ABS theoretical basis and principle of action
The tangential (tangential direction) friction force of two objects is composed of the vertical force (the gravity of the car and the ground) and the friction Determined by the coefficient, the sliding friction coefficient is generally a fixed value, and static friction is the action range from zero to the maximum static friction coefficient. Except for the difference between tangential relative motion and static state, the maximum static friction coefficient is greater than the sliding friction coefficient when other conditions are the same, that is, the maximum static friction force. It is greater than the sliding friction force (the braking force that is achievable for the car).
The ABS anti-lock braking system tries to convert the sliding friction between the wheels and the ground into static friction as much as possible when the vehicle is braking (at this time, the contact part between the wheels and the ground remains relatively stationary, and the vehicle does not stop. It is not locking but rolling. The braking system can implement a braking force on the wheel that is bounded by the maximum static friction. At this time, the static friction between the rotating wheel and the ground is rolling friction) to increase the braking force.
ABS anti-lock braking system "ABS" is translated as "anti-lock braking system" in Chinese. It is a car safety control system with the advantages of anti-skid and anti-lock. ABS is an improved technology based on conventional braking devices, and can be divided into two types: mechanical and electronic. Anti-lock braking systems are installed on a large number of modern cars. ABS not only has the braking function of ordinary braking systems, but also prevents the wheels from locking, allowing the car to still turn under braking and ensuring the stability of the car's braking direction. , to prevent sideslip and deviation, is currently the most advanced braking device with the best braking effect in automobiles.
In the past, when consumers bought cars, whether they had ABS was an important indicator. With the development of technology, currently, most cars in my country have ABS as standard configuration. However, many drivers are still not very clear about ABS and how to use it correctly, and there are even some misunderstandings about ABS. Some drivers think that ABS is a device that shortens the braking distance. The braking distance of a vehicle equipped with ABS on any road surface is definitely shorter than that of a vehicle without ABS. Some even mistakenly believe that the braking distance on icy and snowy roads can be shorter. The braking distance is equivalent to that on asphalt roads; some drivers believe that as long as ABS is equipped, the vehicle will not lose control even when driving at high speed on rainy days or icy and snowy roads. ABS does not greatly improve the limits of a car's physical performance as some people think. Strictly speaking, the function of ABS is mainly to ensure the maneuverability and stability of the vehicle itself during braking within the performance of physical limits. At the same time, it can also prevent pure tire slippage when accelerating, improving acceleration performance and operational stability.
Application of ABS
The full name of ABS is Anti-lock Brake System (anti-lock braking system) or Anti-skid Braking System (anti-skid braking system). It can effectively control the wheels to keep in a rotating state, improve the stability of the car during braking and the braking performance of the car under poor road conditions. ABS continuously detects the rotation speed of each wheel through the rotation speed sensor installed on each wheel or transmission shaft. The computer calculates the wheel slip rate at that time and compares it with the ideal slip rate to increase or decrease the brake braking. The pressure is determined and the actuator is ordered to adjust the brake pressure in time to keep the wheels in an ideal braking state.
ABS was first patented in 1906, and in 1936 Bosch registered a "mechanical" patent for preventing wheel locking of motor vehicles. All early designs had the same problems: they were too complex and prone to failure, and they were too slow. In 1947, the world's first ABS system was first used on the B-47 bomber. Teldix began research on this project in 1964, and its ABS research was soon taken over by Bosch. Within two years, the first ABS test vehicles had the function of shortening braking distances. Vehicle steering and stability were also ensured when turning, but the approximately 1,000 analog components and safety switches applied at the time meant that the reliability and durability of the electronic control unit, known as the ABS 1 system, was not yet sufficient for large-scale Production requirements need to be improved. The technology acquired by Bosch during the development of electronic engine management, digital technology and the arrival of integrated circuits (ICs) allowed the number of electronic components to be reduced to 140.
In 1968, ABS began to be studied and applied to automobiles.
Due to the passage of the U.S. Federal Motor Vehicle Safety Standard No. 121 in 1975, many heavy-duty trucks and buses were equipped with ABS. However, due to many technical problems with the braking system and opposition from the truck industry, this standard was withdrawn in 1978. . In the same year, Bosch, as the first company in the world to launch an ABS system with electronic control functions, began to install this ABS 2 system as an optional configuration and installed it on the Mercedes-Benz S-Class, and then soon equipped it. On the BMW 7 Series luxury sedan. After this period, further research and design work on ABS in the United States decreased, but European and Japanese manufacturers continued to carefully develop ABS.
After entering the 1980s, because cars imported into the United States were equipped with ABS, American automobile manufacturers showed new interest in ABS in the American automobile market. With the rapid development of microelectronics technology and people's strong requirements for automobile driving safety, ABS devices have been further widely used in the world's automobile industry. In 1987 about 3% of cars in the United States were equipped with highly reliable ABS. In the ensuing period, developers focused on simplifying the system. In 1989, Bosch engineers succeeded in attaching a hybrid control unit directly to the hydraulic module. This eliminates the need for wiring harnesses and connectors to connect the control unit to the hydraulic module, significantly reducing the overall weight of the ABS 2E.
Bosch engineers created ABS 5.0 using a new solenoid valve in 1993, and developed versions 5.3 and 5.7 in subsequent years. The main features of the new ABS 8 generation are once again a significant reduction in weight, volume and memory, while adding further functions such as electronic distribution of brake pressure, which replaces the mechanical reduction of brake pressure on the rear axle. mechanism. The predictions made by some automobile industry analysts at that time were confirmed: by the mid-1990s, most cars and trucks on the world market will be equipped with ABS.
Function of ABS
The main function of ABS is to improve the braking performance of the vehicle, improve driving safety, and prevent the wheels from locking (that is, stopping rolling) during the braking process. This ensures that the driver can still control the direction when braking and prevents the rear axle from slipping. Its working principle is: during emergency braking, it relies on the highly sensitive wheel speed sensor installed on each wheel. Once a wheel is found to be locked, the computer immediately controls the pressure regulator to release the pressure of the brake cylinder of the wheel, causing the wheel to brake. Restore rotation to prevent wheel locking. The working process of ABS is actually a cyclic working process of "locking - releasing - locking - releasing", which keeps the vehicle in a critical locking clearance rolling state, effectively overcoming the vehicle's risk of wheel locking during emergency braking. Deviation phenomenon, preventing the car body from losing control and other situations.
There are two types of ABS: mechanical and electronic. Mechanical ABS has a simple structure and mainly uses its own internal structure to achieve the effect of simply adjusting the braking force. The working principle of the device is simple. There are no sensors to feedback signals such as road friction and wheel speed. It relies entirely on preset data to work. Whether it is a waterlogged road, an icy road, a muddy road or a good cement asphalt road, it The way they work is the same. Strictly speaking, this kind of ABS can only be called "Advanced Brake System". At present, only some low-end pickup trucks and other models in China still use mechanical ABS.
Mechanical ABS only uses the physical characteristics of components to perform mechanical movements, while electronic ABS uses computers to analyze and calculate various data to obtain results. Electronic ABS consists of wheel speed sensors, wiring harnesses, computers, ABS hydraulic pumps, indicator lights and other components. According to the signal of the wheel speed sensor of each wheel, the computer applies different braking force to each wheel respectively, thereby achieving the effect of scientific and reasonable distribution of braking force.
The earliest ABS system was a two-wheel system. The so-called two-wheel system is to install ABS on the two rear wheels of the car. Since the two rear wheels share a brake hydraulic pipeline and a control valve, it is also called a "single-channel control system." This system selects the brake pressure based on the state of the wheel with the smaller adhesion of the two rear wheels, which is called the "low selection principle." In other words, when one rear wheel of an ABS vehicle using the low-selection principle has a tendency to lock, the system can only relieve pressure on both rear wheels at the same time. And since the front wheel does not have an anti-lock function, it is difficult for the two-wheel system to achieve the best braking effect.
With the development of related technologies, the "three-channel control system" later appeared. This system is based on the two-wheel system, and the two front wheels are independently controlled by two separate pipelines. Although the rear wheel still adopts the "low selection principle", this system has the main characteristics of modern ABS because it realizes the steering function during emergency braking and the function of preventing the rear axle from slipping. So far, there are still vehicles on the market that use this three-channel controlled ABS system.
Currently, the most common equipment on vehicles is the four-sensor four-channel ABS system. Each wheel is controlled by an independent hydraulic pipeline and solenoid valve, which can achieve independent control of a single wheel. This structure can achieve good anti-lock braking function.
Two control methods of ABS
1. Dual-parameter control
Double-parameter control ABS consists of a vehicle speed sensor (speed radar), a wheel speed sensor, a control device (computer) and an actuator. Its working principle is that the vehicle speed sensor and wheel speed sensor input the vehicle speed and wheel speed signals into the computer respectively, and the computer calculates the actual slip rate, compares it with the ideal slip rate of 15% to 20%, and then increases or decreases it through the solenoid valve. The braking force of the brake.
This type of drag speed sensor commonly uses Doppler speed radar. When the car is driving, the Doppler radar antenna continuously emits electromagnetic waves to the ground at a certain frequency, and at the same time receives the reflected electromagnetic waves. By measuring the difference between the car radar transmission and reception, the car speed can be accurately calculated. The wheel speed sensor is installed in the transmission housing and driven by the transmission output shaft. It is a pulse motor, and the frequency generated is proportional to the wheel speed.
The actuator is composed of solenoid valves and relays. A solenoid valve adjusts the braking force to maintain the ideal slip rate. This kind of ABS can ensure ideal control of slip rate and good anti-lock braking performance, but due to the addition of a speed measuring radar, the structure is more complex and the cost is higher. For example, the patent (Patent No. 92221809.9) reviewed by Shen Shusheng, an automotive magazine.
2． Single parameter control
It aims to control the angular deceleration of the wheel, control the braking force of the wheel, and achieve anti-lock braking. Its structure mainly consists of a wheel speed sensor, a controller (computer) and a solenoid valve. composition.
Attachment 1: The wheel speed sensor is composed of a sensor and a ring gear steel ring (see Figure 2)
1. Cable 2. Permanent magnet 3. Housing 4, sensing coil 5, polar axis 6. Ring gear In order to accurately measure wheel speed, there should be a 1mm gap between the sensing head and the wheel ring gear. In order to avoid the influence of water, mud and dust on the sensor, the sensor should be filled with butter before installation.
Solenoid valves are used to regulate wheel brake pressure. For a four-channel braking system, there is one solenoid valve for each wheel rim; for a three-channel braking system, there is one for each front wheel and one for both rear wheels. The solenoid valve has three hydraulic holes, which are connected to the brake master cylinder and the wheel brake branch cylinder respectively, and can realize the pressure regulation functions of pressure increase, pressure maintenance and pressure reduction. Here's how it works.
1) Boost: When the solenoid valve is not working, the brake master cylinder interface and each brake branch cylinder interface are connected directly. Due to the strong strength of the main spring, the oil inlet valve opens and the brake pressure increases.
2) Pressure maintenance: When the pressure in the brake cylinder of the wheel increases to a certain value, the oil inlet valve is cut off and closed. The bracket remains in the middle, and the three holes are sealed against each other to maintain braking pressure.
3) Pressure reduction: When the solenoid valve works, the bracket overcomes the elastic force of the two springs and opens the unloading meat to reduce the pressure of the brake cylinder. Once the pressure is reduced, the solenoid valve switches to the pressure maintenance state or the pressure increase preparation state.
The main task of the control device ECU is to calculate, analyze, amplify and discriminate the signals returned by the sensors of each wheel, and then the output stage outputs the command signal to the solenoid valve to perform brake pressure adjustment Task. The electronic control device consists of four parts, input stage A, controller B, output stage C, voltage stabilization and protection device D.
The electronic controller drives the solenoid valve at a frequency of 4 to 101tz. It's something the driver can't do.
This type of single-parameter control ABS is currently widely used due to its simple structure and low cost.
Most Chrysler luxury cars in the United States are equipped with this single-parameter control ABS. It is equipped with wheel speed sensors on all four wheels of the car. The distribution valve (see Figure 5) is a three-channel distribution valve located below the brake oil pump assembly.
Attachment 2: A 45-tooth or 100-tooth ring gear is installed on the wheel axle, and the sensing head of the wheel speed sensor is installed on the top of the ring gear. When the wheel rotates, the sensor continuously generates voltage signals and inputs them into the computer. Compared with the ideal speed in the RoM, the speed increase or deceleration of the wheel is calculated, and a command to increase or decrease the pressure is sent to the solenoid valve to control the brake cylinder system. power. Get out of ABS misunderstandings
The misunderstandings about ABS mentioned in the opening chapter need to be explained. If a car's wheels lock while braking, the car will have minimal lateral traction. At this time, due to factors such as the imbalance of road adhesion coefficient, the imbalance of the car's own braking force, the imbalance of the suspension, the tire pressure of the car, the curvature of the road, bumps or slopes, etc., the car may slip, drift or lose control. . In addition, due to the locking of the front wheels of the vehicle, the car will lose its steering ability. A car anti-lock braking system with excellent performance can control the slip rate of the car wheels between 20% and 30% during braking. In this state, the wheels can take into account the relatively maximum longitudinal braking force and The lateral grip effectively ensures that the vehicle will not lose control. In addition, when the front wheels are not locked, due to a certain amount of grip, the car can also turn according to the driver's wishes, thereby controlling the vehicle. In order to control the wheel slip rate under an ideal state and pursue vehicle stability, some longitudinal braking force may be sacrificed. Therefore, when ABS is activated, the braking distance will not be shortened on all road surfaces.
On ice and snow roads, the adhesion provided by the ground is much smaller than that of ordinary roads. ABS can only adjust the car's braking force based on this adhesion, and will not generate additional braking factors. Therefore, the braking distance on ice and snow roads can only be said to be shorter than when the wheels are locked, but still much longer than the braking distance on ordinary roads.
The actual road is actually very complex, such as: unbalanced road adhesion coefficient, road curvature or road cross slope, and even car tire pressure and other reasons of the car itself. There are many factors that can cause the car to brake when braking. These factors that produce a tendency to slide sideways cannot be overcome by ABS itself. Therefore, if you brake suddenly when the vehicle speed is too fast on an icy or snowy road, and encounter one of the above factors, when the centrifugal force of the vehicle is greater than the maximum lateral force that the ground can provide, the vehicle will tend to lose control, which is very dangerous.
In short, no equipment is omnipotent, and drivers must use their own subjective initiative to achieve safe driving. Even ABS with excellent performance has limited effect in stabilizing the vehicle under working conditions. Especially when driving on gravel roads or ice and snow roads, you should maintain sufficient distance between vehicles, slow down and drive slowly, and do not rely entirely on the ABS system.
Get out of ABS misunderstandings
The misunderstandings about ABS mentioned in the opening chapter need to be explained. If a car's wheels lock while braking, the car will have minimal lateral traction. At this time, due to factors such as the imbalance of road adhesion coefficient, the imbalance of the car's own braking force, the imbalance of the suspension, the tire pressure of the car, the curvature of the road, bumps or slopes, etc., the car may slip, drift or lose control. . In addition, due to the locking of the front wheels of the vehicle, the car will lose its steering ability. A car anti-lock braking system with excellent performance can control the slip rate of the car wheels between 20% and 30% during braking. In this state, the wheels can take into account the relatively maximum longitudinal braking force and The lateral grip effectively ensures that the vehicle will not lose control. In addition, when the front wheels are not locked, due to a certain amount of grip, the car can also turn according to the driver's wishes, thereby controlling the vehicle. In order to control the wheel slip rate under an ideal state and pursue vehicle stability, some longitudinal braking force may be sacrificed. Therefore, when ABS is activated, the braking distance will not be shortened on all road surfaces.
On ice and snow roads, the adhesion provided by the ground is much smaller than that of ordinary roads. ABS can only adjust the car's braking force based on this adhesion, and will not generate additional braking factors. Therefore, the braking distance on ice and snow roads can only be said to be shorter than when the wheels are locked, but still much longer than the braking distance on ordinary roads.
The actual road is actually very complex, such as: unbalanced road adhesion coefficient, road curvature or road cross slope, and even car tire pressure and other reasons of the car itself. There are many factors that can cause the car to brake when braking. These factors that produce a tendency to slide sideways cannot be overcome by ABS itself.
Therefore, if you brake urgently when the vehicle speed is too fast on an icy or snowy road, and encounter one of the above factors, when the centrifugal force of the vehicle is greater than the maximum lateral force that the ground can provide, the vehicle will tend to lose control, which is very dangerous.
In short, no equipment is omnipotent, and drivers must use their own subjective initiative to achieve safe driving. Even ABS with excellent performance has limited effect in stabilizing the vehicle under working conditions. Especially when driving on gravel roads or ice and snow roads, you should maintain sufficient distance between vehicles, slow down and drive slowly, and do not rely entirely on the ABS system.
Common sense on the use of ABS
Nowadays, almost all passenger cars are equipped with ABS systems, which plays a great role in improving the active safety performance of the vehicle. However, if used improperly , the effect will be greatly reduced. Here, our principles for using ABS are summarized as “four do’s and seven don’ts”.
Four essentials
1. Always press the brake pedal and never relax, so as to ensure sufficient and continuous braking force and make ABS work effectively.
2. Keep a sufficient safe distance between vehicles. Under normal circumstances, the minimum distance between vehicles should not be less than 50m. When the vehicle speed exceeds 50km/h, the minimum distance between vehicles is the same as the vehicle speed. For example, at 100km/h, the minimum distance between vehicles is 100m, and at 120km/h, the minimum distance between vehicles is 120m. .
3. Be familiar with ABS in advance so that you can be prepared and adaptable to the vibration of the brake pedal when ABS is working.
4. Read the car driver's manual in advance to further understand the various operating instructions provided by the car manufacturer that installs ABS.
Seven Don’ts
1. Don’t think that you can drive as you like with ABS. ABS is not absolutely safe. When the vehicle speed is too high and the turn is too sharp, if the vehicle brakes too quickly or too hard, the car will still slide. Therefore, even if your car is equipped with ABS, you still need to drive carefully.
2. Do not use "point braking" braking. When vehicles without ABS are braking on slippery roads and at high speeds, they need to use "point braking" to achieve safe braking. After installing ABS, because ABS can automatically adjust the braking force, when performing emergency braking, you can press the pedal to the bottom without releasing it. Don't worry about the wheels locking and slipping, otherwise the braking distance will be greatly extended.
3. Don’t be frightened by the vibration of ABS. When ABS is working, you will hear the noise it makes. This noise is produced by the solenoid valve and hydraulic pump in the hydraulic control system. Do not panic because you think there is something wrong with the braking system, and do not move your feet from it. Remove the brake pedal, but still depress the brake pedal and ignore it.
4. Do not ignore the inspection of the ABS indicator light. Under normal circumstances, this light should come on after the ignition switch is turned on; it will automatically go out after about 3 seconds. This process is essentially the electronic control device performing a power check on the wheel sensor and the control valve of the hydraulic regulator according to the self-test procedure. If the light remains off, it means that the ABS is faulty.
5. Don’t panic if the ABS indicator light does not go out. When the ABS fails while driving, the anti-lock braking system automatically connects the oil circuit of the original braking system. The original braking system on the car still works, but without ABS. Just pay attention to maintenance.
6. Do not replace the ABS computer unit without permission. If the computer detects a fault, the entire ABS unit should be replaced.
7. For vehicles that are equipped with ABS but wish to be modified, do not disassemble and install the nuts connecting the brake pipes to the ABS unit.
ABS is divided into electronic ABS and mechanical ABS
1. Electronic ABS is designed according to different models. Its installation requires professional technical force. If it is replaced, To use it in another vehicle, you must change its circuit design and battery capacity, and there is no universality; mechanical ABS is highly versatile and can be used on any vehicle with a hydraulic brake device, and can be changed from one vehicle to another. on the car, and installation only takes 30 minutes.
2. Electronic ABS is large in size, and the finished vehicle may not have enough space to install electronic ABS. In contrast, mechanical ABS is smaller in size and takes up less space.
3. Electronic ABS starts working when the wheels are locked, and can work 60 to 120 times per second depending on the vehicle speed; mechanical ABS starts working when the brakes are applied, and can work 60 to 120 times per second depending on the vehicle speed. The clock works 6 to 12 times.
The applicable characteristics of mechanical ABS need to be set in advance. On watery roads, ice and snow roads, gravel roads, and asphalt roads, the friction coefficients of tires are different, the vehicle speeds are different, and the required braking force is also different. . There is no real-time measurement feedback system, and it only relies on preset threshold values. The applicable range is narrow and the braking effect will be reduced.
Be very careful when purchasing a mechanical ABS anti-lock braking system. Counterfeit ABS products are similar in appearance and structure to genuine products, but inferior products cannot withstand the corrosion and high pressure of brake fluid for a long time. Over time, the rubber will age and deform, losing its proper performance.
The genuine rubber valve bag can withstand a high pressure of 11,000 pounds per square inch when soaked in brake fluid and will not deform for a long time. The price of imported mechanical ABS is around 2,000 yuan, while the domestic one only costs more than 200 yuan.
What is ABS?
ABS can be installed on any car with hydraulic brakes. It uses a rubber airbag in the valve body. When the brakes are stepped on, the brake fluid pressure is given and filled into the ABS valve body. At this time, the airbag uses the air barrier in the middle to return the pressure, allowing the wheels to avoid the locking point. When the annual wheel is about to reach the next locking point, the pressure of the brake fluid causes the air bag to act repeatedly. For example, it can act 60120 times in one second, which is equivalent to braking and relaxing continuously, that is, similar to It is a mechanical "point brake". Therefore, the ABS anti-locking system can avoid directional control and wheel sideslip during emergency braking, prevent the wheels from being locked during braking, and prevent the tires from colliding with each other at a [1] point. Ground friction, thereby increasing friction, making the braking efficiency reach more than 90%. It can also reduce braking consumption and extend the service life of brake drums, discs and tires by two times. Vehicles equipped with ABS can operate on dry asphalt roads and rainy days. The anti-skid performance of roads such as roads and snowy days reaches 80%90%, 30%10% and 15%20% respectively.