| As the improvement of national highway transportation, tractor seimi-trailer has become one of the main vehicles for freight, because of its flexibility and mobility. Stock of tractor semitrailer is 210,000, and volume of production every year is 25,000. Howerver, they tend to rollover or loss of lateral stability, even cause severe traffic accidents and huge loss, with its big load capacity, large size, high center of gravity. There are two ways to improve the stability of the current tactor semitrailer, mainly through optimization of structural parameters or installation of stability control systems, but there is little margin for optimization of vehicle parameters to enhance its stability. So development of stabiliyt control systems becomes an urgent task for OEMs'customers.There are many stability products for tractor semitrailer available on the maket, such as rollover stability advisor (RSA), rollover stability control (RSC), electronic stabibility control system (ESC) and active anti-roll control (ARC). All the stability systems enhance vehicle stablility mainly through reducing the output torque of engine, applying braking pressure on selected wheels, or modifying characteristic of suspensions.ESC is developed based on antilock braking system and equipes a steer wheel angle sensor, a yaw rate sensor, a lateral acceleration sensor, etc. ESC also gets the infromation of engine, transmission and suspension through control area network. With these signals, ESC can detect vehicle state in realtime. When vehicle loses its lateral stabiliy or has the trends of rollover, ESC reduces the output torque of engine and applies brake on selected wheels to modify vehicle state.Combined with signaficant projects of 863 special project and international cooperation of ministry of science and technology and technology support program of Jilin provence, cooperated with Rearch center of FAW, according with the acquirements of FAW, a stability control system for tractor semi-trailer with 3 axles is researched in this dissertation. There are big differences between stability control system for tractor semitrailers and that for passenger cars. The stability control system for tractor semitrailers can be divided into two main parts: antirollvoer control and yaw stability control. An integrated structure of control algrithms is proposed in this dissertation, based on which the algorithms of estimation of vehicle states are designed. The algorithms mainly focus on estimation of vehicle mass, positon of center of gravity, rollover state, articulation angle, nomial yaw rate, and vehicle velocity, etc. The conditions of start and quit of stability control are determined, and the thresholds for different driving conditions are set, then the strategy of pressure control is used to control the pressure of the wheels. An offline simulation platform and a hardware-in-the-loop test bench are developed under the environment of Matlab to validate the stability control algrithms under different driving conditions.The main contents of the dissertaion are:1. According to the kinematic principle of tractor semitrailer, a 25 degree-of -freedom nonlinear dynamic model of triaxial tractor-semitrailer is built. The vehicle model includes the dynamics subsystem models and vehicle dynamics system models. The subsystem models include engine model, transmission model, brake model, suspension model, tire model, fifth wheel constraint model, model for calculation of wheel vertical load and auxiliary compute model. The vehicle dynamic models include tractor and trailer kinematics model and the handling stability model. The tractor semitrailer model can simulate braking, steering, accelerating and combined operating conditions on different roads, and perform the desired simulation for vehicle stability control system.2. An integrated structure of stability control algrithm is proposed. The control target of stability control system for tractor semitrailer is determined. The control system is divided into two parts, which are anti-rollover control and yaw stability control. With the analysis of different conditions of rollover, the rollover caused by steering is selected to study in this dissertaiton.3. As the great fluctuation of vehicle load, lots of vehicle paprameters change with load conditions, which may have some effects on stability control, so, some parameters used must be estimated to modify the control parameters. Multi-function is designed to estimate vehicle mass in which the information of engine and wheel speed under starting acceleration conditon is used. The distances between every axle and center of gravity are calculated using pressure signals of air suspension. Height of center of gravity is estimated using the signals of lateral acceleration and roll rate from lateral acceleration sensor and roll rate sensor available in stability control system. Vehicle velocity is estimated using wheel speed signals. A state observer is designed to estimate tractor roll angle using signals of roll rate lateral acceleration and a 4-DOF roll plane model. Extended Kalman Filter and a 3-DOF vehicle model are used to estimate articulation angle of tractor semitrailer. A 2-DOF linear model is built to calcualte the nominal yaw rate of tractor in order to identify the steering characteristics of the tractor.4. According to the estimated vehicle state parameters and the vehicle steering characteristics, which aim to identify the steady state of the vehicle, to determine the start and exit conditions of stability control. When the vehicle has the trend of rollover, firstly a test brake pressure is applied on the tralier wheels to determine whether the wheels lift off the ground, then, according to the different degree of rollover and the steering conditions, it sets different control thresholds and pressure control algorithms. For the backup steering condition, full wheel brake pressure is applied on left and right wheels while the wheels are fully contacted with ground.5. When loss of yaw stability occurs, in accordance with the identified under-or over steering characteristics, the stability control system reduces the engine output torque and actively applies brakes on the selected wheels, to correct the vehicle instability state. When jackknife risk occurs, the pressure control strategy must be adjusted, and anti-jackknife control must be performed. If it triggers anti-roll control during yaw stability control, anti-rollover control will take the priorty.6. According to the tractror semitrailer dynamics model and control algorithms presented above, an offline simulation platform under Matlab environment is built, and the platform includes vehicle model, controller model and the graphical user interface. With this platform, off-line simulations of steering angle of open loop as step input, fishook input, sinusoidal input and closed-loop as double lane etc. on high, medium and low adhesion coefficient road, are perfomed. The simulation results show that the off-line simulation platform is able to complete the validation of stability control algorithm and the algorithm can prevent vehicle from rollover and loss of lateral stability and the stability control system can enhance driving stability under many conditions.7. Based on the offline simulation platform, a hardware-in the-loop test bentch is developed using the toolbox of Matlab / xPC. The test bentch includes xPc Target real-time platform, actuators, sensors, data acquisition, processing system and simulation software. The hardware of test bentch includes three parts, manipulation bentch, tractor bentch and semitrailer bentch. Tests are perfomed on the test bentch in different road conditions to validate the developed stability control algorithm furtherly. The test results show that the stability of control algorithms developed in the dessertation can prevent vehicle from rollover and yaw instability, and improve vehicle stability in a variety of driving conditions.
|
PDF Full Text Request