ABS - Anti-lock braking system. Antilock Braking System - ABS Generation abs

Almost every owner of movable property has come across the abbreviation ABS (in Russian it looks like this - ABS). But not every motorist knows what ABS is needed for in a car. This is especially interesting for novice drivers. They don't even know how everything works. It is worth opening this veil of mystery.

It is worth noting that nowadays almost every car that comes off the assembly line is equipped with ABS, which has become a prerequisite for most manufacturers. And if earlier such equipment was available as an additional option, now ABS is already installed on cars. basic configuration... An example of this is a brand or any other model.

In order to stop the vehicle, it is not enough just to press the brake pedal in time. Yes, the car will stop, but how long will it take and what distance will it cover in case of normal braking? Everything here mainly depends on the speed - if it is small (say, up to 20-30 km / h), then the transport will stop quite quickly, without having overcome even a few tens of meters. It is quite another matter when it is necessary to resort to emergency braking when driving more than 60-100 km / h.

If you hit the brake pedal sharply, the wheels will immediately lock, but the car will still move as if it were on skis - the tires will slide along the road. Also, under all 4 wheels there may be a heterogeneous surface - accordingly, the sliding speed will be different, which in itself becomes dangerous. Control over the car is lost and it will be carried into a skid. And unmanaged vehicles are a source of increased danger for other participants. road traffic.

What is the conclusion from this? That's right - to prevent hard blocking of the wheels in order to avoid slipping! To achieve this, there is one proven trick - braking must be intermittent. To do this, it is not necessary to keep the brake pedal constantly depressed; you need to release it from time to time, and then press it again. We do approximately the same in the case of lifting the machine with a foot jack.

Such uncomplicated actions ensure the preservation of vehicle control - the tires do not lose traction. However, once in an extreme position, not every driver is able to avoid the human factor. It's very easy to get confused and forget about all the rules. And for this very reason, an assistant was invented in the person of the ABS.

Definition of ABS

We all know the importance of the braking system. The safety of not only the driver and his passengers, but also other road users depends on how good it is. ABS in a car in full decoding sounds like an anti-lock braking system (or a whole complex), which does not allow the wheels to lock in case of emergency braking.

Structurally, the unit is represented by an electromechanical unit, which takes over braking in difficult road conditions.

Complex device

From a constructive point of view, the ABS anti-lock braking system is as follows:

• electronic control unit (BU);
• speed control sensors;
• valve body.

BU is the "brain" of the entire system or a computer. In fact, he manages all the work, based on the signals received from the ABS sensors. Other components also deserve special attention.

Sensors

Each sensor is attached directly to the wheels and records the speed. The principle of operation of the sensor is based on the physical phenomenon of electromagnetic induction. The coil itself, equipped with a magnetic core, is fixed on the wheel hub immovably, and in some cars it is located in the drive axle gearbox.

A toothed ring is attached to the hub, rotating with the wheel, as a result, the magnitude of the magnetic field changes. As a result, an electric current is created, and its strength directly depends on the rotational speed. As a result, a signal of a certain magnitude is generated, which is then sent to the control unit.

Valve body

As for this element, the valve body, in turn, is also arranged in a peculiar way:

1. Solenoid valves - inlet, outlet. - due to them, the pressure in the brake cylinders is regulated. Their number for each type of ABS is purely different.
2. Pump - has a return flow function. Its task is to build up pressure, ensuring the supply of brake fluid from the accumulator, and when necessary, it takes it back.
3. The accumulator is the storage where the brake fluid is located.

In cars with ABS, the valve body is sequentially integrated into the general brake system, that is, it is located immediately behind the main brake cylinder.

How everything works

How does ABS work? The principle of operation is as follows. When the sensor (the one in the wheel hub) detects its sharp deceleration or complete stop, the control unit gives a control signal that opens the exhaust valve for a short period of time. As a result, the pressure in the system decreases and nothing prevents the wheel from rotating. But after exceeding its speed limit, it is the turn of the opening of the inlet valve - the pump builds up pressure again, which leads to the operation of the brakes.

Everything looks like it was previously described in the section on correct braking - briefly press the brake pedal, then release. This buildup continues until the car stops. But unlike humans, electronics work much faster - in just one second, the number of repetitions can be from 4 to 10!

As a result, tire adhesion to the road is maintained, due to which the braking distance is actually reduced. In addition, control of the machine is not lost, that is, during braking, there is always the opportunity to go around an obstacle that has arisen.

What is the need?

Even some seasoned drivers, let alone newbies, have the misconception as to what exactly ABS is for. That is, they are firmly convinced that the anti-lock braking system can only shorten the braking distance. In fact, its main role is reduced to maintaining control of the car when it is necessary to resort to emergency braking.

How does a car stop without an ABS equipment? It simply slides and therefore has a long stopping distance. And the higher the speed, the longer it is. In this case, even if you turn the steering wheel in any direction in order to go around the obstacle, the car will still move straight ahead!

ABS solves the problem with wheel blocking, which means that the tire grip is preserved. That is, control over the car is not lost. Yes, the wheels lock, but for a short time - so they do not slip.

Now it is clear what ABS is in the full sense of the word. But besides this, the system provides another equally useful function - to provide straight-line braking on a road with a dissimilar surface. Consider an illustrative example, when one side of a car runs into a wet, slippery (ice, etc.) area, and another clean surface under the wheels. In this case, without ABS, braking on one side will be much more effective than on the other, which leads to an uncontrollable skid. This is especially critical when cornering, when lateral force is also acting on the machine.

With regard to the reduction of the braking distance, this statement is true, but only partly and is more likely a consequence of the work of ABS.

ABS problems

In the absence of mechanical stress, there are usually no problems with this braking system. The whole ABS complex is arranged quite simply and is reliable in operation. But even despite the protective measures in the form of a fuse, sometimes breakdowns cannot be avoided. The reasons for this may be different circumstances:

1. Exposure to the environment on an ongoing basis and at times they are quite aggressive.
2. Battery charge level.
3. Unsatisfactory condition of the wiring of the on-board network.

If the voltage drops below 10.5 Volts, the device shuts down spontaneously. To prevent this from happening, you should adhere to simple recommendations:

1. First, avoid lighting the battery from another car. It is also not necessary to use your own battery for such purposes.
2. Secondly, when the ignition is on, do not disconnect any connectors.

In other words, in order to maintain performance ABS systems and extend its resource (as far as possible), you should monitor the technical condition of your own machine. If you have problems with the ABS, you need to contact the nearest car service, where the malfunction will be detected and eliminated at a professional level.

Sensor check

It is important not only to understand what kind of system this ABS is, but also to treat it with due respect. To do this, react in time to alarming "signals", and not ignore them. A faulty sensor is not able to transmit a signal to the system and the auto-blocking complex stops working. As a result, the wheels are blocked during braking. This may cause an indicator on the dashboard to come on to indicate a problem with ABS. And if the icon lights up and does not go out, this is a serious reason to contact a car service, and as early as possible.

Often the most common problem is a wire break. It is easier to identify with a tester. First you need to connect the pins to the connectors, and then measure the resistance with the device. If it is within the permissible limits, which are indicated in the vehicle manual, then everything works.

A significant discrepancy in values ​​indicates an obvious problem, which can be of a different nature. In particular, we are talking about the desire for resistance in one direction or another:

1. To zero - indicates a short circuit.
2. To infinity - the presence of an open circuit in the electrical circuit.

You also need to measure the resistance when the wheel rotates - it should change, which will show the sensor is working properly. The detected breaks should be eliminated, and the break points should be restored only by soldering - the usual twisting is inappropriate here and will not give the desired result. Also, do not mix up the polarity when connecting the wires.

If the sensor is broken, you need to figure out how to remove the rear or front sensors. And here it is better to contact a car service, where they will do everything at the proper level, since all sorts of difficulties and nuances may arise.

Lit indicator

On the dashboard, when the ignition is turned on, several indicator lights turn on immediately. This is evidence that all vehicle systems are self-diagnosed. After a while, they go out, which indicates their full performance. If the ABS light comes on, don't worry, it's just that the test is complete.

Today's new cars are equipped with a variety of systems that even novice drivers can handle with ease. One of the very first systems is the anti-lock braking system. The ABS system is installed even in basic vehicle configurations. It is an electromechanical unit that controls the braking of the vehicle in difficult road situations such as slippery, wet or icy roads. In fact, this is the right hand of a driver, especially a beginner.

Correct braking without ABS

Every driver should be aware that it is not enough just to use the brake pedal in time. Since if you press the brake sharply at high speed, the wheels of the car are blocked, as a result of which there will be no coupling of the wheels with the road surface. The road surface can be different, therefore, the sliding speed of the wheels will be different. As a result, the vehicle is no longer steerable and can easily skid. If the owner of the car is inexperienced, then controlling the direction of the car may not be able to him.

The most important thing in such braking is to prevent the wheels from locking rigidly, causing the vehicle to skid. In order to avoid such cases, it is recommended to use the intermittent braking technique. To implement such correct braking, it is necessary to periodically press and release the brake pedal at short intervals, and in no case should the brake pedal be kept pressed until it stops completely. With such a simple braking technique, it is possible to control the vehicle regardless of the quality of the road surface.

However, it is necessary to take into account a simple human factor - the driver in an unforeseen situation is able to get confused and all the braking rules can simply fly out of his head. To control the vehicle in such emergency situations, the anti-lock braking system was developed.

What is the secret of ABS work

It is important to know on what principle the ABS works, because it has a close connection with the control system, which means that, accordingly, with the safety level of the driver and passenger. So, the main idea of ​​the system is that when the driver presses the brake pedal, instant control occurs, and braking force is redistributed to the wheels. Thereby, the vehicle is steerable in all conditions, and a speed reduction effect is achieved. However, you cannot rely only on various additional systems because the driver has to master his own car - the length of the braking distance and the behavior in emergency situations. It is recommended to test the ability of the car on specialized race tracks, in order to prevent awkward situations on the road in the future.

There are still some features of the ABS. For example, when the driver decides to stop the movement of a car equipped with the ABS system, then when the brake pedal is pressed, a slight vibration is felt on the pedals, and an accompanying sound similar to a "ratchet" can be heard. Vibration and sound are a sign that the system is working. In the meantime, the sensors read the speed values ​​and the control unit monitors the pressure inside the brake cylinders. Thus, it does not allow wheel blocking, but slows down with quick jerks. Due to this, the speed of the car drops, and at the same time does not go into a skid, which allows you to drive the vehicle until the very stop. Even on slippery roads, with ABS, the driver only needs to keep the direction of the vehicle under control. This perfect and controlled braking is only possible thanks to the ABS system.

The following steps should be emphasized:

1. Release of pressure in the brake cylinder.
2. Maintain continuous cylinder pressure.
3. Increase the pressure to an appropriate level in the brake cylinder itself.

It is important to know that the valve body in the vehicle is mounted in the brake system in a row right after the main brake cylinder. As for the solenoid valve, this is a kind of valve that admits and blocks the flow of liquid substance to the brake cylinders themselves.

Monitoring, as well as the working processes of the vehicle braking system are carried out in accordance with the information that came to the ABS control unit from the speed sensors.

During the braking process, ABS decodes information from the wheel speed sensors, due to which the vehicle's speed decreases evenly. In the event of a stop of any wheel, the signal is instantly sent from the speed sensors to the control unit. Upon receipt of such a signal, the control module releases the blockage by activating the exhaust valve, which blocks the entry of liquid into the wheel brake cylinder. At this moment, the pump returns the liquid to the accumulator. When the wheel rpm increases to the permissible speed, the control unit will give the command to close the exhaust valve and open the intake valve. After that, the pump starts, which will build up pressure into the brake cylinder, as a result of which the wheel will continue to brake. These processes are carried out instantly, and last until the final stop of the vehicle.

The discussed essence of the ABS operation represents the newest four-channel system in which all wheels of the vehicle are monitored.

Other notable types

1. The single-channel consists of a sensor located on the rear axle, the task of which is to distribute the braking force synchronously to four wheels. This kind of system has only one pair of valves, due to which, the pressure is varied simultaneously throughout the entire circuit.
2. Two-channel - it carries out paired control of the wheels, which are located on one side.
3. The three-channel consists of three speed sensors: one is mounted on the rear axle, and the rest are mounted on the front wheels separately. In the mentioned type of system, there are three pairs of valves (inlet and outlet). The action of this type of ABS is to individually control the front wheels and a pair of rear wheels.

Comparing different types of ABS, we can conclude that their difference is manifested only in a different number of valves themselves and speed control sensors. However, the essence of the system in the vehicle, as well as the order of the ongoing processes, is identical for all types of systems.

System implementation history

Engineers from leading automotive companies worked diligently to develop ABS in the first half of the 70s. Even the very first systems were quite successful, and already in that decade, similar systems began to be installed in mass-produced cars.

Initially, mechanical sensors were mounted on cars only on one axle, which sent data to the control module about the change in pressure in the brake circuits. Developers from Germany took this area one step further and began to use sensors without contacts, and this, in turn, catalyzed the transfer of information to the logic block. In addition, the number of false alarms has been reduced, and due to the fact that rubbing surfaces have been eliminated, wear has disappeared. The modern system works according to the same principle that was used in the first anti-lock braking systems.

Anti-lock braking system components

Hypothetically, the structure of the ABS is absolutely simple, and consists of the following devices:

• valve body
• speed sensors
• electronic control unit

The latter plays the role of the "intelligence" of the system (computer), so it is not difficult to imagine what role it plays. As for the speed control sensors and valve body, a deeper analysis is needed.

How the speed sensor works

The sensors that control the speed work on the principle of electromagnetic induction. A coil with a magnetic core is rigidly fixed in the drive axle gearbox. Also, a gear ring is fixed in the hub, which rotates in parallel with the wheel. Then this rotation changes the parameters of the magnetic field, which in response causes the appearance of a current. The strength of the electric current will increase in direct proportion to the speed of rotation of the wheels. Based on this force, in turn, a signal is generated and transmitted to the electronic control unit. The pulses are transmitted from four speed sensors, which are of two types: active and passive, and also differ in design.

The active type of sensor operates with a magnetic sleeve. The transmission of a binary signal is carried out by reading its label. Thanks to the rotational speed, there are no errors and, as a result, accurate pulse data.

The passive type uses a specific comb in the hub block. Thanks to such signals, the sensor is able to determine the rotational speed. It is important to take into account one drawback of this design - at low speed, inaccuracy may result.

Valve body

The valve body includes:

• a reservoir for storing brake fluid - a hydraulic accumulator;
• intake and exhaust solenoid valves, which regulate the pressure supplied to the brake cylinders of the vehicle. Each type of ABS differs in the number of valve pairs;
• thanks to the universal pump, the required pressure in the system is generated, as a result of which the brake fluid is supplied from the accumulator, and, when necessary, takes it back.

Some disadvantages of ABS

One of the biggest disadvantages of anti-lock braking systems is that their effectiveness depends on the quality and condition of the road surface. If the road surface is not good enough, the braking distance is much longer. This is due to the fact that from time to time the wheel loses contact or grip on the asphalt and stops rotating. ABS detects this kind of wheel stop as a blockage, and thus stops braking. At the moment of coupling the wheels to the asphalt, the programmed command does not agree with the one necessary in this case, and the system itself needs to rebuild again, which takes time and increases the braking distance. This effect can be minimized only by reducing the vehicle speed.

In the case of an uneven road surface, for example, snow - asphalt or ice - asphalt, getting on a wet or slippery road section, ABS evaluates the surface and adjusts the braking process for this road. At the same time, when the wheels hit the asphalt, the ABS is rebuilt again, due to which the length of the braking tupi again increases.

On unpaved roads, conventional braking systems perform much better and more reliably than anti-lock braking systems. Indeed, during normal braking, the locked wheel pushes the ground, creating a small hill, which does not allow the vehicle to move further. Thanks to this, the car stops very quickly.

Another flaw in the anti-lock braking system is that at low speeds, the system is completely disabled. In the case when the road is sloping and at the same time slippery, you need to remember that a reliable hand brake may be required for braking. Therefore, it must always be in working order.

Regular deactivation of the anti-lock braking system in cars is not provided. Sometimes drivers want to disable this system. To do this, pull the plug out of the block. It is also necessary to take into account that in new cars the redistribution of inter-axle braking forces depends on ABS. Therefore, by braking, the rear wheels are completely locked.

It is important to note that the ABS system is an excellent addition to the vehicle's braking system, thanks to which it is possible to control the vehicle in the most difficult and unusual situations. Despite this, it should not be forgotten that it is impossible to rely entirely on the machine. On the driver's side, too, you need to make great efforts to keep the situation under control.

Video

The technical equipment of modern cars is so diverse that it never ceases to delight with a variety of useful and convenient options, and many motorists are no longer surprised by the presence of such delights that they had only dreamed of before.

One of the systems that are equipped with machines equipped with the latest technology is the ABS anti-lock braking system. It appeared on the automotive market quite a long time ago, but nevertheless, for many Russian car enthusiasts, ABS is still a novelty.

Anti-lock braking system is installed on the vehicle as an option, the task of which is to prevent the wheels from locking up under heavy braking. Thanks to ABS, when the brake pedal is firmly pressed, the car will not skid both on dry asphalt and on a wet road.

And yet, what is abs in a car? This system is a complex electronic device that includes a central unit and speed sensors installed on each wheel. At the moment the brake is applied, they determine the speed at which each wheel rotates. Then the clever electronics removes the excess pressure from the brake line into a special accumulator.

In this case, the rotation of the wheel begins to recover, as the brake pads are unclenched. If the pressure is still high enough, the process described above is repeated again until it returns to normal.

Purpose of ABS

The anti-lock braking system, according to the principle of its operation, imitates the actions of an experienced motorist who uses intermittent braking on a slippery road so that the car does not skid. And then the question arises: why do we need an electronic system when the driver himself can perform these actions? Cars that do not have ABS are difficult to control during hard braking, and in many situations it is simply impossible to do without it.

The anti-lock braking system is automatically activated 15-20 times per second, so that the braking distance in the event of emergency braking will be minimal. A person is physically incapable of working at such a speed.

Perhaps the main advantage of ABS is that the driver, even with strong braking, can make the car obey the steering wheel turns. When this device is absent, during braking, the car will uncontrollably slide along a straight trajectory, despite all the efforts of the driver to align the movement of personal vehicles by turning the steering wheel.

When the ABS is turned on, you can hear a quiet crackling sound in the passenger compartment, indicating the functioning of the executive unit, and feel slight and frequent jolts on the brake pedal. For those motorists who have not previously dealt with such a system, it will take some time to get used to this innovation.

The car's anti-lock braking system has proven itself perfectly, but, nevertheless, it has not been without a number of disgruntled motorists who claim that no electronics can replace an experienced driver, because he is much better oriented in a difficult situation on the road and will take the right action.

ABS does not interfere in the braking process at all, but serves as an irreplaceable assistant in preventing an irreparable road situation. Thanks to the anti-lock braking system, the uncontrollable mountain of iron, which the car turns into when braking hard, begins to obey the steering wheel and is able to perform at least some maneuvers.

Tasks performed by ABS

Thus, the anti-lock braking system performs a number of very important tasks:

• ensures safety when braking, both for the driver himself and for his passengers;
• shortens the braking distance on slippery or wet road surfaces;
• does not allow the driving wheels to lock during heavy braking, which leaves the ability to maneuver and avoid an obstacle.

In some cases, the use of ABS is not justified. So, driving down a steep slope in conditions of strong snow or mud swamp, the inclusion of the anti-lock braking system can lead to sad consequences. Due to gravity, the car rolls downward, while the ABS does not allow the wheels to lock, in this regard, the car, although slowly, still continues to move forward even with the brake pedal fully depressed.

Despite the fact that such situations are extremely rare, all modern SUVs are equipped with additional sensors that determine the degree of slope of the road track. If the descent is too steep, the anti-lock system becomes inactive.

Video

The principle of operation of the ABS system is clearly shown in this video:

Modern cars are replete with various systems that help the driver to drive safely on any road. One of the first such "helpers" was the Anti-lock Brake System. Almost all leading car companies have recently been installing it not as an additional bonus, but in the basic equipment package.

The principle of operation of the ABS on a car is not to unambiguously block the wheels upon the driver's request after pressing the brake pedal. Instead, instant control and redistribution of the braking force on the wheels is carried out. Thanks to this algorithm, the car remains controllable, directional stability is ensured, while the vehicle safely reduces speed.

Automotive engineers have been developing ABS since the late 1960s on safety-related tasks. Some of the first versions of these systems proved to be quite successful. In the next decade, such blocks were already introduced on production vehicles.

The leading company in this industry was the German Mercedes. After experiments with mechanical sensors, which were installed on only one axis and transmitted information to the control unit about the pressure change in the brake circuits, German engineers switched to contactless sensors. This helped speed up the transfer of information to the logical block. The number of false alarms has also decreased, wear has disappeared due to the elimination of rubbing surfaces.

The modern ABS system works on the same principle as was laid down in the first anti-lock braking systems being developed.

It was the second generation of this block, released in the late 70s, that began to be installed as a basic configuration on the prestigious Mercedes-Benz 450 SEL cars of that time.

Wheel work in a car

The main task entrusted to the ABS is to maintain the vehicle's controllability as much as possible during braking. Since a locked wheel has significantly worse adhesion to the road surface, then its braking forces will be much lower than that of a rotating wheel. The driver does not have control of the non-rotating wheel at all. The most acceptable option for moving a braking car will be straight-line movement, however, the car often goes into an uncontrolled trajectory.

Installed unit ABS contributes to efficient wheel operation at this point in time, balancing between blocking (not completely stopping rotation) and the greatest level of traction. An icy road and a dry dirt road will give different inputs, but in both cases the system will strive to maintain maximum control.

It is believed that a well-tuned anti-lock braking system is capable of driving a car at a higher level than a highly skilled driver in a car without ABS.

The anti-blocking system calculates its work based on the wheel slip coefficient. It is found as the quotient of the difference between the speeds of the car and the peripheral speed of the wheels to the linear speed of the car. The indicator changes with different driving modes.

Intense acceleration gives a high peripheral speed to the wheel at a low line speed of the machine, while hard braking promotes the reverse process. An example of 100% slippage is two options: a locked braking wheel and the process of slipping without moving, for example, in a mud or puddle.

How ABS works on a car

In modern braking systems, anti-blocking manipulates electronic valves and a pump. The driver starts braking by pressing the pedal, and if slippage does not occur, then the anti-lock braking system is not connected to these actions.

The new machines are equipped with four-channel circuit layouts. Thus, the control is carried out for each of the wheels. Each circuit is capable of functioning in three modes:

• increased pressure;
• retention of its value;
• decrease in pressure.

When the wheel locking moment approaches, the pressure is kept at the same level. At this time, the fluid supply from the brake cylinder is cut off and further pressing on the pedal does not lead to blocking. If the calculated slip coefficient takes on values ​​of more than 20%, then the pressure is released through the pump. Then slippage can, on the contrary, decrease by more than 15%, then the valves open to increase pressure when the pedal is depressed.

There is an alternating on / off of such modes. The process most often stops after a significant decrease in speed to 5-15 km / h. The driver can hear the recoil of the anti-lock braking system from the brake pedal. Even the greatest professional cannot repeat the rate at which such cycles change.

ABS elements

If the car hits a dry surface with one wheel and the other on a slippery one, then the ABS vehicle will be kept straight by balancing the pressure for each of the circuits. If a car hits such a road without this block, the car will leave the side of the dry track, and during a sharp blocking of the wheels, the situation will lead to a sharp U-turn with unpredictable consequences.

Built-in speed sensor operation

The information on which further actions on the operation of the anti-lock braking system are based is received by the control module from the speed sensors. To fully understand what ABS is in a car, you need to understand their work. The pulses come from four of these sensors, which may differ in design and be active or passive.

Passive option execution implies the presence of a comb in the hub block. The sensor determines the rotation speed using analog signals. However, such a design at low speeds can give an error.

Active option the sensor works with a magnetic ring. Reading its labels, a binary signal is transmitted. There are no errors due to rotation speeds in this case. All that remains is the exact momentum diagram.

Non-standard ABS kit for truck

You need to know that cars with four-wheel drive equipped with an additional G-sensor, which corrects the vehicle speed and acceleration readings for the ABS module.

This sensor is mounted with an accelerometer.

Riding with and without ABS

Young drivers are often interested in how to brake a car with anti-lock braking. After all, the kickback to the pedal at the first sensation pushes the foot off the brake. This is not worth doing. Intermittent braking is typical for cars without ABS, as the driver tries to avoid complete wheel locking and loss of control. On modern cars, you can safely drown the pedal, and the automation will do its job.

Regular shutdown of this system in vehicles not provided. For various reasons, car owners sometimes want to do this. To do this, it is enough to remove the fuse from the block, but you need to remember that in modern transport, the ABS is also responsible for the redistribution of the inter-axle braking forces. Accordingly, pressing the pedal leads to complete blocking of the rear wheels and undesirable consequences.

You also need to understand that you cannot shift everything to the machine. The driver must control the situation himself, and the car with its various elements will remain only a high-quality tool.

When driving in a straight line during braking of the car, different forces act on it: the weight of the car, the braking force and the lateral force. The magnitude of the forces depends on many factors, such as the speed of the vehicle, the size of the wheels, the condition and design of tires and road surfaces, the design of the braking system and its technical condition.

Rice. Forces acting on the wheel during braking:
G is the weight of the car; FB - braking force; FS - lateral force; νF - vehicle speed; α - withdrawal angle; ω - angular velocity

During the rectilinear movement of the vehicle at a constant speed, there is no difference in the wheel rotation speeds. In this case, there is also no difference between the superficial vehicle speed νF and the averaged wheel rotation speed νR coordinated with it, i.e. νF = νR. The average speed of rotation of the wheels is understood as the value

νR = (νR1 + νR2 + νR3 + νR4) / 4,
where νR1… νR4 are the rotation speeds of each wheel separately.

But as soon as the process of intensive braking begins, the superficial speed of the car νF begins to exceed the average speed νR of rotation of the wheels, since the body "overtakes" the wheels under the action of the inertial force of the mass of the car, i.e. νF> νR.

In such a situation, a phenomenon of uniform moderate slip occurs between the wheels and the road.This slip is an operating parameter of the brake system and is defined as:

λ = (νF - νR) / νF 100%

Physically, working sliding, in contrast to emergency skid, is realized due to the deflection of the tread of the wheel tires, the shift of fine fractions on the road surface, and due to the amortization of the car suspension. These factors keep the vehicle from skidding and reflect the usefulness of wheel slip when braking. It is clear that in this case, the deceleration of the wheel rotation occurs gradually and in a controlled manner, and not instantaneously, as in the case of blocking.

The λ value is called the slip coefficient and is measured as a percentage. If λ = 0%, then the wheels rotate freely, without the effect of road friction resistance on them. The slip coefficient λ = 100% corresponds to the skid of the wheel when it enters the locked state. At the same time, the braking efficiency, stability and controllability of the vehicle during braking are significantly reduced.

With the appearance of the working slip effect, in which there is still normal rolling of the wheels between them and the road, a uniformly increasing friction resistance arises, expressed by the coefficient of adhesion in the direction of travel μHF, which is a function of the working slip γ and creates the vehicle braking force FB = K μHFG. K - constructive proportionality coefficient, depending on the state of the tire tread, brake pads, brake discs and brake calipers.

The figure shows the dependence of the value of the relative wheel slip on the coefficient of adhesion in the direction of travel μHF and the coefficient of adhesion in the transverse direction μS when braking on a dry concrete surface.

Rice. Dependence of the coefficient of adhesion on wheel slip.

As can be seen from the figure, the relative wheel slip λ reaches its maximum value at certain values ​​of the coefficient of adhesion in the direction of travel μHF, with a decrease in the coefficient of adhesion in the transverse direction μS. For most road surfaces, at γ values, and hence the braking force, in the range from 10% to 30% μHF, reaches a maximum value and this value is called critical (λ) cr. Within these limits, the coefficient of adhesion in the lateral direction μS is also sufficiently high, which ensures stable movement of the vehicle when braking if a lateral force acts on the vehicle.

The shape of the curves for the friction coefficient in the direction of travel, μHF, and the coefficient of traction in the lateral direction, μS, depend largely on the type and condition of the road surface and tires.

It is important to note that for small γ (from 0% to 7%), the braking force linearly depends on the slip.

During emergency braking, excessive force on the brake pedal can cause the wheels to lock. In this case, the tire adhesion to the road surface is sharply weakened, and the driver loses control of the car.

Purpose and structure of the ABS

Anti-lock braking systems(ABS) brakes are designed to provide constant control over the force of adhesion of the wheels to the road and accordingly regulate at any given moment the braking force applied to each wheel. The ABS redistributes the pressure in the branches of the hydraulic drive of the wheel brakes so as to prevent blocking of the wheels and, at the same time, to achieve the maximum braking force without loss of vehicle control.

The main task of the ABS is to maintain in the process of braking the relative slip of the wheels within narrow limits near λcp. This ensures optimum braking performance. For this purpose, it is necessary to automatically regulate the braking torque supplied to the wheels during braking.

There are many different ABS designs that solve the problem of automatic control of the braking torque. Regardless of design, any ABS must include the following elements:

• sensors, the function of which is to issue information, depending on the adopted control system, about the angular speed of the wheel, the pressure of the working fluid in the brake drive, the deceleration of the car, etc.
• a control unit, usually electronic, where information from sensors is received, which, after logical processing of the received information, gives a command to the actuators
• actuators (pressure modulators), which, depending on the command received from the control unit, reduce, increase or maintain a constant pressure in the brake drive of the wheels

Rice. ABS control scheme:
1 - actuator; 2 - the main brake cylinder; 3 - wheel brake cylinder; 4 - control unit; 5 - wheel speed sensor

The process of regulation with the help of the ABS wheel braking is cyclical. This is due to the inertia of the wheel itself, the drive, as well as the ABS elements. The quality of regulation is assessed by how much the ABS ensures the sliding of the braking wheel within the specified limits. With a large range of cyclical pressure fluctuations, the comfort during braking "jerking" is disturbed, and the elements of the car experience additional loads. The quality of the ABS operation depends on the adopted regulation principle, as well as on the speed of the system as a whole. The speed determines the cyclic frequency of the braking torque change. An important property of the ABS should be the ability to adapt to changing braking conditions (adaptability) and, first of all, to a change in the coefficient of adhesion during braking.

A large number of principles (algorithms of functioning) have been developed, according to which the ABS works. They differ in complexity, cost of implementation, and in the degree to which the requirements are met. Among them, the most widely used is the algorithm for decelerating the braking wheel.

The braking dynamics of a car with ABS depends on the adopted scheme for installing the elements of this system. From the point of view of braking efficiency, the best scheme is with autonomous regulation of each wheel. To do this, it is necessary to install a sensor on each wheel, and in the brake drive - a pressure modulator and a control unit. This scheme is the most complex and expensive.

There are simpler ABS schemes. Figure b shows a diagram of an ABS with controlled braking of the two rear wheels. For this, two wheel speed sensors and one control unit are used. In such a scheme, the so-called low or high threshold regulation is used. Low threshold regulation provides for the control of the braking wheel, which is in the worst grip conditions ("weak" wheel). In this case, the braking capabilities of the "strong" wheel are underutilized, but equal braking forces are created, which contributes to maintaining directional stability during braking with a slight decrease in braking efficiency. High threshold regulation, i.e. steering the wheel in better traction conditions, gives higher braking efficiency, although stability is somewhat reduced. A "weak" wheel is cyclically blocked with this control method.

Rice. ABS installation diagrams on a car

An even simpler diagram is shown in Figure c. It uses one yaw rate sensor located on the propeller shaft, one pressure modulator and one control unit. Compared to the previous one, this scheme has a lower sensitivity.

Figure d shows a diagram in which angular velocity sensors are used on each wheel, two modulators, and two control units. In such a scheme, both low and high threshold regulation can be applied. Often in such schemes, mixed control is used (for example, a low threshold for the front axle wheels and a high threshold for the rear axle wheels). In terms of complexity and cost, this scheme occupies an intermediate position between those considered.

The ABS operation process can take place in a two- or three-phase cycle.

With a two-phase cycle:

• second phase - pressure release

With a three-phase cycle:

• first phase - pressure build-up
• second phase - pressure release
• third phase - maintaining pressure at a constant level

When installed on passenger car ABS are possible closed and open brake hydraulic drives.

Rice. Hydrostatic brake pressure modulator circuit

Closed or closed (hydrostatic) drive works on the principle of changing the volume of the brake system during braking. Such an actuator differs from the usual installation of a pressure modulator with an additional chamber. The modulator operates on a two-phase cycle:

• The first phase - the increase in pressure, the winding of electromagnet 1 is disconnected from the power source. The armature 3 with the plunger 4 is under the action of the spring 2 in the extreme right position. Valve 6 is pressed from its seat by spring 5. When the brake pedal is depressed, the fluid pressure generated in the master cylinder (port II) is transmitted through port I to the working brake cylinders. The braking torque increases.
• The second phase is pressure relief: the control unit connects the coil of electromagnet 1 to the power source. Armature 3 with plunger 4 moves to the left, increasing the volume of chamber 7. At the same time, valve 6 also moves to the left, blocking output I to the working brake cylinders of the wheels. Due to the increase in the volume of the chamber 7, the pressure in the working cylinders drops, and the braking torque decreases. Further, the control unit gives a command to increase the pressure, and the cycle is repeated.

An open or open hydraulic brake actuator (high pressure actuator) has an external power source in the form of a high pressure hydraulic pump, usually in combination with a hydraulic accumulator.

At present, the preference is given to a high-pressure hydraulic drive, which is more complex than a hydrostatic one, but has the necessary speed.

Rice. Dual-circuit brake actuator with ABS:
1 - wheel sensor of angular speed; 2 - modulators; 3 - control units; 4 - hydraulic accumulators; 5 - check valves; 6 - control valve; 7 - high pressure hydraulic pump; 8 - drain tank

The brake actuator has two circuits, therefore it is necessary to install two autonomous accumulators. The pressure in the accumulators is maintained at the level of 14 ... 15 MPa. Here, a two-piece control valve is used, which provides a follow-up action, i.e. proportionality between the effort on the brake pedal and the pressure in the brake system. When the brake pedal is pressed, the pressure from the accumulators is transmitted to the modulators 2, which are automatically controlled by the electronic units 3, which receive information from the wheel sensors 1. The figure shows a diagram of a two-phase slide pressure modulator for a high-pressure hydraulic brake drive. Consider the phases of this modulator:

• Pressure build-up phase 1: the ABS control unit disconnects the solenoid coil from the power source. The spool and the solenoid armature are spring force moved to the upper position. When you press the brake pedal, the control valve communicates the accumulator (terminal I) with the discharge channel of the pressure modulator. The pressurized brake fluid flows through port II to the working cylinders of the brake mechanisms. The braking torque increases.
• Pressure relief phase 2: the control unit communicates the solenoid coil with the power supply. The solenoid armature moves the spool to the down position. The supply of brake fluid to the working cylinders is interrupted: outlet II of the working brake cylinders is in communication with drain channel III. The braking torque is reduced. The control unit gives a command to increase the pressure, disconnecting the solenoid coil from the power source, and the cycle repeats.

Rice. Scheme of operation of a two-phase high pressure modulator:
a - phase 1; b - phase 2

Currently, the more common ABS operating in a three-phase cycle. An example of such a system is the rather common 2S ABS system from Bosch.

This system is integrated as an add-on to the conventional braking system. Pressure (H) and unload (P) solenoid valves are installed between the main brake cylinder and the wheel cylinders, which either maintains a constant level or reduce the pressure in the wheel drives or in the circuits. The solenoid valves are driven by a control unit that processes information from the four wheel sensors.

The control unit, which continuously receives data on the rotation speed of each wheel and its changes, determines the moment of blockage, then, if necessary, releases the pressure, turns on the hydraulic pump, which returns part of the brake fluid back to the feed tank of the master cylinder.

Rice. Functional diagram of ABS Bosch 2S:
1 - control unit; 2 - modulator; 3 - the main brake cylinder; 4 - tank; 5 - electrohydraulic pump; 6 - wheel cylinder; 7 - wheel sensor rotor; 8 - wheel inductive sensor; 9 - signal lamp; 10 - brake force regulator; N / R - discharge and discharge solenoid valves; - .-. control unit input signals; - - - - control unit output signals; –––– brake line

The ABS modulator contains solenoid valves, a hydraulic pump with fluid pressure accumulators, a solenoid valve relay and a hydraulic pump relay.

Rice. Electro-hydraulic modulator:
1 - solenoid valves; 2 - hydraulic pump relay; 3 - solenoid valve relay; 4 - electrical connector; 5 - hydraulic pump electric motor; 6 - radial piston pump element; 7 - pressure accumulator; 8 - muffler

The system operates according to a program divided into three phases: 1 - normal or conventional braking; 2 - keeping the pressure at a constant level; 3 - pressure relief.

Normal braking phase

During normal braking, there is no voltage on the solenoid valves, from the master cylinder, the brake fluid under pressure freely flows through the open solenoid valves and activates the wheel brakes. The hydraulic pump is not working.

Rice. Braking phases:
a) normal braking phase; b) the phase of keeping the pressure at a constant level; c) pressure release phase; 1 - wheel sensor rotor; 2 - wheel sensor; 3 - wheel (working) cylinder; 4 - electrohydraulic modulator; 5 - electromagnetic valve; 6 - pressure accumulator; 7 - delivery pump; 8 - the main brake cylinder; 9 - control unit

Pressure holding phase at a constant level

When signs of blocking of one of the wheels appear, the control unit, having received the corresponding signal from the wheel sensor, proceeds to the program of the cycle of maintaining the pressure at a constant level by disconnecting the main and corresponding wheel cylinders. A current of 2 A is applied to the coil of the solenoid valve. The valve piston moves and blocks the flow of brake fluid from the master cylinder. The pressure in the wheel slave cylinder remains unchanged even if the driver continues to press the brake pedal.

Pressure relief phase

If the danger of wheel blocking persists, the control unit supplies the coil of the solenoid valve with a current of a larger sip: 5 A. As a result of additional movement of the valve piston, a channel opens through which the brake fluid is discharged into the fluid pressure accumulator. The pressure in the wheel cylinder drops. The control unit issues a command to turn on the hydraulic pump, which removes part of the fluid from the pressure accumulator. The brake pedal is raised, which is felt by the brake pedal beat.

The inductive wheel sensor consists of a winding 5 and a core 4. The gear 6 has a rotational speed equal to that of the wheel. When the wheel 6, made of ferromagnetic iron, rotates, the magnetic flux changes depending on the passage of the rotor teeth, which leads to a change in the alternating voltage in the coil. The frequency of the voltage change depends on the speed of the gear wheel, that is, the speed of the wheel of the car. Air gap and tooth size have a large effect on signal amplitude. This allows you to determine the position of the wheel at the intervals between the teeth within half or a third. The signal from the inductive sensor is transmitted to the electronic control unit.

Rice. Inductive sensor:
1 - permanent magnet; 2 - case; 3 - sensor mounting; 4 - core; 5 - winding; 6 - a gear wheel

Inductive sensors can be mounted on the wheel drive shaft, on the bevel gear drive shaft for RWD car models, on the stub axles and inside the wheel hub.

Rice. Fastening the inductive sensor to the stub axle:
1 - brake disc; 2 - front hub; 3 - protective casing; 4 - screw with internal hexagonal gearing; 5 - sensor; 6 - pivot pin

Rice. Mounting the inductive sensor inside the wheel hub:
1 - wheel mounting flange; 2 - balls; 3 - ABS sensor ring; 4 - sensor; 5 - flange for attaching to the suspension.

More advanced are active sensors used to measure the wheel speed. The sensitive element of the electronic cell 2 of such a sensor is made of a material, the electrical conductivity of which depends on the strength of the magnetic field. When the master disk 3 rotates, changes in the magnetic field occur. The fluctuations in the current flowing through the sensor, caused by the changing magnetic field, are converted electronically into fluctuations in the voltage output to the external contacts of the sensor. When the master disk rotates, the sensor installed near it generates rectangular pulses, the frequency of which corresponds to the rotational speed of the disk. The advantage of this sensor in comparison with previously used systems is the accurate registration of the speed when it decreases until the wheel stops.

Rice. Active sensor:
1 - sensor body; 2 - electronic sensor cell; 3 - master disk

As a rule, there should be a control lamp on the instrument panel, which should go out when the engine is running or if the vehicle speed exceeds 5 km / h. It also lights up if one of the wheels slips for more than 20 seconds or if the power supply is supplying less than 10 volts. The system warning light warns the driver that due to a malfunction of the system, it has been automatically turned off, however, the braking system continues to function as a conventional braking system without ABS.

A similar operating principle is applied to the Bosch ABS 2E, but this system uses an equalizing cylinder to equalize the pressure in the rear wheel brakes, which allows three valves to be used instead of four solenoid valves. The modulator thus includes not four, but three solenoid valves, an equalizing cylinder, a two-piston pressure hydraulic pump, two pressure accumulators, a pump relay and a solenoid valve relay.

The system works as follows. During normal braking, brake fluid under pressure from the master cylinder enters the working cylinders of both front wheels and the right rear wheel through three solenoid valves, which are closed in the initial position. The brake fluid is supplied to the slave cylinder of the rear left wheel through the open bypass valve of the equalizing cylinder. When there is a danger of blocking one of the front wheels, the control unit issues a command to close the corresponding solenoid valve, preventing an increase in pressure in the wheel cylinder. If the danger of wheel blocking is not eliminated, a current is applied to the solenoid valve to open the section of the line between the wheel slave cylinder and the pressure accumulator. The pressure in the brake drive drops, after which the control unit issues a command to turn on the hydraulic pump, which distills fluid into the master cylinder through the equalizing cylinder.

Rice. Bosch ABS 2E in the normal braking phase:
1 - the main brake cylinder; 2 - solenoid valve; 3 - pressure accumulator; 4 - rear axle solenoid valve; 5 - delivery pump; 6 - bypass valve; 7 - balancing cylinder piston; Ппр - front right wheel; Pl - front left wheel; Зпр - rear right wheel; Evil - rear left wheel

When there is a danger of locking one of the rear wheels, the pressure will be regulated in both rear brakes at the same time in order to prevent the rear wheels from skidding.

The solenoid valve for the right rear brake actuator is set to the constant pressure hold position and closes the section of the line between the master cylinder and the wheel cylinder. On the opposite end surfaces of the piston 7 of the equalizing cylinder pressure of various magnitudes begins to act, as a result of which the piston with the rod will move to the side of the lowest pressure (in the figure - up) and close valve 6, disconnecting the master cylinder and the wheel cylinder of the left rear brake. Due to the resulting pressure difference in the working chambers above and below it, the piston of the balancing cylinder is always set to such a position in which the pressure in the drives of both rear brakes is the same.

If there is still a danger of blocking the rear wheels, the control unit supplies the solenoid valve in the rear wheel circuit with a current of 5 A. The solenoid valve spool moves and opens the section of the circuit between the working cylinder of the right rear brake and the fluid pressure accumulator. The pressure in the circuit decreases. The hydraulic pump pumps the brake fluid into the master cylinder through the equalizing cylinder. As a result of a decrease in pressure in the space above the piston 7, its next movement occurs, the spring of the central valve is compressed, and the volume of space under the upper piston increases. The pressure in the left wheel brake cylinder is reduced. The balancing cylinder piston is again set to the position corresponding to the equality of the pressures in the drives of both rear brakes. After removing the threat of wheel blocking, the solenoid valve returns to its original position. The balancing cylinder piston also returns to its original lower position under the action of the spring.

More advanced is the Bosch 5-series ABS with block 10, which belongs to a new generation of ABS systems, representing a closed hydraulic system that does not have a channel for returning brake fluid to the reservoir that feeds the main brake cylinder. A diagram of this system is shown using the example of a Volvo S40.

Rice. Diagram of Bosch 5-series ABS:
1 - check valves; 2 - plunger pump valve; 3 - hydraulic accumulator; 4 - chamber for suppressing pulsation in the system; 5 - an electric motor with an eccentric plunger pump; 6 - a reservoir for brake fluid; 7– service brake pedal; 8 - amplifier; 9 - the main brake cylinder; 10 - ABS block; 11 - controlled outlet valves; 12 - inlet controlled valves; 13 - throttling valve; 14-17 - brakes

Electronic and hydraulic components are assembled as a single unit. These include, in addition to those indicated in the diagram: a relay for turning on the plunger pump motor 5 and a relay for turning on the inlet 12 and outlet 11 valves. The external components are: an ABS warning light in the dashboard, which comes on in the event of a malfunction in the system, as well as when the ignition is turned on for four seconds; brake light switch and wheel speed sensors. The block has an output to the diagnostic connector.

The throttle valve 13 is installed to reduce the braking force on the rear wheels in order to avoid blocking them. Due to the fact that the brake system has a setting for the "weaker" rear wheel (this means that the brake pressure of the rear wheels is the same, and its value is set according to the wheel closest to blocking), the throttle valve is installed one per circuit.

Brakes 14-17 include brake discs and single-piston calipers with floating calipers and brake pads equipped with lining wear control calipers. The brakes of the rear wheels are similar to the front ones, but they have solid brake discs (ventilated on the front) and a parking brake actuator mounted in the caliper.

When the brake pedal 7 is pressed, its lever releases the brake light switch button, which, when triggered, turns on the brake light bulbs and brings the ABS into standby state. The movement of the pedal through the rod and the vacuum booster 8 is transmitted to the pistons of the master cylinder 9. The central valve in the secondary piston and the collar of the primary piston block the communication of the circuits with the reservoir 6 for brake fluid. This leads to an increase in the pressure in the brake circuits. It acts on the brake cylinder pistons in the brake calipers. As a result, the brake pads are pressed against the discs. When you release the pedal, all parts return to their original position.

If, during braking, one of the wheels is close to blocking (as reported by the speed sensor), the control unit closes the inlet valve 12 of the corresponding circuit, which prevents a further increase in pressure in the circuit, regardless of the increase in pressure in the master cylinder. At the same time, the hydraulic plunger pump 5 starts to work. If the wheel continues to slow down, the control unit opens the outlet valve 11, allowing the brake fluid to return to the accumulators 3. This reduces the pressure in the circuit and allows the wheel to rotate faster. If the rotation of the wheel is excessively accelerated (in comparison with other wheels) to increase the pressure in the circuit, the control unit closes the outlet valve 11 and opens the inlet valve 12. The brake fluid is supplied from the main brake cylinder and by means of the plunger pump 5 from the accumulators 3. The damper chambers 4 are smoothed ( suppress) pulsations arising in the system during the operation of the plunger pump.

The brake light switch informs the control module of braking. This allows the control module to more accurately control the wheel rotation parameters.

The diagnostic socket is used to connect the Volvo System Tester when performing diagnostics.

If the vehicle is equipped with DSA (Dynamic Stability Control), the DSA control module receives the wheel speed data, which is necessary for measuring wheel slip. The DSA control module receives this information from the ABS control module. There are three communication lines for this purpose. DSA does not use brakes to control slip.

Internal relays (for pump and valves) have separate connections protected by fuses.

When the ignition is turned on, the system checks the electrical resistance of all components. During this test, the warning light is on. After completing the test (4 s), the lamp should go out.

When the car is moving, the pump electric motor, its relay, intake and exhaust valves are checked at a speed of 6 km / h. At a speed of 40 km / h, the operation of the wheel sensors is checked. During system operation, the pump runs continuously.

While driving in rain or snow at a speed of more than 70 km / h and the windshield wiper is on, the front brake linings periodically (every 185 seconds) briefly (for 2.5 seconds) are pressed against the brake discs with a minimum pressure (0.5 ... 1.5 kgf / cm2). As a result, the linings and discs are cleaned and the braking performance is improved.