Driving Directional Stability And Brake Compatibility Control Strategy Of Tractor-Semitrailer Braking In A Turn

With the increasing of highway construction investment and stable growth of national economy fast development to the material requirement as well as the freight vehicle technical level progressing continually, our country road freight transportation industry entered a high-speed development stage. The road freight transportation is very important in the mixed transportation services system, and has become the large artery for the economic development. Tractor-semitrailer has become one of main vehicle style in road freight transportation for its advantages, and played a major role in the road freight transportation. Tractor-semitrailer is consisted of a tractor and a semitrailer based on the coupling of joint elements such as electrical connections, fifth wheel and fifth wheel kingpins. Comparing with the single vehicle, more complex dynamics system is formed by coupling between tractor and semitrailer. Some factors such as the irrational use of design parameter, operating parameter and controlled parameter as well as tractor and semitrailer's improper matching, result in some unsteady state while breaking, for example jackknifing, swing and side-slipping, etc. When Tractor-semitrailer is braking in a turn, unsteady state will cause severe traffic accident, posing serious threat to the traffic participants. Therefore, it is very important to the traffic safety that studying on the lateral stability of Tractor-semitrailer when braking in a turn. Studying on Tractor-semitrailer from the designing aspect and utilization aspect affecting lateral stability of the tractor-trailer braking in a turn, and researching into braking force control strategy based on theory, whick providing the theory and the technical support for tractor-semitrailer lateral stability, that is very important to the traffic safety.The dissertation supported by The West Traffic Construction Science and Technology Project'Study on Technical Specifications of Safe Operation and Inspection Method for Goods Vehicle'(200531822322) and National Key Technology R&D Program of China'Technologies Research of Safe Operation for Heavy Commercial Vehicles and Buses'(2007BAK35B04) aiming at security when tractor-semitrailer braking in a turn, combining with related cybernetics technology, makes an in-depth research on tractor-semitrailer when braking in a turn. The detailed contents are as following:(1) To meet the requirement of research, the models of meridian pneumatic tire braking mechanical model of the heavy truck and the braking moment dynamic characteristic model are established. Based on testing data, concrete parameter that describes tire model is identified according to the actual test data, and this parameter makes the tire model concretization. Then it analyzed relations between braking force, the side force, the slip angle of deflection, the vertical load and adhesion coefficient of the tire and ground as well as the slipping rate and so on. The braking moment dynamic characteristic model contains the influence of brake delay time, describing dynamic changeable characteristics of the wheel braking moment and the brake pressure along with the control pressure and the delay time. The establishment of tire model and the braking moment model has provided theoretical foundation for the study on lateral stability and control strategy of tractor-semitrailer braking in a turn.(2) This dissertation put forward the method of calculating air resistance coefficient and rolling resistance coefficient based on two-stage coasting-down test data. Because tractor-semitrailer affects by air resistance and rolling resistance when it brakes in a turn, whether it is accurate or not will affect the correctness of the simulation of tractor-semitrailer braking stability. Traditional sliding test limits by the test sites, and results in the waste of time and resources. This dissertation presents the method of calculating air resistance coefficient and rolling resistance coefficient based on two-stage coasting-down test data. According to the real vehicle test data of two-stage coasting-down test, rolling resistance coefficient and air resistance coefficient are calculated. According to the achieved data, sliding distance and velocity data compare with multi-sample coasting-down data prove the two-stage coasting-down test data right and feasible. The reference for calculation of vehicle rolling resistance coefficient and air resistance coefficient of vehicle dynamics is given.(3) By the analysis of dynamics of tractor-semitrailer system in a turn, establish a nonlinear mathematical model of vehicle breaking in a turn. According to the standards of ISO and the actual situation, the evaluation indicators of braking performance of tractor-semitrailer in a turn are proposed, and the test specifications are established. Tractor-semitrailer performance test equipment, which supported by The West Traffic Construction Science and Technology Project, is used for real vehicle road braking performance tests. By comparing and analyzing the data between real vehicle road test and the simulation test based on model, prove that the model of tractor-semitrailer breaking in a turn is correct, and lay a foundation for the accurate analysis for effect of operation parameters and structural parameters to lateral stability of tractor-semitrailer braking in a turn.(4) On the basis of the model which has been built for the tractor-semitrailer braking in a turn, the effect of breaking performance in a turn has been analyzed and researched from the aspects of the structural parameters and the operation parameters, and put forward measures for improvement. In this thesis, the operation parameters and structural parameters of fifth wheel lead, semi-trailer wheelbase, brake lag time, brake-force distribution coefficient and the speed are chosen to analyze their influence of tractor-semitrailer braking performance in a turn。In addition, braking distance, hinge angle, hinge angular velocity and tractor yaw rate are chosen as the indicator that can reflect direction stability of tractor-semitrailer braking in a turn. The research and analysis show that an increase on the fifth wheel lead and semi-trailer wheelbase is conducive to improve the brake stability of tractor-semitrailer in a turn; and excessive large semi-trailer brake lag time and speed are disadvantage. The braking force distribution coefficient should be designed that the semi-trailer rear axle lock firstly, followed by semi-trailer tractor front axle, while tractor rear axle lock firstly is a dangerous situation, which can led to tractor-semitrailer in a jacknifing dangerous conditions, that should be avoided.(5) To improve the stability of tractor-semitrailer braking in a turn, the strategy of breaking control has been given. By control the breaking force, the wheels slip ratio can maintained at a relatively low level, so that tractor-semitrailer can maintain a good ability to resist lateral force in the case of a higher braking deceleration. In this dissertation, the braking force control strategies are studied by means of the software named TruckSim software. Based on comparing and analyzing advantages/disadvantages and simulation results of the control strategy such as logic threshold value, slip ratio optimal fuzzy control and slip ratio optimal PID control, it shows that the breaking force control system can significantly improve the direction stability of tractor-semitrailer braking in a turn. It provides a technical reference for improving lateral stability of tractor-semitrailer braking in a turn and protecting the road traffic safety by controlling the braking force.