21.2. Anti-blocking system of brakes (ABS)
Efficiency of braking the greatest, 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 kept about optimum. Action of this system has to be immediate and 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, especially at bad coupling with a paving at a rain or snow. Force of adhesion between wheels and the road in this case is more when when braking a wheel continue rotation. In addition, even at full braking the car remains operated. Rotation frequency sensors, on one on each wheel, measure the speed of rotation of a wheel. On signals from sensors of frequency of rotation of a wheel 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 frequency of rotation transmits a signal of blocking of the corresponding wheel, the electronic control unit immediately gives a closing signal to the corresponding inlet electromagnetic valve which immediately blocks supply of brake fluid via the pipeline to a wheel brake – force of braking remains to a constant. If sliding continues, then the final valve opens, pressure in hydraulic system of this brake decreases – the wheel is not braked (surplus of brake fluid comes back to a tank). As soon as the wheel starts over again rotating, 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 (several times in a second) and proceeds to a stop of the car or to a brake pedal otpuskaniye.
Process repeats at sharp braking separately for each wheel until the pedal of a brake is released or at reduction of speed of the car to 2–3 km/h.
The system of emergency shutdown provides shutdown of ABS at any malfunction or at low voltage in onboard network of the car (lower than 10 V). The electronic control unit performs also diagnostic function, warning the driver fire of a control lamp about registration of malfunction in electric chains or elements of anti-blocking system. Malfunction of ABS does not exert impact on work of brakes, they function as if this system is not in the car.