21.2. Antiblocking system of brakes (ABS)
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 district speed of a wheel becomes less speeds of the car.
The antiblocking system limits the pressure created in a hydraulic drive of brakes so that size of sliding was kept about the optimum. Action of this system should be immediate and separate for each wheel. The system should answer immediately each change of a surface (coupling factor) and loading on the car.
The antiblocking system interferes with blocking of wheels at sharp braking thanks to what the brake way decreases, especially at bad coupling with a road covering at a rain or snow. Cohesive force between wheels and road in this case is more, when at wheel braking continue rotation. Besides it, even at full braking the car remains operated. Gauges of frequency of rotation, on one on each wheel, measure speed of rotation of a wheel. On signals from gauges of frequency of rotation of a wheel 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 a separate wheel and by that establishes, what wheel is in a preblocking condition.
When one of four gauges of frequency of rotation 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 immediately blocks giving of a brake liquid through the pipeline to a wheel brake – force of braking remains to a constant. If sliding proceeds, the final valve opens, pressure in hydraulic system of the given brake decreases – the wheel is not braked (surpluses of a brake liquid come 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 is again braked.
Such cycle of braking and free rotation of a wheel occurs very quickly (some times in a second) and proceeds to a stop of the car or to release brake pedals.
Process repeats at sharp braking separately for each wheel until the pedal of a brake will be released or at reduction of speed of the car to 2–3 km/h.
The system of emergency switching-off provides switching-off ABS at any malfunction or at a low voltage in an onboard network of the car (more low 10). The electronic block of management carries out also diagnostic function, warning the driver fire of a control lamp about malfunction registration in electric chains or elements of antiblocking system. Malfunction ABS does not render influence on work of brakes, they function as if this system is not present in the car.