braking force.
Four-wheel steering control is necessary to control the lateral stability of the vehicle by controlling the steering angle of the wheel. At present, the vehicle stability control system with application value mostly uses the control of braking moment and engine output torque to achieve the purpose of controlling the yaw rate and the side-slip angle of the vehicle. The ECU of vehicle stability control judges the driver’s driving intention through the information obtained by steering wheel angle sensor and brake main cylinder pressure sensor, and decides the ideal vehicle running state, such as the ideal yaw angular speed. The actual vehicle state detected by ECU is compared with the ideal vehicle state, and the control logic determines how much yaw moment should be applied to the vehicle to restore the stability of the vehicle. Then the brake wheel cylinders of the brake system are adjusted by hydraulic regulator to generate the required yaw moment of the vehicle. When necessary, it communicates with EMS and is managed by the engine management system.
The driving force of the driving wheel is changed to change the running state of the vehicle. After the change, the running state of the vehicle is measured by sensors to ECU, and then controlled by the next cycle, so as to keep the vehicle stable. This is the general principle of vehicle stability control. The basic idea of vehicle operation stability control is to control the critical steady state condition to organize the vehicle into uncontrollable unsteady state. The critical steady state is determined by the deviation between the actual driving state variable value and the reference variable value obtained by using the linearized tire model.
The vehicle operation stability directly receives the influence of yaw moment and can be adjusted by adjusting the wheel lengths. The directional force is used to control the yaw moment. According to this idea, the operation stability of automobile under extreme conditions is improved by applying braking force to a certain wheel. Firstly, the sensor detects the running state of the car and calculates the actual yaw angular velocity. Then, the expected value of the running state of the car is calculated by the ideal state model. The deviation between the actual value and the expected value is calculated by the controller to output the yaw moment needed to control.
Then, the yaw moment output by the controller is used to distribute the braking power of the driving wheel, that is, by applying braking to the output wheel. Force is used to control the yaw moment and improve the operation stability of the vehicle. Because the sideslip angle of the center of mass can not be measured directly in the actual control system, considering the problem of sampling time, that is, the simpler the vehicle model is, the shorter the calculation time of sampling, the two-degree-of-freedom vehicle model is selected to estimate the sideslip angle of the center of mass. In the design of the PID control system, based on the linear two-degree-of-freedom measurement model, the ideal vehicle model parameters, i.e.
the desired vehicle yaw rate and the sideslip angle of the center of mass, are obtained. Gamma d = ul(1 Au2)delta. In the steering of automobile, the two parameters of sideslip angle of mass center or yaw angular velocity of automobile are selected to control respectively. Finally, compared with the reference value, the error value after comparison is output to the brake force distributor through PID control, and then the brake force distributor adjusts the wheel to achieve the purpose of controlling the vehicle. In the PID control of yaw angular speed, the error signal is output to the brake force distributor after passing through the PID controller. The brake force distributor needs to combine the difference between steering wheel angle and yaw angular speed (gamma-gamma d) to determine the over-steering and under-steering, so as to determine which wheel to control best.
The PID control of the sideslip angle of the center of mass is similar to that of the yaw speed control. In fact, it is the error between the actual value of the sideslip angle of the center of mass and the ideal value (beta-beta d). The ideal value of the sideslip angle of the center of mass is beta d=0. The actual value is calculated by the linear two-degree-of-freedom vehicle model. When the error is output to the brake force distributor of the vehicle by the controller, the difference between steering wheel angle and yaw angular velocity (gamma-gamma d) should be combined to judge the excessive steering and insufficient steering, so as to determine which wheel to control is the best. Automotive Electronic Stability Control System (ESP) can improve vehicle stability under extreme conditions by adjusting the braking power of four wheels. It has yaw moment control system, which provides strong support for safe and stable driving of vehicles under braking and steering conditions.
During steering, the key parameters of vehicle operation, such as center of mass sideslip angle and yaw angular velocity, are analyzed and controlled. The deviation between the actual value and the reference value, the output yaw moment is distributed to the tire in the form of appropriate braking force through the braking force distribution system, so that the vehicle can offset or reduce the yaw moment of excessive and insufficient steering, so as to achieve the purpose of vehicle turning stability control.