Detailed explanation of automobile anti-lock braking system
One is classification by manufacturer, and the other is classification by control channel. The following mainly introduces the method of classifying by channel.
In ABS, the brake pipeline that can independently regulate brake pressure is called a control channel.
The control channels of the ABS device are divided into four channels, three channels, two channels, and one channel.
(1) The four channel four channel ABS has four wheel speed sensors, and in the pipelines leading to the four wheel brake cylinders, each is equipped with a brake pressure regulator device for independent control, forming a four channel control form. However, if the adhesion coefficients of the left and right wheels of the car differ greatly (such as ponding or icing on the road), the ground braking forces of the two wheels differ greatly during braking, so yaw moment will be generated, which will make the body deviate to the side with larger braking force, and the car cannot be kept driving in the predetermined direction, which will affect the braking Directional stability of the car. Therefore, when driving on wet and slippery roads such as partially frozen or waterlogged roads, drivers should reduce their speed and not blindly believe in ABS devices.
(2) The three channel ABS system independently controls the two front wheels, and the two rear wheels are controlled together according to the principle of low selection (that is, the two wheels are controlled by one channel to ensure that the wheels with low adhesion do not lock up), also known as hybrid control.
Performance characteristics: when the two rear wheels are controlled together according to the principle of selecting the lower one, the braking force of the left and right rear wheels can be guaranteed to be equal under various conditions. Even if the adhesion coefficient of the wheels on both sides is greatly different, the braking force of the two wheels is limited to the level of small adhesion, so that the braking force of the two rear wheels is always balanced, and the vehicle has good Directional stability when braking under various conditions.
Independent control of the two front wheels is mainly considered for small cars, especially those with front wheel drive. The braking force of the front wheels accounts for a large proportion of the total braking force of the car (up to about 70%), which can fully utilize the adhesion of the two front wheels. However, due to the imbalance of the braking force of the two front wheels, the impact on the Directional stability of the vehicle is relatively small, and the resulting impact can be corrected through the driver's steering control. Therefore, three channel ABS is widely used in small cars.
(3) Two channel two channel ABS is difficult to take into account Directional stability, steering control and braking efficiency, and is rarely used at present.
(4) One channel one channel ABS, commonly known as single channel ABS, is a brake pressure regulator installed in the rear wheel brake manifold and a wheel speed sensor installed on the rear axle final drive (or one installed on each rear wheel).
What is the function of the ABS system? The anti lock braking system can improve the safety factor of emergency braking when driving. In other words, cars without ABS are prone to tire lockup when emergency braking is applied, which means the steering wheel cannot rotate. This increases the risk factor and can easily lead to serious consequences.
Single channel ABS generally controls both rear wheels together according to the principle of low selection. Single channel ABS cannot fully utilize the adhesion of the two rear wheels, so the braking distance may not be significantly shortened. In addition, the front wheel braking is not controlled, and the front wheel will still lock when braking, so the steering control ability has not been improved. However, because the two rear wheels will not lock when braking, it can significantly improve the Directional stability when braking, which is a major advantage in safety, simple structure, low cost and other advantages, so it is widely used in light trucks.
In summary, although ABS devices can shorten the braking distance, different types of ABS devices also have significant price differences due to their composition and structure. Therefore, when choosing a car, one should not only consider the high and low prices, but also the type of ABS device installed.
Limitations: When the wheels are about to reach the next locking point, the pressure of the brake fluid causes the airbag to repeatedly act, which can be applied 60-120 times in a second, equivalent to constantly braking and releasing, similar to mechanical "braking". Therefore, the ABS anti lock system can prevent directional loss and wheel slip during emergency braking, so that the wheels are not locked during braking and the tires do not rub against the ground at one point, thereby increasing friction and achieving braking efficiency of over 90%. At the same time, it can reduce braking consumption and extend the service life of the brake drum, disc, and tire by twice. Vehicles equipped with ABS have anti slip performance of 80% -90%, 10% -30%, and 15% -20% on dry asphalt roads, rainy days, and snowy days, respectively.
The ABS system itself also has limitations, and it still cannot escape certain physical laws. In both cases, the ABS system cannot provide the shortest braking distance. One method is to brake directly by experienced drivers on smooth main roads. Another scenario is to brake on loose gravel, loose soil, or deep snow covered roads. In addition, usually on dry roads, the latest ABS system can control the slip rate within the range of 5% -20%, but not all ABS brakes at the same rate or degree. Although the four-wheel anti lock braking system can enable the car to brake within the shortest possible distance, if the brake is applied too late and cannot completely stop before colliding with an obstacle, it still cannot prevent the occurrence of an accident.
During braking, ABS can quickly determine the lock up status of the wheels based on the speed signal transmitted by each wheel speed sensor. It closes the normally open input solenoid valve on the starting lock up wheel to keep the braking force unchanged. If the wheels continue to lock up, it opens the normally closed output solenoid valve. The braking pressure on this wheel rapidly moves down due to the presence of a pipeline directly connecting to the brake fluid storage tank, It prevents the wheels from completely locking due to excessive braking force. When keeping the braking state at its optimal point (with a slip rate of S of 20%), the braking effect is maximized and driving is safest.
The brake fluid in the front chamber of the brake master cylinder is dynamic pressure brake fluid, which drives the reaction sleeve to move to the right. The reaction sleeve in turn pushes the power piston, causing the brake pedal push rod to move to the right. Therefore, when ABS is working, the driver can feel the vibration of the pedal on their feet and hear some noise.
After the car slows down, once the ABS computer detects that the wheel lock state disappears, it will close the main control valve, causing the system to switch to a normal braking state for operation. If the pressure of the accumulator drops below the safe limit, the red brake fault indicator light and the amber ABS fault indicator light will light up. In this case, the driver needs to use a deep pedal style braking method with greater force to effectively brake the front and rear wheels.
In ABS, a speed sensor is installed on each wheel to input signals about the speed of each wheel into the electronic control device. The electronic control device monitors and determines the motion status of each wheel based on the signals input from each wheel rotation sensor, and forms corresponding control commands. The hydraulic solenoid valves at all locations are not energized and in a closed state, and the electric pump is also running without electricity. The brake pipelines from the brake master cylinder to each brake wheel cylinder are in communication, while the brake pipelines from each brake wheel cylinder to the reservoir are closed. The brake pressure of each brake wheel cylinder will change with the output pressure of the brake master cylinder, and the braking process at this time is completely the same as that of a conventional braking system.
During the braking process, the electronic control device determines that when a wheel tends to lock based on the wheel speed signal input from the wheel speed sensor, ABS enters the anti lock brake pressure regulation process. For example, when the electronic control device determines that the right front wheel tends to lock up, the electronic control device energizes the inlet solenoid valve that controls the brake pressure of the right front wheel, causing the right forward solenoid valve to turn off. The brake fluid output from the brake master cylinder no longer enters the right front brake wheel cylinder. The electronic control device then de-energizes both the right forward solenoid valve and the outlet solenoid valve, causing the inlet solenoid valve to turn on and the outlet solenoid valve to turn off, At the same time, it also energizes the electric pump and sends brake fluid to the brake wheel cylinder. The brake fluid output from the brake master cylinder and the brake fluid pumped by the electric pump enter the right front brake wheel cylinder through the open right forward fluid solenoid valve, causing the brake pressure of the right front brake wheel cylinder to rapidly increase, and the right front wheel begins to slow down and rotate again
ABS controls the sliding rate of the wheel towards lockup within the vicinity of the peak adhesion coefficient sliding rate by cycling the braking pressure towards the wheel towards lockup through a process of maintaining decreasing increasing. In this ABS, there is a pair of inlet and outlet solenoid valves corresponding to each brake wheel cylinder, which can be controlled separately by electronic control devices. Therefore, the braking pressure of each brake wheel cylinder can be independently adjusted, This ensures that all four wheels do not experience brake lock up.
Although the structural forms and working processes of various ABS systems are not entirely the same, they are all controlled by adaptively cycling the brake pressure that tends to lock the wheels to prevent the controlled wheels from locking. Moreover, various ABS systems are the same in the following aspects.
(1) ABS only adjusts the anti lock brake pressure on the wheels that tend to lock during the braking process when the car's speed exceeds a certain level (such as 5km/h or 8km/h). When the speed of the car is reduced to a certain level by braking, ABS will automatically stop the adjustment of anti lock braking pressure. Afterwards, the braking process of cars equipped with ABS will be the same as that of conventional braking systems, and the wheels will lock up when the car brakes. This is because when the car's speed is very low, the impact of wheel lockup on the car's braking performance is already very small, and to make the car brake and stop as soon as possible, it is necessary to make the wheels brake and lock up.
(2) During the braking process, only when the controlled wheel tends to lock up will ABS adjust the braking pressure of the wheel towards lock up; When the controlled wheels have not yet reached lockup, the braking process is completely the same as that of a conventional braking system.
(3) ABS has a self diagnostic function that can monitor the operation of the system. Once a fault is found that affects the normal operation of the system, it will automatically turn off the ABS and light up the ABS warning light, sending a warning signal to the driver. The car's braking system can still brake like a conventional braking system.
① Fully utilize the effectiveness of the brake to shorten braking time and distance.
② It can effectively prevent the vehicle from slipping and shaking its tail during emergency braking, and has good driving stability.
③ It can turn during emergency braking and has good steering maneuverability.
④ It can avoid severe friction between the tires and the ground, reducing tire wear.
ABS warning light
The ABS warning lamp monitors the anti-lock braking system. This warning light will remain on for four seconds when the ignition switch is turned to the ON position.
If the ABS warning lamp is on, repair the brake system as soon as possible to restore the function of the anti-lock braking system. If the ignition switch is turned to the ON position and the ABS warning light does not turn on, please repair this bulb as soon as possible.
If both the brake warning lamp and the ABS warning lamp are on, the anti-lock braking system (ABS) and the Electronic Brake Force Distribution (EBD) will lose their functions. The anti-lock braking system must be repaired immediately.
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