Any modern vehicle is subjected to a strict check of the controlling services before entering the territory of the dealership. The most significant criterion in the framework of the audit is the safety of the driver and passengers. Inside the cabin, they have long been installed, firing at the moment of impact. But for the driver, they came up with a lot of additional systems through which the safety of driving increases. ABS is one of them. In this article, we will tell what is ABS system, we will analyze its key features, the principle of application and touch on other important issues.
What's this?
ABS is an auxiliary system, the purpose of which is to prevent the car wheels from locking when the brake pedal is pressed a. In such a situation, the use of the system helps to reduce the distance from the moment when the driver applied the brakes to the time of a complete stop. As a result, the controllability of the machine during hard braking is increased. It should be noted that the system is designed to eliminate the possibility of a car getting into a skid when it encounters an uncontrolled slip.
At the moment, ABS is an auxiliary element of the stopping system controlled by the electronic unit. It is characterized by a large number of additional technologies. Here you can add traction control, ESC (electric stability control) and help during an emergency stop.
Due to its proven high performance, ABS is installed almost everywhere these days. First, it was invented for cars, then introduced into passenger buses and minibuses. In an almost identical period, ABS began to be used on trucks and cars, trailers, and motorcycles. To understand how effectively ABS functions in transport, it can be noted that now it is present even on the retractable landing gear of large passenger or cargo airliners.
The device and principle of operation of ABS
This includes such main components:
- deceleration or speed sensors mounted on the hub of the machine;
- a set of control valves acting as auxiliary elements for the pressure modulator. They are pressed into the line hoses where the brake fluid is located. At the same time, they are integrated into all circuits;
- a control unit that receives and processes signals from sensors. Based on the information received, he independently controls the operation of the valves in real time.
During the movement of the vehicle, the wheels have a fixed contact patch relative to the roadbed. In other words, the wheel encounters a frictional force at rest. Because it is larger when compared with the sliding friction force, in the process of decelerating the wheels spinning at the same speed, the stop becomes faster when compared with the stop of the wheels that are slipping. In parallel, it should be noted that if one or more wheels on the machine slip into a slip, there are increased chances of losing control.
As soon as braking begins, ABS starts constantly, while quite accurately fixing the speed of rotation of each wheel. Since the speedometer usually takes into account the intensity of operation of the wheelset not involved in acceleration, ABS is not connected to it. After all, if the car is front-wheel drive, it is enough to press the handbrake to confuse all the sensors. It is for this reason that the sensors are integrated into each wheel hub individually. If any wheel rotates at a much lower speed compared to others (indicating being in a state close to blocking), the internal valves of the line reduce the amount of braking force on the selected wheel. After restoring the normal speed of its rotation, the system automatically resumes the optimal level of braking force.
The procedure discussed above can continue more than 20 times in one second. In the vast majority of cars, this behavior of the sensors leads to the fact that the brake pedal begins to pulsate. Accordingly, the driver independently understands exactly when the anti-lock braking system automatically works.
It is noteworthy that the transmission of braking force can be regulated in the entire braking system, or on one of the circuits. In modern vehicles a separate wheel is subject to monitoring. Based on this behavior, the system is usually divided into:
- single-channel - the entire trunk is analyzed;
- two-channel - one of the boards is analyzed;
- multi-channel - each wheel is limited individually.
single channel the system is characterized by a fairly effective level of deceleration, but provided that the grip of each wheel is at an identical level. Multichannel the design is characterized by an increased level of complexity, so its cost is an order of magnitude higher. At the same time, the level of efficiency increases significantly if the car is operated on heterogeneous surfaces. For example, when the car moves on ice, the roadside, or a wet road section.
In the current ABS design, a self-diagnostic module was added in parallel, capable of automatically checking the health and accuracy of all system components for a number of physical characteristics. The self-diagnosis is also responsible for activating the ABS lamp on the instrument panel if it detects that the system has become faulty. The received information is additionally sent to the control unit in the form of a special combination, which is stored in the internal memory. Once the fault is identified, the component will not function at all, or the entire system will become inactive. But this will not affect the serviceability of the brakes themselves.
Among modern cars, mechanisms that run on electrics are very popular. Their advantage lies in the following - the brake mechanism independently serves its own wheel, not being dependent on the rest. In such a situation, ABS is used as one of the safety elements regulated by the ECU. It is noteworthy that the anti-lock does not affect the handle or pedal.
Why is ABS needed?
In most situations, it contributes to a reduction in the stopping distance, when compared with a car without it. Also one of the basic tasks it is considered to maintain a high level of control over the machine during an emergency stop maneuver. In other words, the driver increases the ability to make a rather sharp maneuver right at the stop. These two factors, combined with each other, make ABS a very useful auxiliary element in terms of increasing the level of safety in vehicle operation.
For drivers with more experience, as practice shows, there is not much difference between the absence or presence of ABS in vehicles, since they are perfectly able to feel the moment when the wheels break off on their own. A similar stopping technique is also used by motorcycle owners. When the effort comes to stop the rotation of the wheels, the motorist does not “drown” the pedal even harder, holding it in an identical position. The advantage of this technique is comparable to the deceleration using a single-channel system. In multi-channel, the advantage lies in controlling the force of individual wheels. Therefore, a high level of efficiency and an increase in the predictability of the response of the vehicle when it passes on a road with portions having an uneven level of grip is ensured.
If a motorist does not have the proper level of experience, having ABS is preferable, regardless of how much time he has already been driving. The fact is that an emergency stop becomes intuitively simple. You just need to press hard on the brake lever or pedal, while maintaining the ability to perform maneuvers. At this time, the ABS will independently determine when the force transmitted to the caliper should be limited.
Sometimes ABS still contributes to increasing the braking distance. On loose surfaces such as deep snow, gravel or sand, locked wheels begin to burrow, thereby enhancing stopping efficiency. But an unlocked wheel in a similar situation will behave differently, stopping the car more slowly. Then the developers allow you to disable ABS.
Do not assume that manufacturers have not provided for such a moment - in some types of ABS there is a specialized algorithm designed for loose surfaces. Its essence boils down to the fact that blocking occurs in large numbers with a minimum period of time between each of them. This technique contributes to effective deceleration while maintaining control, as is often the case with total blocking. The driver can independently choose the type of surface. But for greater convenience, the software selects it automatically, analyzing the behavior, or by using sensors that determine the road surface.
conclusions
Based on the foregoing, the following conclusions can be drawn. The ABS system is an indispensable element of the safety of any vehicle. It contributes to a more efficient stop, and also prevents the vehicle from stalling into a skid. The principle of operation is that when the wheels stop, they do not block, but continue to scroll on the verge of a stall moment. The system can control four wheels at once, two or each separately. For winter operation, a choice is provided in favor of completely disabling the operation of ABS, or using several coverage modes. The car can switch the latter on its own, or entrust the choice to the driver.
ABS: What is it for?
It is known: if during emergency braking “to the floor” the front wheels are blocked, then the car becomes uncontrollable. Turning the steering wheel in this case is completely useless. An experienced driver brakes intermittently, allowing the wheels to turn, which allows you to steer in the right direction and, perhaps, go around an unexpected obstacle. But how many of us have enough self-control even for a moment to ease the pressure on the brake pedal when the car flies with a squeal, perhaps on its last journey?
What is difficult for a person, the power of dispassionate electronics. And now the pedal responds to blocking the wheels with frequent sharp shocks, which indicates: the ABS is working, and now you have the opportunity to turn away from danger!
ABS: HOW IT IS DESIGNED
On fig. 1 shows a functional diagram of a typical ABS. Let's immediately pay attention to the electric pump 1 and the pressure accumulator 2: these nodes are necessary so that smart electronics can control the braking force regardless of the driver's reaction (recall - in an emergency, he, as a rule, simply presses the pedal "to the stop"). In addition, the electronic control unit 3 (ECU) needs to “know” whether the wheels are currently rotating and at what speed. This information is provided by 4 sensors that control each wheel. After all, a situation may arise when a slippery road surface under one of the wheels provokes its early blocking. Then the ECU, on a signal from this wheel, issues a command to weaken the braking force, preventing the car from skidding and turning around. True, in this case, the braking distance will be the same as if all the wheels were on a slippery road. But the developers believe that it is more important in any case to maintain controllability and maneuverability.
The last of the ABS units is a block of solenoid valves 5, which, in fact, control the fluid pressure. Each of the circuits of the brake system has two valves - an inlet valve, which opens the way for fluid from the pressure accumulator to the working cylinder, when it is necessary to increase the braking force, and an exhaust valve, which allows the fluid to go back into the reservoir when the pressure needs to be relieved. These valves, when the ABS is working, either open alternately or are closed if the pressure in the circuit should remain unchanged. Finally, it is important to know that when de-energized, the intake valves are open and the exhaust valves are closed. This allows, in case of ABS failure, simply turn it off (for example, by removing the F54 fuse (Fig. 2) or removing the connector from the computer) and brake, as in a conventional car.
ABS: In health and in sickness
Schematic diagram of connections of the Teves ABS, installed, in particular, on Volkswagen Passat cars manufactured in 1990 as additional equipment, is shown in fig. 2. As you can see, it is not too complicated. Nevertheless, it is worth making a few important remarks for those who dare to self-repair the ABS.
1. Before removing the battery and carrying out welding work on the vehicle, be sure to disconnect the connector from the ABS computer with the ignition off. This unit in the Passat is located under the rear seat cushion.
2. When carrying out painting work, the ECU should not be exposed to a temperature of 85 ° C for more than two hours.
3. Before any work on the brake system, discharge the pressure accumulator by pressing the brake pedal at least 20 times with the ignition off, otherwise the system will retain a pressure of about 180 atm.
4. Be careful, turning on the ignition when the hydraulic system is depressurized, as in this case the brake fluid pump will work.
Now let's start checking the ABS nodes. For employees of service centers, readers are produced that allow you to remove information from the self-diagnosis system. The thing is expensive and almost inaccessible to the motorist. We will manage with an ordinary amateur radio tester, which measures voltage and resistance in electrical circuits. You will have to connect the terminals of the device to the contacts of the computer connector, which requires accuracy and skill. Therefore, we will carry out most of the checks with the ignition off and the ECU connector removed, then the tester can be easily connected to the terminals of the block on the wiring harness. So, we study the ABS check table.
Explain the rules for using the table. It is necessary to measure voltages or resistances between the terminals of the ABS computer connector indicated in the second column - in the block located on the wiring harness. However, only pp. 35–40 we check with the connected computer, in other cases the connector is removed. The last column indicates the possible cause of the malfunction if the measurement results do not match those indicated in the fifth column. In this case, we consider only cases of defects in the nodes of the system, assuming that the electronic unit is in good order. This is all the more justified since it is impossible to repair an ECU at home, and a handy and savvy car enthusiast is quite capable of replacing any node as a whole. If you do everything correctly, then the ABS warning lamp on the instrument panel will go out some time after the ignition is turned on - as it should be with a working system.
And in conclusion - a few recommendations for replacing nodes. We repeat once again that before disconnecting the hydraulic system, it is necessary to relieve the pressure in its accumulator by pressing the brake pedal 20 times with the ignition off. Pumping circuits connected to the pump has its own characteristics. So, you put a transparent tube on the fitting and lowered its end into a jar of brake fluid. Now press the brake pedal, unscrew the bleeder screw and turn on the ignition. This will turn on the ABS pump, which will expel air from the system. As soon as the bubbles stop coming out, release the pedal, tighten the fitting and turn off the ignition.
Before installing a new wheel speed sensor, apply a layer of grease to the seat surface and install a new O-ring.
Of course, the ABS of other models differs from the one described above and the fault table may not be suitable for them. But the general principles of building systems are the same, and if you manage to find an ABS diagram for your car, then by comparing it with Fig. 2, it is easy to adjust the diagnostic table. Therefore, our recommendations, we hope, will be useful in any case.
| ABS TEST CHART | |||||
| Connector pins | Checked node | Verification conditions | Control value | Possible cause of malfunction | |
| 1 | 2 and 1 | Eg. ABS supply | Turn on the ignition | About 12V | chain break |
| 2 | 3 and 1 | ABS relay K79 | Connect terminals 2 and 8 with a jumper and turn on the ignition. Remove the jumper at the end of the test. | About 12V | Open circuit or malfunction. relay. See point 3 |
| 3 | 1 and 8 | Same, winding | Turn off the ignition | R=50–100 Ohm | Open circuit or winding |
| 4 | 12 and 1 | Stoplight switch | Switch off the ignition, press the brake pedal | About 12V | F20 burned out, open circuit, faulty. off stop sign |
| 5 | 4 and 22 | Right rear wheel sensor | Hang the right rear wheel and rotate it at a speed of about 60 rpm | ~U>75 mV | An open in the circuit, the installation of the sensor is knocked down, its malfunction |
| 6 | 4 and 22 | Same | – | R=0.8–1.4 kOhm | Sensor defect, open circuit |
| 7 | 6 and 24 | Left rear wheel sensor | Hang the left rear wheel and rotate it, as in paragraph 5 | See point 5 | See point 5 |
| 8 | 6 and 24 | Same | – | See point 6 | See point 6 |
| 9 | 7 and 25 | Right front wheel sensor | Hang the right front wheel and rotate it as in paragraph 5 | See point 5 | See point 5 |
| 10 | 7 and 25 | Same | – | See point 6 | See point 6 |
| 11 | 5 and 23 | Left front wheel sensor | Hang the left front wheel and rotate it as in paragraph 5 | See point 5 | See point 5 |
| 12 | 5 and 23 | Same | – | See point 6 | See point 6 |
| 13 | 1 and 3 | Normally closed relay contacts K79 ABS | Ignition off | R<1,5 Ом | Break in the wires or defect in the relay |
| 14 | 3 and 20 | Relay circuit K79 ABS | Same | R<1,5 Ом | Break in the chains |
| 15 | 1 and 11 | Common valve wire | Same | R<1,5 Ом | Violation of the connection with the "mass" of one of the conclusions |
| 16 | 1 and 18 | main valve | Same | R=2–5 ohm | Open circuit or valve winding |
| 17 | 11 and 17 | Rear intake valve | Same | R=5–7 Ohm | Same |
| 18 | 11 and 15 | Inlet valve right front circuit | Same | R=5–7 Ohm | Same |
| 19 | 11 and 35 | Intake valve left front circuit | Same | R=5–7 Ohm | Same |
| 20 | 11 and 33 | Rear outlet valve | Same | R=3–5 ohm | Same |
| 21 | 11 and 34 | Exhaust valve right front circuit | Same | R=3–5 ohm | Same |
| 22 | 11 and 16 | Exhaust valve left front circuit | Same | R=3–5 ohm | Same |
| 23 | 1 and 14 | High pressure sensor S01 | Ignition off, depress the brake pedal 20 times | R<1,5 Ом | Open circuit or sensor defect |
| 24 | 1 and 4 | Right Rear Sensor Wire Shield Insulation | Ignition off | R>100 kOhm | Violation of the screen insulation, broken capacitor C |
| 25 | 1 and 6 | Same for left rear sensor | Same | R>100 kOhm | Same |
| 26 | 1 and 7 | Same for right front sensor | Same | R>100 kOhm | Same |
| 27 | 1 and 5 | Same for left front sensor | Same | R>100 kOhm | Same |
| 28 | 2 and 14 | Same | R=50–100 Ohm | Open circuit or relay winding | |
| 29 | 2, 17 and 33 | Rear circuit valves | Connect the terminals with a jumper, hang the rear wheels, apply the brake with the ignition off | Rear wheels must brake | Valve box failure |
| 30 | 2, 17 and 33 | Same | Rear wheels must turn | Same | |
| 31 | 2, 15 and 34 | Right front valves | Same as in paragraph 29, but hang the right front wheel | The wheel must stop | Same |
| 32 | 2, 15 and 34 | Same | Same with ignition on. | The wheel must turn | Same |
| 33 | 2, 16 and 35 | Left front valves | Same as in paragraph 29, but hang the left front wheel | The wheel must stop | Same |
| 34 | 2, 16 and 35 | Same | Same with ignition on. | The wheel must turn | The same, after checking, do not forget to remove all jumpers! |
| 35 | 32 and 1 | Relay K78 for turning on the ABS pump | This and subsequent checks are carried out with the ABS computer connector connected. Turn off the ignition, disconnect the ABS pump connector, press the brake pedal 20 times and turn on the ignition | A voltage of about 12 V should appear between the terminals. | F53 burned out, open circuit, defective relay. After checking, connect the pump connector |
| 36 | 9 and 10 | Sensors for emergency pressure S02 and liquid level in the ABS reservoir S03 | Check the fluid level in the tank, turn on the ignition and wait for the pump to turn off | R<1,5 Ом | Breaks in circuits or defects in sensors |
| 37 | 9 and 10 | Emergency pressure sensor S02 | Turn off the ignition and apply the brake 20 times | R>100 kOhm | If there is low resistance between terminals 3 and 5 of the five-pin connector on the hydraulic unit, a defect in the high pressure sensor |
| 38 | 9 and 10 | ABS fluid level sensor S03 | Turn on the ignition, wait for the pump to turn off, turn off the ignition and remove the sensor from the tank | R>100 kOhm | Faulty fluid level sensor in the tank |
| 39 | – | Pump M ABS | Turn off the ignition, press the brake 20 times, note | The liquid level should drop by about 1 cm. | If the pump worked, there is a mechanical defect in it, if it did not turn on, an open in the circuits, F53 or a relay defect is possible |
| 40 | 2 and 18 | main valve | Connect the terminals with a jumper with the ignition off, press the brake pedal to the full and, without releasing the pedal, turn on the ignition | You should feel pressure on your leg. | Valve defective |
The concept and principle of operation of the anti-lock braking system - ABS. ABS operation diagram for a Volkswagen car.
The content of the article:
In an extreme situation on the road, one or several wheels are blocked in the car. In this case, the grip of the car with the road is very poor. Locked wheels no longer keep the car on a straight path, and the car starts to roll on the road. That is, the driver loses control over the car, while the transport begins to actively skid in different directions.
What is anti-lock braking system ABS
A system such as the Anti-lock Braking System, or better known to everyone as ABS technology, prevents the wheels from locking when the car is braked, which allows the driver to maintain excellent control over his car. This technology also improves braking performance itself by reducing stopping distances on both wet and dry roads. The advantages of this system include also uniform tire wear.
This system only has disadvantages (see video below) on surfaces such as sand and gravel, because the use of ABS on such surfaces, on the contrary, will only increase the braking distance. To drive on such a surface, you need to turn off the ABS and this will speed up the stopping distance of the car, due to the wedge formed from the soil. Modern ABS systems automatically detect the surface and act differently in different situations.
Video about the pros and cons of ABS:
What does this traction control system allow?
- Effective braking on slippery, wet roads.
- Gives the driver more control over the car.
- Prevents car skid.
Modern ABS technology consists of the following components:
- Wheel speed sensors.
- Brake pressure sensor.
- Hydraulic block.
- A light bulb in the passenger compartment (mainly on the instrument panel).
The correct functioning of the ABS system is considered essential to protect both passengers and people outside the vehicle. In general, the operation of the ABS system consists of an electronic unit, also known as an ECU (electronic control unit), which collects data from sensors and controls the hydraulic control unit, mainly consists of valves that regulate brake pressure on the wheels.
The communication between the control unit and the sensors must be very fast. Tire position sensors are usually located on the wheel axle. The sensor must be stable and maintenance-free. These tire position measurements are processed by the control unit for calculation.
The hydraulic control unit is usually located in close proximity to the ECU (or vice versa), and consists of a series of valves that control pressure. All these valves are placed close to each other and packed in a solid block.
The central control unit usually consists of two microcontrollers. These two microcontrollers interact and check each other in operation. The software that runs in the ECU has a number of functions. In particular, algorithms that control the HCU based on inputs or control the brakes based on recorded wheel spin. This is obviously the main task of the entire ABS system. In addition, the software processes the information coming from the sensors. There are also some programs that constantly check every component of the ABS system for its correct operation.
The principle of operation of the ABS system
The anti-lock braking system (ABS) works as follows:
When braking, fluid is forced out of the cylindrical brake ports to the HCU intake ports. This pressure is transmitted through four normally open solenoid valves located inside the HCU through the HCU exhaust ports to each wheel. If the anti-lock brake control module senses that the wheel is about to lock, based on the signal data to the sensor, it closes the open solenoid valve for that circuit. This prevents more fluid from entering this circuit. If that wheel is still slowing down, it opens the solenoid valve for that circuit. After the wheel is rotated to its normal position, the anti-lock brake control module resets the solenoid valves to normal to allow flow affected by the brake. The anti-lock brake control module controls the electromechanical components of the system. Loss of hydraulic fluid in the brake cylinder will disable the anti-lock system. There are many different options and algorithms for ABS control. The computer monitors the speed sensors all the time. He's looking for wheel slip. The drive wheels will lock up if the computer experiences rapid deceleration and a hard skid.
When the ABS system is in operation, the driver feels a pulsation in the brake pedal, this comes from the rapid opening and closing of the valves. This pulsing signal also tells the driver that the ABS has been activated.
Modern ABS system is reliable and durable. Electronic sensors and system blocks have many fuses and specials. relay. Breakdowns are often associated with improper operation. Wheel sensors have the greatest influence, which will have to be changed from time to time.
- Do not separate electrical connectors when the ignition is on or the engine is running.
- Do not connect your car's battery to another car.
- Carefully monitor the contacts on the generator, they must always be in good condition.
- If you need to weld something in the car, do not forget to disconnect all the wiring to the ABS.
- Do not heat the ABS control unit above 85 degrees for 2 hours. This is necessary if you are going to paint a car and at the same time dry it with a hot method.
- If this happened on the road, then stop and measure the voltage on the battery.
- If below 10.5 V, then this is the first sign that the battery needs to be charged.
- If the light is flashing, this is a sign that something is wrong with the ABS, namely, something is wrong with the wiring.
- If this is not the reason, contact the service station, it may be time to replace it.
The system we are interested in took root on cars back in the late 1970s, so it has passed the test of time. Currently, the absence of ABS as standard is a rarity. It significantly improves road safety and partly reduces the requirements for driver skills. In any case, under the control of ABS, even an inexperienced person is more likely to avoid an emergency.
IN THE POWER OF INTEREST
The task of ABS is to maintain controllability during emergency braking. It is known that a blocked wheel has lower adhesion to the road surface than a rolling wheel - the braking forces created by it are less, and there are no control forces at all. At best, the car slides straight, at worst - on an uncontrolled trajectory with an unpredictable result. ABS, on the other hand, controls the operation of the wheel on the border between the maximum possible (in specific conditions) grip and a breakdown in blocking, preventing it from developing. Of course, the tire grip coefficient itself does not depend on ABS. On ice, it can be ten times lower than on dry pavement, which means that the car's handling will be different. But in both cases, ABS provides the maximum possible. With sufficient fine tuning, it is able to operate even more efficiently than an ace driver.
All schemes, tables and graphs open in full size on a mouse click.
The work of ABS is based on the coefficient of wheel slip - the ratio of the difference between the speed of the car and the circumferential speed of the wheel to the speed of the car. In different driving modes, the forward speed of the vehicle and the circumferential speed of the wheel may not match. With intensive acceleration, the circumferential speed of the drive wheel is higher than the speed of the machine, with deceleration - vice versa. Naturally, two modes correspond to 100% slippage - wheel locking during braking or slipping in place. Meanwhile, the best grip of the tire with the coating and, consequently, the maximum transfer of braking forces is achieved at a degree of wheel slip of about 20%. Here ABS also maintains this value at the level of 15-20%.
ANATOMY
The hydraulic circuit of the ABS module includes solenoid valves and a pump. During normal braking, the valves are not activated, the necessary pressure is controlled by the driver's foot. But if there is a slip with the risk of blocking the wheel, the ABS is activated.
Modern ABS is four-channel: this arrangement makes it possible to control the pressure in the brake system separately for each wheel. All circuits of the system work in a similar way in three modes - holding pressure, reducing it and increasing it. When the wheel is close to blocking, the system switches to pressure hold mode. The valves cut off the wheel caliper from the master brake cylinder - now the fluid pressure on the pistons is constant, regardless of the force on the pedal. But at slippage above 20%, the system reduces pressure with a pump, dumping some of the fluid from the caliper to the brake master cylinder. When the slip falls below a certain threshold, the system proceeds to increase the pressure: the valves open - and when the pedal is depressed, the pressure increases. These modes alternate until the situation changes: braking is interrupted or significantly reduced and there is no slip, or the vehicle speed has dropped below 5–15 km/h (depending on system settings). This alternating change of operating modes causes itching on the brake pedal. The frequency is high - the foot of even the best pro driver cannot compete in speed with ABS! When braking, the ABS maintains the slippage of all wheels at the same level to maintain directional stability. In mixed driving (for example, the left wheels of the car are on asphalt, and the right wheels are on ice), the system will maintain a straight-line movement by adjusting the pressure in the circuit of each wheel depending on the adhesion of this wheel to the surface. Braking without ABS will cause the car to pull towards the surface with better grip, and if the wheels lock up, it will come to a turn.
Perhaps the most important elements of ABS are wheel speed sensors. Based on their impulses, the speed of each wheel is calculated and compared with the speed of the car. Based on this information, the ABS module calculates and maintains the slip of each wheel at the required level.
At the choice of the designer, passive or active sensors are used. Passive is easy to identify by the gear ring (comb) on the wheel drive. It is very simple: when the comb is rotated, the sensor produces an analog voltage signal. But, alas, at a low wheel speed, such a sensor does not give a clear signal, it may be wrong.
The active sensor reads the magnetic ring marks on the wheel bearing. It is characterized by a clear digital signal in the form of successive voltage pulses, the magnitude of which does not depend on the speed of rotation of the wheel. But the pulse frequency reflects this speed.
For all-wheel drive vehicles, the ABS includes an additional G-sensor with an accelerometer for longitudinal accelerations. It sends an acceleration or deceleration signal to the ABS module, which is taken into account when calculating the vehicle speed correction factor. After all, under certain circumstances it is impossible to measure the speed with the required accuracy.
FORCE MAJEURE
Nothing in the world is perfect, and ABS is no exception. Maintaining controllability is sometimes paid for by increasing the braking distance. If ABS is effective with good adhesion of all four wheels to the road, then emergency situations are possible on problematic surfaces. Irregularities in the roadway (comb, tram tracks, etc.) cause wheel rebound, and if the suspension malfunctions, even a temporary separation of the wheel from the surface is possible. At such moments, the wheels are heavily unloaded, which leads to their early blocking during forced braking and, accordingly, to early ABS activation. The same early response effect is observed on asphalt areas covered with sand, mud, gravel or on bare ice. Worst case scenario - off the road. Without ABS, locked wheels could bite into the surface, somehow dampening speed. With ABS, the braking distance is greatly increased, and in the event of braking in a skid, the car strongly leads away in an arc. The December issue of the RFP for 2012 described a special test that compared braking performance from a speed of 60 km / h on flat asphalt and on a comb. Two out of three tested cars had a 40% increase in braking distance on the comb!
BETTER NOT RISK
Disabling ABS is not provided. But you can get rid of it by removing the fuse. Most often, they do this when they go to practice on the ice track. However, it should be remembered that modern ABS is also responsible for the distribution of braking forces along the axles during normal braking (previously, independent mechanical regulators were in charge of this). If the ABS is disabled, any normal braking action can cause the rear wheels to lock up, with all the ensuing consequences.
THERAPIST'S OFFICE
ABS has a failure indicator lamp. Reading fault codes is also provided. You can also monitor the parameters of the elements and control some of them - for example, the valves and pump of the ABS module. It is best to use dealer diagnostic equipment. The system is quite reliable and does not include too many elements. Most of the ABS malfunctions are related to external influences.
Control module errors.
Most often, these are internal electronic malfunctions of the module. Sometimes such errors are random, that is, after deletion, they no longer occur. If the errors are not removed or reappear, the control module must be replaced: no repair is provided.
Wheel speed sensor errors.
Possible causes range from faulty wiring to failure of the sensor itself. If an active sensor is used, then the malfunction may be due to increased play of the wheel bearing (too large air gap between the sensor and the magnetic ring on the bearing) or the fact that when replacing the bearing it was simply put on the wrong side. When using a passive sensor, a comb on the drive can create a problem: during the replacement of the wheel bearing or when removing and installing the drive, it could be slightly displaced from the seat. The signal from this sensor is sometimes weak due to accumulated dirt or metal particles on the comb. Both sensors are afraid of strong vibrations, but especially the active one. Because of this, the sensor is sometimes impossible to remove without damage, because hammer blows are not even on it, but nearby! - able to destroy it.
Modern cars are equipped with a significant number of active safety systems, the task of which is to prevent the driver from losing control of the car in different traffic situations. These include the anti-lock braking system (ABS).
It should be noted that the ABS is the first among the systems related to active safety, which has been massively used on cars. At the same time, it also acts as a base for.
The first working samples on cars began to be used more than 40 years ago. As technology has developed, it has been improved and refined. For example, the first systems included more than a hundred components, and the latest versions of the ABS system consist of only 18 elements.
Features of the system
ABS is installed on the brake system and makes its own adjustments to its operation. By the name itself, you can understand that its task is to prevent the wheels from locking during braking.
The peculiarity of the wheels of a car is that the rolling friction force is higher than the sliding friction. That is, a wheel that is rolling adheres better to the road surface than a sliding one, which happens if it is completely blocked. As a result, the stopping distance of the car increases.
Also, the sliding of the wheel does not always occur in a rectilinear direction, since the lateral forces can prevail over the longitudinal ones, due to which the trajectory of the movement of such a wheel changes. The result of this is unpredictable and uncontrolled movement of the machine.
But if you create a force on the brake mechanism that will slow down the speed of rotation as much as possible, but without blocking it (keeps it on the verge), then the braking distance will be reduced and the car will not lose controllability.
In cars without this system, experienced drivers use the method of repeatedly pressing the pedal (intermittent braking) to get the maximum effect when braking. So that the wheels do not turn out to be blocked, the driver, when braking, presses the pedal, then releases and repeats this many times.
The essence of this method is very simple - to catch the moment on the brakes when they slow down the wheels as much as possible without breaking them into blocking, but this is not always possible, especially if the wheels move on different surfaces.
Intermittent braking (pushed-released) does not completely block the wheels, since the driver simply periodically eases the force on the brake mechanism. The same principle applies to ABS.
The design and purpose of the constituent parts
The anti-lock braking system device consists of three main components:
- Wheel speed sensors
- Control block (module)
- Executive device
Car ABS elements
As noted, this system is often used as a base for others. At the same time, the components of a number of other systems are only an addition to the ABS.
Sensors
Speed sensors are very important components, since the operation of the ABS system is based on their readings. According to the impulses that they give, the control module calculates the rotation speed of each of the wheels, and based on the calculations, the actuator is controlled.
Location of the speed sensor on the wheel hub
The ABS design uses two types of sensors. The first are called passive sensors. These elements are of the inductive type.
Their design includes the sensor itself, consisting of a winding, a core and a magnet, as well as a ring gear used as a setting element. The ring gear is mounted on the hub, so it rotates with the wheel.
Inductive type sensor
The essence of the functioning of the passive element is very simple - the winding generates a magnetic field through which the ring gear passes. The existing teeth, when passing through the field, influence it, which ensures the excitation of voltage in the sensor. Alternating teeth with cavities provides the creation of voltage pulses, which allow you to calculate the speed of rotation of the wheel.
The negative quality of passive sensors is the lack of measurement accuracy when driving at low speeds, which can cause incorrect operation of the ABS system.
Now, due to an existing drawback, passive sensors are not used in the anti-lock braking system and they have been replaced by so-called active elements.
As in the first option, active sensors consist of two main components - the sensor itself and the setting element. But in active elements, sensors are built either on the magnetoresistive effect or on the Hall effect. Both options require power to work (passive elements generated it themselves).
As for the master element, here the design uses a ring with magnetized sectors (multipole).
The device and principle of operation of the active speed sensor
The essence of the work of active elements is different. In the magnetoresistive version, the constantly changing field (from the drive ring) leads to changes in the resistance readings in the sensor. In the Hall element, this field changes the voltage itself. In both cases, an impulse is created from which the rotation speed can be calculated.
Active type elements are widely used due to the high measurement accuracy at any speed.
Control block
The ABS system control module, like other ECUs involved in auto systems, is needed to receive and process pulses transmitted from wheel sensors. It contains tabular data, on the basis of which it controls the actuator. That is, after receiving a signal from each sensor, it compares it with the information entered in the table, and based on the results obtained, it will determine what it should do.
In a car with a number of systems built on the basis of ABS, the control unit has additional modules that are responsible for the operation of their systems.
Actuating mechanism
The actuator (it is also called a valve body or ABS module) is the most complex in design and consists of a number of elements:
- solenoid valves (inlet, outlet);
- pressure accumulators;
- return pump;
- damping chamber.
ABS block device
In the classical scheme, only one line goes to the working mechanism of the brakes, through which fluid is supplied from the master cylinder. In ABS, the return line is cut into it, but it passes only inside the module.
The inlet valve is the only element installed on the main supply line. Its task is to shut off the fluid supply under certain conditions, by default it is open.
The tie-in of the return flow line is carried out behind the inlet valve. An exhaust valve is installed at the inlet to it, which is normally closed.
If the volume of the accumulator is not enough to accept all the liquid, a pump is switched on, which pumps the excess into the main line.
But the pumping process is accompanied by pulsation, and in order to dampen the fluctuations of the liquid, it first enters the damping chambers and only after that - into the line.
Generations and species
The modern system installed on a car is four-channel. It includes two valves per wheel, as well as one pressure accumulator and shock absorber chamber per circuit (and there are two of them).
In general, this system already has 5 generations. The first of them appeared in 1978, the second came to replace it in 1980 and it was installed until 1995, after which the 2nd generation replaced the 3rd. The current 4th generation of the system appeared in 2003, and now the 5th generation is being used, which continues to be used to this day.
As for the design features, the four-channel system is the latest and technologically advanced. But it was preceded by:
- single-channel system (it used only two valves that regulated the pressure in all lines at the same time. It is noteworthy that in the single-channel type, the system usually made adjustments only in the drive axle mechanisms, that is, the ABS worked with only two wheels);
- two-channel (in this type of ABS, the brake mechanisms were divided along the sides, each of which has its own set of valves. That is, one channel combined the mechanisms of the front and rear wheels of one side);
- Three-channel (in it, one set of valves was provided for the wheels of the rear axle, and the front ones were each equipped with their own channel).
Now these three types of ABS systems are found only on older cars.
Operating modes
The anti-lock braking system can operate in three modes:
- Injection. In this mode, the brakes work in the usual way. After pressing the pedal, the liquid goes to the mechanisms, the wheel slows down the rotation. In this mode, the inlet valve is open, and the outlet valve is closed, that is, the liquid moves only along the supply line;
- Retention. If the signal unit calculates that one of the wheels is reducing rotation faster than the others, then it will close the intake valve. As a result, the force of the mechanism will stop increasing, so the deceleration of the wheel stops at a certain level. On other mechanisms, the force will continue to grow;
- Pressure relief. If, even after switching to the hold mode, the wheel still continues to slow down, the control unit activates the exhaust valve (closes the inlet valve) and part of the liquid goes into the pressure accumulator, thereby reducing the pressure in the mechanism (the wheel is released and starts to increase speed). As stated above, one battery is for two brakes (included in the circuit). There are situations when pressure is released from these two mechanisms at once, so the volume of the battery may simply not be enough. And then the pump turns on, pumping the excess into the main line.
Diagram of the ABS system
During braking, the system changes the operating mode repeatedly, which ensures effective braking. At the same time, the driver does not need to “play” with the pedal himself in order to prevent the wheels from locking, the system does everything itself.
Advantages and disadvantages
Other benefits of this system include:
- maintaining the trajectory of movement during braking at the entrance to the turn;
- when braking, maneuvering is allowed;
- convenience for novice drivers.
But ABS isn't perfect. Under certain conditions, this system may not function correctly and make errors. And this affects the effectiveness of braking and can somewhat disorientate the driver.
These conditions are:
- road with problematic surface;
- sand;
- coating with potholes, "comb".
In general, ABS only works well on flat roads with good grip. In other cases, the ABS system may make errors.
For example, on a problematic track with often alternating coverage (asphalt changes with gravel or other bulk material), the system will not be able to select the optimal force on the mechanisms, which increases the braking distance.
When going off the road, the ABS is also not an “assistant”. Here, locking is the best way to stop the car as quickly as possible.
The features of the anti-lock braking system also include some delay in switching on when driving at high speeds (over 130 km / h). It's just that the control unit under such conditions needs some time to make calculations and activate the valve body.
At low speeds (10-15 km / h), the system is completely disabled. If this is a stop on a flat surface, then disabling the ABS does not affect in any way, but when braking on a descent, deactivating the system can have a negative impact.
Note that turning off the ABS is a conditional concept, since the system works constantly and it is impossible to turn it off. Here, deactivation should be understood as a transition to "standby mode". That is, it is activated again and will begin to perform its function the next time you press the brake pedal. The only time it won't turn on is braking when driving at low speeds.
Improvements and improvements
Engineers have brought the ABS design to a high level and there is practically nothing to improve. Only some of the constituent elements are subject to modifications. So, wheel sensors now not only measure the speed of rotation, G-sensors and accelerometers are additionally integrated into them.
Also, improvements include an increase in the functionality of the electronic unit (the very use of ABS as the basis for other systems). For example, the ABS control unit is involved in the traction control and brake force distribution.
Autoleek