| The function of Anti-lock Braking system(ABS) is to prevent wheels from locking when braking, keep the wheel slip ratio close to the optimal. In this way, vehicle handling stability and direction stability could be ensured while braking performance could be enhanced. ABS is very important for commercial vehicles, which are equipped with pneumatic braking system and always carry groups of people and cargo. The ABS control strategies are different in hydraulic braking system and pneumatic braking system, because the medium used in these two systems(liquid and air) and the structures of these two systems are different. The ABS control strategy used in pneumatic braking system would be proposed in this paper. The main research contents are as follows:Firstly, a test bench for pneumatic ABS was built for the design of ABS control strategy. Experiments were conducted on the bench to test the performance of an ABS magnetic regulating valve. The pressure in the brake chamber was regulated by sending different control signals to the ABS magnetic regulating valve, and the change curves of the pressure were recorded. Then, the pressure-regulating performance of the ABS valve was obtained through the recorded curves. After this, to design the ABS control strategy, a pressure model was built by the method of curve-fitting in Simulink based on the recorded curves.Secondly, the ABS control strategy was proposed based on state machine theory, using the logic threshold control method. The control strategy was designed to fit pneumatic braking system, according to the performance of the ABS valve. In the control strategy, braking process of a wheel is divided into 5 states based on whether the slip ratio is in stability region. In different states, the ABS valve is controlled in different ways to change the braking torque and the real-time kinesiology signals of the wheel(wheel angle deceleration, slip ratio). Once the real-time kinesiology signals are changed enough to trigger the logic conditions which contain threshold values, the state of the wheel will transfer to another. Through transitions between states, the wheel slip ratio would be kept close to the optimal. In the process of control, pressure-increasing, pressure-holding and pressure-decreasing are performed in the brake chamber periodically. In the process of pressure-decreasing, the pressure would be estimated before decreasing and after decreasing. In the process of pressure-increasing, before the pressure begin to increase, duration of this process would be estimated by the pressure estimated in the process of pressure-decreasing. In this way, an effective control for increasing the pressure could be achieved. The road friction coefficient could be recognized by vehicle deceleration, then threshold values would be adjusted in real time to adapt different road friction coefficients.Thirdly, to testify the validity of the control strategy, the offline joint simulation method of Truck Sim and Simulink was used. The strategy model was built in Simulink while the vehicle model was set in Truck Sim. Then, the emergency braking processes of the vehicle were simulated. In simulation, 4 kinds of roads were chosen: the road of high friction coefficient, the road of low friction coefficient, the bisectional road and the joint road. The simulation results indicate that on all kinds of roads the control strategy has good control effect.Finally, based on the pneumatic ABS test bench, a Hardware-in-the-loop Simulation(HILS) test bench was established with the Simulator and Micro Auto Box which are both produced by the company d SPACE. The control strategy was downloaded into Micro Auto Box, and the vehicle model was downloaded into Simulator. The validity of the control strategy was testified again by HILS solution. The emergency braking processes on different kinds of roads were simulated. The roads chosen in HILS were the same as the roads in offline simulation. And the simulation results also indicate the control strategy has good control effect. |
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