Study On The Road Condition Identification Of Traction Control System For Commercial Vehicle

With the development of automobile industry, more and more demands to vehicle safety capability are taken into account. When vehicle starts or accelerates on the road with low coefficient of friction, the wheels will slip excessively because of the low adhesion that the road surface provides. It causes the longitudinal and lateral adhesions to further reduce. And the vehicle stability,handling and acceleration will be greatly reduced. Traction Control System (TCS) is coming to be a popular feature on many vehicles because it can prevent the driven wheels from slip during vehicle accelerates especially when vehicle starts,accelerates and improve vehicle's acceleration stability and handling. It is based on the principle that maintaining the driving slip ratio value round the optimum slip ratio so that traction can not only be fully used but also the lateral stable potentiality is guaranteed. The optimum slip ratio and control algorithm is very important for TCS, so the two problems are what this paper will discuss. In this paper, the characteristics of the vehicle's parameters, especially the speed and acceleration of the driven and driving wheels, will be analyzed while the vehicle is speeding up in different roads. Then a new method of road identification is supplied for TCS, and TCS control algorithm is also improved duo to the control purpose under different conditions.This paper includes five parts:(1) Model of vehicle dynamic: The math models of the vehicle's dynamic systems are built, including: engine, transmission, tire, whole vehicle, brake system and so on. Engine model contains two parts, which are static state model and dynamic model. The former is modeled according to the speed characteristic experiment, then polynomial fit these experiment data (output torque and engine speed); the latter is modeled by simplifying the engine dynamic process as once inertia. The system with a transport delay. In the case of transmission, we regard clutch,gear-box,power transfer,transmission shaft and half shaft as the components whose function are only transferring the torque. Double index model is used for the tire model, which has simple structure, fewer parameters needing fitting, litter computer task and can well describe the nonlinear law for longitude adhesion coefficient changing duo to the slip ratio and other factors. The whole vehicle model, having seven freedom, reflects control system's influence on the vehicle dynamic and observes vehicle's running state, including longitudinal displacement, lateral displacement, yaw angle and the spin of the four wheels. Brake system is a once inertia system with a transport delay, by which the code (increase, decrease or hold) is transformed into the ratio of the air pressure. Besides, according to those, simulation model is built under the environment of Matlab/simulink.(2) The characteristic of the vehcile's parameters while speeding in different roads and the road condition identification method: Having analyzed the characteristic of vehicle's parameters, the conclusions come:While the driving wheels slip in different roads, the acceleration of the driven wheels is different when the vehicle is speeding up. But in the same road the acceleration of the driven wheels are equal, in spite of the gears and the accelerograph. According to longitudinal adhesion coefficient computing method of the double-index model, the road adhesion coefficient is related with the vehicle speed. So according to the average acceleration of the driven wheels during the first TCS control cycle and the vehicle speed, the road style can be identified.The difference of the left and right driving wheel also serves as the parameter to identify theμ-split road. But different rules are used to set the threshold according to the vehicle speed. When the vehicle is starting, the threshold is a small constant. While low speed, it could be a rate of the low wheel speed. While high speed, it also could be a constant, but larger. Simulation model is built according to the road condition identification and the validity is identified.(3)Control algorithm with mutative threshold for TCS: PID control for accelerograph and logic threshold for braking is used generally. Considering the following factors : Wheel velocity sensors'signals are not accurate enough, when velocity is lower than 1m/s. While the speed is more high than 1m/s but lower than 10m/s improving vehicle's power is important. While speed is over 10m/s, the stability should be considered. So, the control law should be as follow: While speed is lower than 1m/s, the target wheel speed is the 1 added by S₀. While speed is more high 1m/s but lower than 10m/s, the target wheel speed is such a speed due to the ratio S₀. Last, while speed is over 10m/s, the target wheel speed is vehicle speed added S₀ ten times. In this way, the misjudgement is reduced while staring, power is improved while lower speed and ensure its stability while high speed. For theμ-split road ,the former method could be used too. But the ratio is not S₀, but a ratio gained by experiments. In theμ-split road, there are two control methods: Consider the difference of the two driving wheels as the threshold. See the acceleration of the high speed wheel as the secondary threshold. The difference of the two driving wheels is limited.(4)Off-line simulation for TCS: TCS simulation model is built under the Matlab/simulink environment. Compare the simulation results with the one while other control algorithm is used in different rod conditions and evaluate them. Comparing results present that this control algorithm is improved. (5) Hardware in loop test-bed: Study on the structure and principle, develop the soft ware for the test-bed and debug it. The following experiments are done: In different roads, while the vehicle is staring and accelerating with low and high speed. The results also have the same conclusions, but in some aspects, the test-bed results have a little lag, which is resulted from the mechanical system. TCS can prevent the slip of the driving wheels.