“The quality of automobile braking performance is mainly evaluated from the following three aspects: 1) Braking efficiency, that is, braking distance and braking deceleration; 2) The constancy of braking efficiency, that is, resistance to heat or water decay; 3 ) The directional stability of the car when braking, that is, the performance of the car not to deviate, side slip and lose the ability to turn when braking.
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Authors: Jiang Kerong, Wang Zhisen, Sun Jun
The quality of automobile braking performance is mainly evaluated from the following three aspects: 1) Braking efficiency, that is, braking distance and braking deceleration; 2) The constancy of braking efficiency, that is, resistance to heat or water decay; 3 ) The directional stability of the car when braking, that is, the performance of the car not to deviate, side slip and lose the ability to turn when braking.
Usually, there are two kinds of forces in the braking process of the car: one is the frictional resistance generated between the brake pads and the brake drum or brake disc, which is called the resistance of the braking system, because it provides braking The braking force when moving, so it is also called the braking force of the brake system; the other force is the frictional resistance generated between the tire and the road surface, also known as the tire-road adhesion. If the braking force of the braking system is less than the tire-road adhesion, the car will maintain a stable state when braking; on the contrary, if the braking force of the braking system is greater than the tire-road adhesion, the wheels will lock when the car is braking. If the front wheels are locked, the car is basically moving forward in a straight line, the car is in a stable state, but the car loses steering control, so that the driver avoids obstacles, pedestrians and the necessary steering on the curve during braking Manipulation control, etc., cannot be achieved. If the rear wheels are locked, the braking directional stability of the car will become poor, and under a small lateral interference force, the car will have a tail flick or even a U-turn and other dangerous phenomena. Especially in some bad road conditions, such as slippery roads or ice and snow, wheel locking will make it difficult to ensure the driving safety of the car. In addition, due to the locking of the wheels during braking, local sharp friction is caused, which will greatly reduce the service life of the tires.
1 How ABS works
ABS (Anti-Lock Braking System) refers to the automatic adjustment of the wheel braking force by controlling the brake line pressure acting on the wheel brake cylinder during the braking process to prevent the wheels from locking and causing damage. The Electronic device that achieves the best braking performance, ABS is one of the effective measures generally recognized in the world to improve the safety of automobile braking.
2 Hydraulic ABS system based on MC9S12DP256 chip
Hydraulic ABS systems are mainly used in braking systems for cars, mini-cars and light-duty vehicles. Unlike the pneumatic ABS system, the controller and regulator of most hydraulic ABS systems are an integrated system. Although it is difficult to manufacture, due to the development of the domestic car market and the improvement of traffic safety requirements in recent years, the hydraulic ABS system has become more and more difficult. More and more manufacturers are paying attention, and many manufacturers have begun to develop hydraulic ABS systems.
The MC9S12DP256 chip of Freescale semiconductor (formerly Motorola Semiconductor) is a single-chip microcomputer with a high-speed CPU12 core (Star Core) as the core, with a bus speed of 25MHz. The MC9S12DP256 single-chip microcomputer consists of standard on-chip peripherals, including a 16-bit central Processor (HCS12CPU), 256K bytes of Flash, 4K of EEPROM, 12K bytes of RAM. 5-channel CAN bus, the temperature range is – 40 ~ 125 ℃, using 5 V power supply, mainly used for industrial control, especially suitable for use in automobiles. The configuration block diagram of hydraulic ABS system based on MC9S12DP256 control is shown in Figure 1.
Due to the built-in bus of the MC9S12DP256 chip, the anti-interference ability is particularly strong. The automatic programming/erase algorithm is used to improve the performance, and the extended instruction set and addressing mode improve the coding efficiency. The background debugging mode module (Background DebuggingMode, BDM) can Allows in-circuit emulation at full speed. In addition, using CodeWarrior integrated development environment (Integrated Development Environment, IDE), through the background debug mode (Background debug mode, BDM), download the control program and modify the relevant parameters, without disturbing the running of the target program, real-time monitoring of each register and memory , to realize the on-board debugging of the control program, thereby improving the development efficiency of the integrated system and the convenience of the test, which can greatly shorten the test cycle.
3 Hydraulic control principle
The hydraulic ABS system adopts an integrated structure, that is, the controller and the regulator are integrated into one, which can make the system structure simple and save the wiring harness. At the same time, due to the use of the MC9S12DP256 chip, it is sufficient to meet the requirements of ABS calculation, real-time control and diagnosis. The hydraulic control part mainly uses the characteristics of the three-position three-way solenoid valve. Its voltage regulation mode and working principle are as follows:
(1) Normal braking (ABS does not work)
During normal braking, the braking force is small, the wheel has not yet locked and slipped, the ABS does not work, and the three-position solenoid control valve is in the first state, that is, the left position of the three-position solenoid valve. The brake fluid from the brake master cylinder directly enters the 4 wheel cylinders through 3 solenoid control valves to generate braking effect. When the brake pedal is released to release the brake, the oil in the wheel cylinder still flows back to the master cylinder through the solenoid control valve. At this time, since the ECU (ABS electronic control unit) has not issued a command, the electric oil pump is in a non-working state.
(2) Depressurized state When the car brakes and the wheels are about to be locked, the ECU sends a command according to the signal of the sensor to pass a larger current to the coil of the three-position solenoid valve, so that the three-position solenoid valve is in the third state. state, that is, the three-position solenoid valve in the figure works in the right position, the oil circuit leading to the master cylinder is closed, and the four wheel cylinders are connected to the pressure accumulator, and the high-pressure oil in the wheel cylinder flows into the pressure accumulator through the solenoid valve, The brake pressure is reduced to prevent the wheels from locking. At the same time, under the command of the ECU, the electric oil pump works to pressurize the oil flowing into the accumulator and send it back to the master cylinder to prepare for the next braking operation.
(3) Pressure-holding state When the brake wheel cylinder is decompressed (or boosted) to the optimum braking state, according to the signal sent by the wheel speed sensor, the ECU will issue a corresponding command to apply a small current to the coil. , the three-position solenoid valve is in the second state, that is, the three-position solenoid valve in the figure is in the middle position, each valve port is closed, and the oil pressure in the wheel cylinder is kept in the best braking state.
(4) Boost state When the braking force is insufficient, the wheel speed sensor sends a signal, the ECU stops supplying power to the coil, the three-position solenoid valve returns to the first state, the master cylinder and the wheel cylinder are connected again, and the high pressure brake in the master cylinder The fluid enters the wheel cylinder through the three-position solenoid valve to rapidly increase the braking force.
ABS controls the above-mentioned states of depressurization, pressure maintenance, and boosting, so that they alternate with each other, so as to ensure the best braking effect of the car. The hydraulic circuit of the ABS system is shown in Figure 2.
4 Simulation test results
SIMUL INK is a software package for modeling, simulating, and analyzing dynamic systems. It supports linear and nonlinear systems, continuous and discrete time models, or a mixture of the two. SIMU2L INK has been widely used in industrial control and other fields. The actual braking process of the car is a very complex process, so it is almost impossible to describe the braking process completely and accurately. Before the simulation, appropriate simplifications and assumptions can be made on the braking process of the car. After simplification and assumptions The rear car actually becomes a two-degree-of-freedom model of linear motion.
The equation of motion of the body when braking is:
In this paper, MATLAB/SIMUL INK software is used for simulation research. The simulation is carried out by the fifth-order variable-step Runge-Kutta method. The minimum simulation step size is 0.001 s, and the maximum simulation step size is 0.01 s. Taking a certain type of car as the simulation test object, the main parameters of the car are: full load mass 1 795 kg; wheelbase 2 800 mm; the height of the center of gravity from the ground is 655 mm; the distance between the center of gravity and the front axle is 1 430 mm; mm; wind resistance coefficient 0. 4; wind resistance center height 965mm; windward area 1 736 × 1 490 mm2; initial braking speed 80 km/h; road surface is ordinary cement road. The simulation results are shown in Figure 3 and Figure 4.
5 Conclusion
(1) It can be seen from the simulation results that the hydraulic ABS system based on the Freescale chip can keep the slip rate between 0.1 and 0.3 during the braking process, which can effectively prevent the locking of the wheels and ensure the safety of the vehicle. Has good braking performance.
(2) When industrial-grade ordinary chips are usually used as ABS control chips, the temperature adaptability and system stability are not satisfactory. The system uses Motorola’s special automotive chip MC9S12DP256, which makes the system more reliable and stable. , which provides a new option for the development of hydraulic ABS system.
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