introduction
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The braking performance of the car is mainly evaluated from the following three aspects: 1) braking performance, that is, braking distance and braking deceleration; 2) braking performance constant, that is, heat resistance or water degradation performance; The directional stability of the car during braking, that is, the car does not experience deviation, side slip and loss of steering ability during braking.
Usually, there are two kinds of forces in the car during braking: one is the frictional resistance generated between the brake lining and the brake drum or the brake disc. This resistance is called the resistance of the brake system, because it provides The braking force at the time of movement is therefore also referred to as the braking force of the brake system; the other force is the frictional resistance generated between the tire and the road surface, also referred to as tire-road adhesion. If the brake system braking force is less than the tire-road adhesion, the car will remain stable when braking; conversely, if the brake system braking force is greater than the tire-road adhesion, the wheel will lock when the car brakes. If the front wheel is locked, the car basically travels straight ahead, the car is in a steady state, but the car loses steering control, so the driver must avoid obstacles, pedestrians and the necessary steering in the cornering process. Manipulation control, etc. cannot be achieved. If the rear wheel is locked, the stability of the braking direction of the car will be deteriorated. Under the small lateral interference force, the car will have a dangerous phenomenon such as tail flies and even turning the head. Especially in some bad road conditions, such as slippery roads or ice and snow, it is difficult to ensure the safety of the car when the wheels are locked. In addition, due to the wheel lock when braking, resulting in local sharp friction, the service life of the tire will be greatly reduced.
1 How ABS works
The anti-lock Braking System (ABS) refers to the automatic adjustment of the wheel braking force to prevent the wheels from being locked by controlling the brake line pressure acting on the wheel brake cylinder during braking. ABS, which is the best electronic braking device for achieving the best braking performance, is one of the most effective measures in the world to improve the safety of automobile braking.
2 Hydraulic ABS system based on MC9S12DP256 chip
The hydraulic ABS system is mainly used in the braking system of cars, mini vehicles and light 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 is more More and more manufacturers have paid attention to it, and many manufacturers have started to develop hydraulic ABS systems.
The MC9S12DP256 chip from Freescale Semiconductor (formerly Motorola Semiconductors) is based on the high-speed CPU12 core (Star Core), with a bus speed of 25MHz. The MC9S12DP256 microcontroller consists of standard on-chip peripherals, including a 16-bit central unit. Processor (HCS12CPU), 256K bytes of Flash, 4K of EEPROM, 12K bytes of RAM. 5-way CAN bus, temperature range -40 ~ 125 °C, 5 V power supply, mainly used for industrial control, especially suitable for use in cars. The block diagram of the 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, and the automatic program/erase algorithm is used to improve the performance. The extended instruction set and addressing mode improve the coding efficiency. The background debug mode module (BDM) can Allow full speed online simulation. In addition, the CodeWarrior Integrated Development Environment (IDE) is used to download the control program and modify related parameters through the background debug mode (BDM) to monitor each register and memory in real time without disturbing the target program. The on-board debugging of the control program is realized, thereby improving the development efficiency of the integrated system and the convenience of the test, and the test cycle can be greatly shortened.
3 Hydraulic control principle The hydraulic ABS system adopts an integrated structure, that is, the controller and the regulator are integrated into one body, which can make the system structure simple and save the wire harness. At the same time, due to the adoption 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 a three-position three-way solenoid valve. Its voltage regulation method and working principle are as follows:
(1) Normal braking (ABS does not work)
The braking force is small during normal braking, the wheel has not been locked and slipped, ABS does not work, and the three electromagnetic control valves are 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 four wheel cylinders through three electromagnetic control valves to generate a 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 via the electromagnetic control valve. At this time, since the ECU (ABS electronic control unit) does not issue a command, the electric oil pump is not in operation.
(2) Step-down state When the brake of the car causes the wheel to be in the state of being locked, the ECU issues a command according to the signal of the sensor, and a large current is applied to the coil of the three-position solenoid valve, so that the three-position solenoid valve is in the third type. The state, that is, the three solenoid valves in the figure are in the right position, the oil passage to the master cylinder is closed, and the four wheel cylinders are in communication with the accumulator, and the high pressure oil in the wheel cylinder flows into the accumulator through the solenoid valve. The brake pressure is reduced to prevent the wheel from locking. At the same time, the electric oil pump is operated by the ECU, and the oil flowing into the accumulator is pressurized and sent back to the master cylinder for preparation for the next operation.
(3) Holding state When the brake cylinder is depressurized (or boosted) to the optimal braking state, according to the signal from the wheel speed sensor, the ECU issues 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 is in the middle position, and the valve ports are closed, and the oil pressure in the wheel cylinder is kept in an optimal 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 communicate again, and the high-pressure brake in the master cylinder The liquid enters the wheel cylinder through the three-position solenoid valve, so that the braking force rises rapidly.
The ABS controls the various states of the above-mentioned step-down, holding pressure, and boosting, so that they alternate with each other, thereby ensuring that the vehicle obtains the best braking effect. 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 a car is a very complicated process, so it is almost impossible to describe the braking process completely accurately. Before the simulation, the braking process of the car can be appropriately simplified and assumed, simplified and assumed. The car afterwards actually becomes a model of two-degree-of-freedom linear motion.
The equation of motion of the body during braking is:
The MATLAB / SIMUL INK software is used for the simulation. The simulation is performed using 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; center of gravity 655 mm from the ground; center of gravity distance from the front axle 1 430 mm; distance from the center of gravity to the rear axle 1370 Mm; wind resistance coefficient 0.4; wind resistance center height 965mm; windward area 1 736 × 1 490 mm2; initial braking speed 80 km / h; pavement is ordinary cement pavement. 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 Freescale chip can keep the slip ratio between 0.1 and 0.3 during the braking process, effectively preventing the wheel from locking and ensuring the car. Has good braking performance.
(2) When industrial grade ordinary chips are used as ABS control chips, there are unsatisfactory aspects in temperature adaptability and system stability. The system is more reliable and stable due to the use of Motorola's special automotive chip MC9S12DP256. It provides a new option for the development of hydraulic ABS systems.
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