Purpose and operation of the anti-lock braking system

7.4.2 Anti-lock braking system operation

The system operates as follows: the control module constantly receives signals from the speed sensors of each wheel. As soon as an emergency situation arises and the driver presses the brake pedal sharply, the wheel or wheels will start to be locked. A signal will be sent from the corresponding sensor or sensors to the electro-hydraulic module. This module will open the required valve or valves, which will reduce the pressure in a particular line or lines, and the wheel or wheels are unlocked. At the same time coupling of wheels with road, and consequently, and controllability at braking remain. But it is impossible to keep the wheel decelerated indefinitely. Thus, in the hydraulic module the valves close again, the pressure increases until the wheel starts to lock, and the process is repeated again.

Note
In case if your car is equipped with an anti-lock braking system, you will feel a slight (and sometimes not very) tremor when you press the brake pedal abruptly in case of an emergency. Under no circumstances you shall release the brake pedal in such a situation. On the contrary, you shall try to push it even more or at least leave it in this position.

Note
The anti-lock braking system, first of all, is called to keep control over driving. Not all surfaces the anti-lock braking system will shorten the braking distance during its operation. In most cases, the braking distance may increase slightly. But most importantly, the driver will be able to maneuver, and avoid the collision.

The anti-lock braking system gave a powerful impulse to the automotive industry, because the active safety systems, that started coping in situations involving the demolition, skidding of the car, control of its stability were invented on its basis (this will be discussed below). In such systems, the basis is the same as in the anti-lock braking system, only the number of sensors was increased.

7.4.3 Brake Assist (emergency braking system)

Numerous studies have shown that in case of emergency braking, many drivers do not press the brake pedal all the way. This does not create enough pressure in the hydraulic drive for the most effective deceleration. Designers came up with and implemented a system of "braking" - Brake Assist – in order to eliminate this unpleasant moment. This system recognizes the moment of emergency braking and in case if it is necessary it develops the maximum possible pressure in mains of a hydraulic drive of brakes.

Note
In case of emergency braking, the pressure in the brake drive system automatically increases significantly within a few milliseconds. This means that the precious time for the operation of the brakes is reduced in situations where everyone decides the moments. In such situations 2 meters of the braking distance decide whether there will be a tragedy or everything will be fine.

7.4.4 EBV (electronic brake force distribution)

The description of the brake force regulator was provided above. But this was a description of an exclusively mechanical controller, which changes the force in the brakes of only the rear wheels. While considering the expansion of features of the anti-lock braking system, the designers concluded that the elements of this system can be used for distributing the braking force directly to each wheel and controlling it according to the traffic conditions.

The operation of this system is that the braking force is transmitted to each wheel individually and strictly dosed. Each wheel brakes individually. All the sensors and the microprocessor on many current parameters estimate behavior of the car in real time, preventing drifts and providing the greatest efficiency of braking.

Note
This system appeared on cars with the fifth generation anti-lock braking system.

7.4.5 ESP (vehicle stability system)

Let’s consider the following situation: in mid-November, you went on a long trip. The road is great as a mirror, but in the blink of an eye your car suddenly falls off the trajectory, it is thrown from side to side. The active rotation of the steering wheel does not provide the desired result, but everything works out, the consequences are expressed by trembling hands due to a surge of adrenaline and shaky nerves. The reason for such an unpredictable car behavior is ice. During the movement, the wheels on the right side hit a thin imperceptible layer of ice. The car differential reacted instantly, "throwing" the moment on the wheel, which lost traction. There was a turning moment and the car started actively "fishtailing" on the road.

The story described above ended without any consequences. But this, unfortunately, is not always the case in real life. In order to eliminate and/or minimize all the factors that influence the change in the trajectory of the car, the designers created and implemented a car stabilization system - ESP. 

This system allows one to virtually eliminate such a dangerous maneuver of the car as skidding (which in most cases cannot be controlled). This system uses all the elements of the anti-lock braking system and additionally has its "own" sensors: the sensor of lateral accelerations, the sensor of car fishtailing, an angle of turn of wheels, etc.

While collecting data from all sensors, in case of a car skid or loss of wheel traction due to weather conditions, the system "decides" which of the wheels shall brake in order to compensate for the torque, and if necessary, reduces engine speed.

Example
In case if the car when passing the turn to the right at high speed front wheels start "carrying away" from the set trajectory in the direction of action of centrifugal forces of inertia, i.e. on radius bigger, then the radius of turn. The ESP system in this case will start braking the rear wheel, which goes on internal radius of turn. This provides the car with big turning ability and directs it, as if screwing it in turn. At the same time in case of the increased danger ESP system will reduce engine turns. In case if the rear side of the vehicle skids while turning, ESP system activates the left front wheel brake on the outer turning radius.

ESP is currently the most effective active safety system; thus most cars do not have the ability to disable it.

Note
The stabilization system has different names for different car manufacturers. Below is a list of names of this system by different companies:
- ESP (Electronic Stability Program) system on most cars in Europe and the USA;
- ESC (Electronic Stability Control) system on Honda, Kia, Hyundai cars;
- DSC (Dynamic Stability Control) system on BMW, Jaguar, Rover cars;
- DTSC (Dynamic Stability Traction Control) system on Volvo cars;
- VSA (Vehicle Stability Assist) system on Honda and Acura cars;
- VSC (Vehicle Stability Control) system on Toyota cars;
- VDC (Vehicle Dynamic Control) system on Infiniti, Nissan, Subaru cars.

7.4.6 ASR (traction control)

This system works in tandem with the anti-lock braking system. As soon as the speed sensors of the wheels detect the slippage of the drive wheels, the anti-slip regulation system automatically reduces the engine speed. In certain cases, it slows down those drive wheels that start slipping (from one to all four). In this mode, the electronics provide the maximum possible acceleration of the car under certain traffic conditions.

Note
The name ASR is used by Audi Company. Cars of other brands may use other names and abbreviations. However, regardless of the name, the system works in a similar way.