Study On Vehicle Dynamic Stability Control On Steering And Braking Maneuvers

Vehicle braking stability is an important performance for vehicle braking. For a long time, a lot of researches about anti-braking system have been done. ABS can improve the braking stability to some extent, and shorten the braking distance. However, it can not resolve the braking stability problem completely. In recent years, the technologies such as four wheel steering (4WS), direct yaw moment control (DYC) have been put forward, but they can only improve the handling stability at high speeds. Vehicle's combined steering and braking maneuvers is a much more complex working condition, which is quite different from the braking on straight road. The load transfer and huge side slip make it difficult to control the stability on combined steering and braking maneuvers. On the other hand, the accompanied braking for vehicle's avoiding obstacle, and the accompanied braking and emergency braking on curved road are quite common in the real driving. Under the conditions mentioned above, the vehicle can easily lose its stability, which is very dangerous, so it is necessary to study stability control on combined steering and braking maneuvers.The four-wheel-eight-degrees-of-freedom vehicle model including Magic Formula tire model is built at the beginning. Based on the model, the simulation under the combinded steering and braking maneuvers is carried out. The simulations show that with slow initial speed, smaller braking torque and minor front wheel steering angle, the vehicle can keep its stability, but if the braking torque increases, the vehicle will lose its stability eventually. What's more, with the high initial speed, even if the braking torque is small, the vehicle will lose its stability easily.The braking simulations on combined steering and braking maneuvers show that the slip ratios of the wheels will increase to 1 rapidly, and cause the unstability. Considering the anti-block braking system (ABS) can control the slip ratio, so it is possible to improve the vehicle's dynamic stability with ABS control on the combined steering and braking maneuvers. In consideration of the strong non-linearity and large parameters change on combined steering and braking maneuvers, fuzzy control which is of high robustness is employed, and the ABS control is accomplished with fuzzy control. The smulation results indicate that with appropriate initial speeds and proper front wheel steering angle, the ABS can make the vehicle maintain its stability. However, with rather larger initial speeds, the ABS control can not satisfy the stability requirements, and the vehicle will lose its stability rapidly.To better the vehicle's stability, the stability control should be exerted. So how to choose the control variables to maintain vehicle's dynamic stability is analysed. Then the vehicle stability control system on combined steering and braking maneuvers is designed. In the control syetem, the gradual change speed model of two degrees of freedom is looked as rederence model. And then the fuzzy yaw rate control, fuzzy sideslip angle control and the control combined with yaw rate and sideslip angle are established. The simulations show that the stability control can improve both the vehicle's stability and braking performance on the combined steering and braking maneuvers. By comparison, we find that the combined control with yaw rate and sideslip angle can get ideal control performance, and can get better control objective between fuzzy yaw rate control and fuzzy sideslip angle control.Finally, the stability control system is applied to lateral road trace control on combined steering and braking maneuvers. The lateral road trace control is built based on driver model. The simulation analysis not only proves that the driver model is correct, but also indicates that the stability control can be used to improve the lateral road control performance,better the driving characteristics of the vehicle, and enhance the vehicle's stability.