The structure and principle of operation of the ABS system

The set of active safety of modern cars includes various assistants and systems that allow either to prevent an emergency situation or to minimize human injuries during an accident.

Among these elements is the anti-lock braking system. What it is? How does modern ABS work? How does the ABS work and how to drive a car when this system is on? The answers to these questions can be found in this review.

What is anti-lock braking system

Anti-lock braking system means a set of electro-hydraulic elements that are installed in the chassis of the car and are associated with its brakes.

It provides better traction on the road surface, preventing the wheels from completely stopping during braking on unstable road surfaces. This often happens on ice or wet roads.

History

For the first time this development was presented to the public in the 1950s. However, it could not be called a concept, because this idea was developed at the beginning of the twentieth century. So, engineer J. Francis in 1908 demonstrated the work of his "Regulator", which prevented wheel slippage in rail transport.

A similar system was developed by the mechanic and engineer G. Voisin. He tried to create a braking system for aircraft that independently regulated the hydraulic effect on the braking elements so that the wheels of the aircraft did not slip along the runway as a result of braking. He carried out experiments with modifications of such devices in the 20s.

Early systems

Of course, as in the case of all the first developments of any inventions, initially the system that prevents blocking had a complex and primitive structure. So, the aforementioned Gabriel Voisin used a flywheel and a hydraulic valve connected to the brake line in his designs.

The system worked according to this principle. The flywheel was attached to a drum on a wheel and rotated with it. When there is no skid, the drum and flywheel rotate at the same speed. As soon as the wheel stops, the drum slows down with it. Due to the fact that the flywheel continues to rotate, the valve of the hydraulic line opened slightly, reducing the force on the brake drum.

Such a system has proven itself to be more stable for the vehicle, since in the event of a skid, the driver instinctively applies the brakes even more, instead of performing this procedure smoothly. This development has increased the braking efficiency by 30 percent. Another positive result - fewer burst and worn tires.

However, the system received due recognition thanks to the efforts of the German engineer Karl Wessel. Its development was patented in 1928. Despite this, the installation was not used in transport due to significant flaws in its design.

A truly working anti-slip brake system was used in aviation in the early 50s. And in 1958, the Maxaret kit was first installed on a motorcycle. The Royal Enfield Super Meteor was equipped with a working anti-lock braking system. The system was monitored by the Road Laboratory. Studies have shown that this element of the braking system will significantly reduce motorcycle accidents, most of which occur precisely due to skidding when the wheel is locked during braking. Despite such indicators, the chief director of the technical department of the motorcycle company did not approve the mass production of ABS.

In cars, a mechanical anti-slip system was used only in some models. One of them is the Ford Zodiac. The reason for this situation was the low reliability of the device. Only since the 60s. the electronic anti-lock braking system has found its way into the famous Concorde aircraft.

Modern systems

The principle of electronic modification was adopted by an engineer at the Fiat Research Center and named the invention Antiskid. The development was sold to Bosch, after which it was named ABS.

In 1971, car manufacturer Chrysler introduced a complete and efficient computer-controlled system. A similar development was used a year earlier by the American Ford in its iconic Lincoln Continental. Gradually, other leading car manufacturers took over the baton as well. By the mid-70s, most rear-wheel drive cars had electronic anti-lock braking systems on the drive wheels, and some were equipped with a modification that worked on all four wheels.

Since 1976, a similar development began to be used in freight transport. In 1986, the system was named EBS, as it worked entirely on electronics.

Purpose of anti-lock braking system

Often, when braking on an unstable surface (ice, rolled snow, water on the asphalt), the driver observes a completely different reaction than expected - instead of slowing down, the vehicle becomes uncontrollable and does not stop at all. Moreover, pressing the brake pedal harder does not help.

When the brakes are applied abruptly, the wheels are blocked, and due to poor grip on the track, they simply stop rotating. To prevent this effect from happening, you need to smoothly apply the brakes, but in an emergency, the driver uncontrollably presses the pedal to the floor. Some professionals press and release the brake pedal several times to slow the vehicle down on unstable surfaces. Thanks to this, the wheels are not blocked and do not skid.

As sad as it may sound, not everyone succeeds in mastering this skill, and some do not even consider it necessary to do this, but simply buy expensive professional tires with greater grip reliability. For such cases, manufacturers equip most of their models with an anti-lock braking system.

ABS allows you to maintain control over the car in an emergency situation, preventing the wheels from completely stopping when the brake is applied.

ABS device

The device of modern ABS includes a small number of elements. It consists of:

• Wheel rotation sensor. Such devices are installed on all wheels. The electronic control unit analyzes the parameters that come from each of these sensors. Based on the received data, the ECU independently activates / deactivates the system. Most often, such tracking devices work on the principle of a Hall sensor;
• Electronic control unit. Without it, it will not work, because it takes "brains" to collect information and activate the system. In some cars, each system has its own ECU, however, manufacturers often install one unit that processes all the elements of the active safety system (directional stability, ABS, traction control, etc.);
• Executive devices. In the classic design, these elements are a block with a set of valves, pressure accumulators, pumps, etc. Sometimes in the technical literature you can find the name hydromodulator, which is applied to these elements.

A feature of the ABS system is that it can be connected to the braking system of not even the newest car. Most often, they are a set that is simply connected to the brake line and the electrical system of the machine.

ABS working principle

Conventionally, the work of the anti-lock braking system is divided into 3 stages:

1. Wheel lock - the ECU sends a signal to activate the system;
2. Actuation of the actuator - the hydraulic block changes the pressure in the system, which leads to the unlocking of the wheels;
3. Deactivation of the system when wheel rotation is restored.

It is worth considering that the whole process is controlled by algorithms embedded in the control unit software. The reliability of the system lies in the fact that it is triggered even before the wheels lose traction. An analogue that works only on the basis of wheel rotation data would have a simpler design and principle of operation. However, such a system would work no better than the first designs of Gabriel Voisin.

For this reason, the ABS does not react to changes in wheel speed, but to the force of pressing the brake pedal. In other words, the system is triggered in advance, as if warning a possible skid, determining both the speed of rotation of the wheels and the force of pressing the pedal. The control unit calculates the possible slip and activates the actuator.

The system works according to the following principle. As soon as an emergency situation arises (the driver has sharply pressed the brake pedal, but the wheels are not locked yet), the hydromodulator receives a signal from the control unit and closes two valves (at the inlet and outlet). This stabilizes the line pressure.

The actuator then pulsates the brake fluid. In this mode, the hydraulic modulator can either provide a slow cranking of the wheel, or independently increase / decrease the brake fluid pressure. These processes depend on the modification of the system.

When the ABS is triggered, the driver will immediately feel it by the frequent pulsation, which is also transmitted to the pedal. You can find out whether the system is active or not by the caress on the activation button. The very principle of operation of the system repeats the skill of experienced motorists, only it does it much faster - about 20 times per second.

Types of anti-lock braking systems

Thanks to the improvement in active safety systems, four variants of ABS can be found in the auto parts market:

• Single channel. The signal to the control unit and back is simultaneously fed through a single wired line. Most often, front-wheel drive cars are equipped with it, and then only on the drive wheels. This system works regardless of which wheel is locked. This modification has one valve at the inlet of the hydromodulator and one at the outlet. It also uses one sensor. This modification is the most ineffective;
• Two-channel. In such modifications, the so-called on-board system is used. It controls the right side separately from the left. This modification has proved to be quite reliable, since in the event of an emergency the car is carried to the side of the road. In this case, the wheels of the right and left sides are on a different surface, therefore, the ABS must also send different signals to the actuators;
• Three-channel. This modification can be safely called a hybrid of the first and second. In such ABS, the rear brake pads are controlled by one channel, as in the first case, and the front wheels work on the principle of an onboard ABS;
• Four-channel. This is the most efficient modification to date. It has an individual sensor and a hydromodulator for each wheel. An ECU controls the rotation of each wheel for maximum traction.

Modes of operation

The operation of a modern ABS system can be carried out in three modes:

1. Injection mode. This is the standard mode, which is used in all classic varieties of the brake system. In an anti-lock braking system, the exhaust valve is closed and the intake valve is open. Due to this, when the brake pedal is pressed, fluid begins to move in the circuit, setting the brake cylinder of each wheel in motion.
2. Hold mode. In this mode, the control unit detects that one of the wheels is decelerating much faster than the others. To prevent loss of contact with the road, the ABS blocks the inlet valve of a particular wheel's line. Thanks to this, there is no force on the caliper, but at the same time the other wheels continue to slow down.
3. Pressure release mode. This mode is activated if the previous one could not cope with the resulting wheel lock. In this case, the inlet valve of the line continues to be closed, and the outlet valve, on the contrary, opens to relieve pressure in this circuit.

The effectiveness of braking when the ABS system is on depends on how effectively it changes from one mode to another. Unlike a standard braking system, with ABS on, there is no need to repeatedly apply the brakes to keep the wheels from losing traction. In this case, the driver must fully depress the brake pedal. The rest of the work will be done by the system itself.

Features of driving a car with ABS

As reliable as the braking system in a car is, it does not eliminate the need for driver attention. Anti-lock braking system has its own characteristics. If they are not taken into account, then the car may lose stability. Here are the basic rules for emergencies:

1. If the car is equipped with a simple ABS, then in order for it to activate, you need to sharply depress the brake pedal. Some modern models are equipped with a brake assistant. In this case, the control unit detects the possibility of loss of traction and activates this assistant. Even with a slight pressure on the pedal, the system is activated and independently increases the pressure in the line to the desired parameter;
2. As already mentioned, when the system is activated, the brake pedal pulsates. An inexperienced driver immediately thinks that something has happened to the car and decides to release the brake;
3. When driving on studded tires, it is better to turn off the ABS, since the studs in the tires have their effectiveness just when the wheel is blocked;
4. While driving on loose snow, sand, gravel, etc. ABS is also more useless than helpful. The fact is that a locked wheel in front of it collects a small bump from the material that makes up the road. This creates additional slip resistance. If the wheel turns, there will be no such effect;
5. Also, the ABS system may not function adequately when driving fast on uneven surfaces. Even with slight braking, a wheel in the air will quickly stop, which will provoke the control unit to activate the device when it is not required;
6. If the ABS is on, the brakes should also be used during the maneuver. In a normal car, this will only provoke a skid or understeer. However, the car with ABS is more willing to listen to the steering wheel when the anti-lock system is active.

Braking performance

The ABS system not only shortens the stopping distance, but also provides maximum control over the vehicle. Compared to a car not equipped with this system, vehicles with ABS will definitely brake more effectively. It doesn't need to be proven. In addition to a shorter braking distance in such a car, tires will wear out more evenly, since the braking forces are distributed evenly to all wheels.

This system will be especially appreciated by drivers who often drive on roads with unstable surfaces, for example, when the asphalt is wet or slippery. Although no system is able to completely eliminate all errors, protect drivers from an emergency (no one has canceled the driver’s attentiveness and foresight), ABS brakes make the vehicle more predictable and manageable.

Given the high braking performance, many experts recommend that beginners get used to driving vehicles with ABS, which will increase safety on the road. Of course, if the driver violates the rules of overtaking and speed limits, the ABS system will not be able to prevent the consequences of such violations. For example, no matter how effective the system is, it is useless if the driver has not winterized the car and continues to drive on summer tires.

ABS operation

The modern ABS system is considered to be a reliable and stable system. It can work properly for a long time, but it still needs proper operation and timely maintenance. The control unit rarely fails.

But if we take wheel rotation sensors, then this is the most vulnerable spot in such a system. The reason is that the sensor determines the speed of rotation of the wheel, which means that it must be installed in close proximity to it - on the wheel hub.

When the car is driving through mud, puddles, sand or wet snow, the sensor becomes very dirty and can either fail quickly or give incorrect values, which will lead to system instability. If the battery is low or the voltage in the on-board system of the car is low, the control unit will turn off the system due to too low voltage.

If the system fails, the car will not lose its brakes. Just in this case, the driver needs to be able to slow down on an unstable road with the help of a classic braking system.

ABS performance

So, the ABS system allows you to perform emergency braking more safely, and also makes it possible to perform maneuvers with the brake pedal fully depressed. These two important parameters make this system an integral part of a vehicle equipped with an advanced active safety system.

The presence of ABS is optional for an experienced motorist. But a beginner has to master many different skills in the first couple of years, so it is preferable that such a driver's car has several systems that provide safety net.

An experienced driver without difficulty (especially if he has been driving his car for many years) will be able to control the moment of wheel stall by changing the effort on the brake pedal. But even with a long driving experience, a multi-channel system can compete with such a skill. The reason is that the driver is not able to control the force on an individual wheel, but the ABS can (a single-channel system works like an experienced driver, changing the force on the entire brake line).

But the ABS system cannot be considered a panacea in emergency situations on any road. For example, if the car skidded on the sand or in loose snow, then, on the contrary, it will cause an increased braking distance. On such a road, on the contrary, blocking the wheels will be more useful - they burrow into the ground, which speeds up braking. In order for the car to be universal on any type of road surface, manufacturers of modern car models equip their products with a switchable ABS.

What are the faults

As for the reliability of the anti-lock braking system, this is one of the most reliable systems in the car. Its elements rarely fail, and most often this is due to violation of the rules of operation and maintenance. All electronic parts are reliably protected from overloads by fuses and relays, so the control unit will not fail.

The most common system malfunctions are wheel sensors failure, since they are located in places where it is extremely difficult to exclude water, dust or dirt from entering them. If the hub bearing is too loose, the sensors will malfunction.

Other problems are already associated more with the accompanying systems of the car. An example of this is the voltage drop in the electrical network of a machine. In this case, ABS will be disabled due to the activated relay. The same problem can be observed with power surges in the network.

If the anti-lock braking system shuts down on its own, do not panic - the car will simply behave as if it did not have ABS.

Repair and maintenance of the brake system of a car with ABS has its own characteristics. For example, before changing the brake fluid, with the ignition off, press the brake several times and release it (about 20 times). This will release the pressure in the valve body accumulator. For information on how to properly replace the brake fluid and then bleed the system, read in a separate article.

The driver will immediately learn about the ABS malfunction by the corresponding signal on the dashboard. If the warning light comes on, then goes out - you should pay attention to the contact of the wheel sensors. Most likely, due to the loss of contact, the control unit does not receive a signal from these elements, and signals a malfunction.

Advantages and disadvantages of the system

There is no need to talk much about the advantages of the anti-lock braking system, since its main advantage is in the stabilization of the car in the event of wheel slip during braking. Here are the advantages of a car with such a system:

• In the rain or on ice (slippery asphalt) the car shows great stability and controllability;
• When performing a maneuver, you can actively use the brakes for better steering response;
• On smooth surfaces, the braking distance is shorter than a car without ABS.

One of the disadvantages of the system is that it does not cope well with soft road surfaces. In this case, the braking distance will be shorter if the wheels are blocked. Although the latest ABS modifications already take into account the characteristics of the soil (the appropriate mode is selected on the transmission selector), and adapt to the given road situation.

In addition, the principle of operation of the ABS and its advantages is described in the following video:

Principles of ABS work

Related videos

At the end of the review, we offer a short video on how to brake on a car with and without ABS:

Braking with or without ABS

Questions and answers:

What does anti-lock braking system mean? It is an electronic system that prevents the wheels from locking during braking by briefly reducing the brake fluid pressure.

What is an anti-lock braking system for? If the brakes are applied sharply, the wheels may lose traction and the car will become unstable. ABS provides impulse braking, allowing the wheels to maintain traction.

How does anti-lock braking system work? Electronics monitors wheel locking and wheel slip. Thanks to the valves on each brake caliper, the TJ pressure on a particular piston is regulated.

How to brake with anti-lock braking system? In cars with ABS, you need to press the pedal all the way, and the system itself will provide impulse braking. There is no need to press / release the pedal during braking.