Efficiency of braking the greatest, when coupling of the tyre with a surface of road the maximum. In the course of braking the tyre slides on a surface, and speed of rotation of a wheel becomes less speeds of the car.
Fig. 11.1. An arrangement of elements of antiblocking system of brakes on cars till 2001: 1, 11 – rotors of gauges of frequency of rotation of a forward wheel; 2 – a diagnostic socket; 3 – the pressure modulator; 4 – electronic control block ABS; 5 – relay ABS; 6 – the delay gauge; 7 – control lamp АBS; 8 – back gauges of frequency of rotation of back wheels; 9 – rotors of gauges of frequency of rotation of back wheels; 10,12 – gauges of frequency of rotation of a forward wheel
Fig. 11.2. An arrangement of elements of antiblocking system of brakes on cars of release since 2001: 1,3, 5, 7 – gauges of frequency of rotation of a wheel; 2 – a diagnostic socket; 4 – the pressure modulator; 6 – the delay gauge
The antiblocking system (fig. 11.1
) limits the pressure created in a hydraulic drive of brakes so that the sliding size was 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 antiblocking system interferes with blocking of wheels at sharp braking thanks to what controllability of the car, even remains at bad coupling with a road covering at a rain and snow. Cohesive force between wheels and road is more, when at wheel braking continue to rotate. Gauges of frequency of rotation, on one on each wheel, measure speed of rotation of wheels. On signals from gauges of frequency of rotation the electronic block of management calculates average speed, about 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 проскальзывания each wheel and establishes, what wheel is in a preblocking condition.
When frequency of rotation any of four gauges will transmit a signal about blocking of a corresponding wheel, the electronic block of management immediately gives out a closing signal to corresponding inlet electromagnetic valve which blocks giving of a brake liquid through 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 the given brake decreases — the wheel is not braked. Surplus of a brake liquid comes back in 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 at braking passes some times for 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 emergency switching-off provides switching-off АBS at any malfunction or at a low voltage in an onboard network of the car (more low 10). Malfunction АBS does not render influence on work of brakes, they function the same as if this system was not in the car.