6.1.6. Anti-blocking system of brakes (ABS)
Braking is most effective when coupling of the tire with the surface of the road maximum. In the course of braking the tire slides on a surface, and the district speed of a wheel becomes less than the speed of the car.
The anti-blocking system limits pressure created in the hydraulic drive of brakes so that the size of sliding was optimum. Action of this system has to be separate for each wheel. The system has to answer immediately each change of a surface (coupling coefficient) and load of the car.
The anti-blocking system interferes with blocking of wheels at sharp braking thanks to what the brake way decreases. Force of adhesion between wheels and the road in this case is more if when braking wheels continue to rotate. Even at full braking the car remains operated. Rotation frequency sensors, on one on each wheel, measure the frequency of rotation of a wheel. On signals of sensors the electronic control unit calculates the average speed which is approximately corresponding to the speed of the movement of the car. Comparing the speed of rotation of each separate wheel to the average calculated speed, the electronic block defines a condition of slipping of a separate wheel and by that, establishes what wheel is in a preblocking state.
When one of four sensors of speed of wheels transmits a signal of blocking of the corresponding wheel, the electronic actuation device immediately gives a closing signal to the corresponding inlet electromagnetic valve which blocks supply of brake fluid via the pipeline to a wheel brake. At the same time force of braking remains to a constant. If sliding continues, then the final valve and pressure in hydraulic system of this brake opens decreases. The wheel does not brake, surplus of brake fluid comes back to a tank. As soon as the wheel starts over again rotating, the inlet valve opens, and final – is closed. Pressure increases in a contour, and the wheel brakes again.
Change of cycles of braking and free rotation of a wheel happens very quickly (several times in a second) and continues to a stop of the car or to a brake pedal otpuskaniye.
Process repeats at sharp braking separately for each wheel until the brake pedal is released.
The emergency system provides shutdown of ABS at any malfunction or low voltage in onboard network of the car (lower than 10 V). Malfunction of ABS does not influence work of brakes.
The hydraulic drive consists of the hydraulic block, supports of brakes and brake tubes. Enter the hydraulic block the electric pump creating pressure, and electromagnetic valves.