Efficiency of braking the greatest, when coupling of the tire with a road surface the maximum. In the course of braking the tire slides on a surface, and speed of rotation of a wheel becomes less speed of the car.
Fig. 11.1. An arrangement of elements of anti-blocking system of brakes on cars till 2001: 1, 11 – rotors of sensors of frequency of rotation of a forward wheel; 2–diagnostic socket; 3–pressure modulator; 4–electronic control ABS block; 5–ABS relay; 6–delay sensor; 7–control lamp of ABS; 8–back sensors of frequency of rotation of back wheels; 9–rotors of sensors of frequency of rotation of back wheels; 10,12-sensors of frequency of rotation of a forward wheel
Fig. 11.2. An arrangement of elements of anti-blocking system of brakes on release cars since 2001: 1,3, 5, 7 – sensors of frequency of rotation of a wheel; 2–diagnostic socket; 4–pressure modulator; 6–delay sensor
The anti-blocking system
(fig. 11.1, 11.2)
limits the pressure created in a hydraulic drive of brakes so that the size of sliding kept about the optimum. Action of this system immediate and independent for each wheel. The system immediately answers each change of factor of coupling of a surface and car loading.
The anti-blocking system interferes with blocking of wheels at sharp braking thanks to what controllability of the car, even remains at bad coupling with a paving at a rain and snow. Force of adhesion between wheels and the road is more, when when braking a wheel continue to rotate. Sensors of frequency of rotation, on one on each wheel, measure speed of rotation of wheels. On signals from sensors of frequency of rotation the electronic control unit calculates the average speed approximately corresponding to speed of movement of the car. Comparing speed of rotation of each separate wheel with the average calculated speed, the electronic block defines a condition of a proskalzyvaniye of each wheel and establishes, what wheel is in a preblocking condition.
When frequency of rotation any of four sensors will transmit a signal about blocking of the corresponding wheel, the electronic control unit immediately gives out a signal of closing to the corresponding inlet electromagnetic valve which blocks supply of brake liquid via the pipeline to a wheel brake, and force of braking remains to a constant. If sliding proceeds, the final valve opens, and pressure in hydraulic system of this brake decreases — the wheel is not braked. Surplus of brake liquid comes back to a tank. As soon as the wheel again will start to rotate, the inlet valve opens, and the final valve is closed. Pressure increases in a contour, and the wheel again starts to be braked.
Such cycle of braking and free rotation of a wheel when braking passes several times in a second and repeats separately for each wheel until the pedal of a brake or before reduction of speed of the car to 2–3 km/h will be released.
The system of an emergency shutdown provides ABS shutdown at any malfunction or at a low voltage in an onboard network of the car (below 10 In). Malfunction of ABS does not render influence on work of brakes, they function the same as if this system was not in the car.