Research On Integrated Control Of Vehicle Braking System Based On Wheel Slip

With the development of the electronic control system, the research on electronic control systems such as ABS,EBD,ESP are more and more mature. In order to improve the performance of the vehicle further, to research the integration of these systems is necessary. To coordinate these subsystems of an integrated control system effectively and rationally and to make them work fully and properly are the most important problems in the integrated control of the vehicle.ABS,EBD,ESP are all the electronic control systems which insecure handling stability of vehicle through controlling the vehicle braking system, the introduction of this paper gives a particular depiction of the configuration,role and dynamic elements of these three systems, the basic goal of ABS is to prevent the wheel from entering into deep slip, and the role of EBD is to insure that the slip of the rear wheel is no more than the slip of the front wheel while ESP keep the stability of vehicle through watching the yaw rate and the sideslip angle of the vehicle. Studies on the integrated control system are recently active in analyzing each system and optimizing the performance. Based on the research production of the past, this paper carries out the research on the integrated control of vehicle braking system.Using the software called veDYNA to build up a parameterized vehicle model, provides a useful simulation tool for researching various control algorithms. veDYNA is a vehicle dynamic simulation software based on Matlab, its vehicle model is built up in form of simulink model, so we can modify or replace any assembly of the veDYNA vehicle model freely. According to the need of the research, the model of HCU is added to the braking system of the veDYNA vehicle model, and the interface of the original vehicle model for adding various control algorithm is found out,both of which provide a typical model for the developing of the control algorithms of ABS,EBD and ESP systems. In order to develop the integrated control algorithm of the vehicle braking electronic control system, we should first obtain the independent algorithms based on wheel slip of the ABS,EBD,ESP. For the control algorithm based on wheel slip of ABS,we use the control method of limit value. Assuming that the optimal wheel slip ratio isλ,we can set a high limitλ+aand a low limitλ?b,when the wheel enters into ABS control algorithm, if the real slip ratio s of the wheel is smaller than the low limit valueλ?b, increase the wheel cylinder pressure; if the real slip ratio s of the wheel is bigger than the low limitλ?bbut smaller than the high limitλ+a, keep the pressure of the wheel cylinder; if s is bigger than the high limitλ+a,decrease the pressure of the wheel cylinder. Through this control method, we can limit the real slip of the wheel to a small range which is around the optimal slip of the wheel, so the wheel can achieve bigger longitudinal and lateral force. To obtain good handling stability on theμ-split road, a control algorithm called"low selected"is added to the ABS control algorithm. In this algorithm, the wheel whose slip is smaller is controlled according to the slip of the wheel whose slip is bigger. For EBD,the control algorithm is implemented by comparing the slip of rear wheel and front wheel. The algorithm maintains the slip of rear wheelλ2near the slip of front wheelλ1 , while it has to make sure thatλ2is not bigger thanλ1 ,through which the brake forces of the rear and front axles are distributed rationally and the road friction coefficient is made full use, the braking stability of the vehicle is improved. We can compare the difference of the front and the rear wheelλ1 -λ2, and we can also compare the ratioλ2/λ1 . Comparing the ratioλ2/λ1 , for example, we can set a high limit s1 and a low limit s 2. If the value ofλ2/λ1 is smaller than s1 ,the cylinder pressure of the rear wheel should be increased. If the value ofλ2/λ1 is bigger than s1 but smaller than s 2,this indicates that the slip ratio of the rear wheel is proper, so we needn't to change the pressure of the rear wheel cylinder. If the value ofλ2/λ1 is bigger than s 2, the cylinder pressure of the rear wheel should be decreased. For ESP,we select yaw rate and sideslip angle as control variable. We can use yaw rate value to judge the under steering and over steering of vehicle when the sideslip angle is relatively small, while the sideslip angle is big we use it to judge the over steering of vehicle. The control algorithm takes the nominal yaw rate and stable boundary of sideslip angle as references to calculate the ideal state of the vehicle under current driver input and outer conditions. Then compare real state of the vehicle with the idea state to figure out the extra yaw moment which can get the vehicle back to stable state. Then the yaw moment is transformed into the change of longitude slip of the given wheels using the HSRI tire model, so we transform the control of yaw moment into the control of the change of wheel slip. After obtaining the relation between the extra yaw momentΔM and the increment of wheel slipΔλ, we can calculateΔλaccording to the value of the extra yaw momentΔM , plusΔλand the original wheel slipλi, we get the nominal slip valueλ(λicorrespond to master cylinder pressure Pi )。According to the value ofλ,we set a high limitλ+a and a low limitλ?b( a >0,b >0) separately,then compare current wheel slip ratio s with these two limit values,if s is smaller thanλ?b,increase the pressure of the corresponding wheel cylinder, if s is bigger thanλ?b but smaller than the high limitλ+a, current cylinder pressure of the corresponding wheel should be kept. If s is bigger thanλ+a,decrease the pressure of the corresponding wheel cylinder. Through these control steps, we constrain the real slip ratio to the range around the nominal slip ratio, and the extra yaw moment we need is produced. Controlling wheel slip directly is the ultimate difference between the ESP control algorithm based on slip and the conventional ESP control algorithm. By acting various open-loop and closed-loop tests, the effectiveness of these control algorithms is proved.After getting the independent control algorithms of ABS,EBD and ESP, we carry out the research on the integrated control of these three electronic control systems. In some complicated maneuver such as cornering braking, the relationship among three systems is coordinated according to the wheel slip. When the brake force is relatively small, EBD is acted to ensure the brake force is distributed between the front and rear wheel rationally. With the increase of the brake force, if any of the four wheels tends to lock, ABS is acted at once; During this process, ESP system watching the yaw rate and the sideslip angle of vehicle carefully, if the vehicle is going to lose stability, the ESP immediately produce a extra yaw moment to get the vehicle back to stable state. When the ESP system is active, ABS system should ensure that none of the wheels get into deep slip area, through these control steps, the relationship among ABS,EBD and ESP under intricacy maneuver is corresponded effectively. A serious of open-loop and closed-loop tests have been done to prove the validity of the control algorithm.