The anti-lock braking system limits the pressure generated in the hydraulic brake actuator so that the amount of slip is kept near the optimum. The action of this system must be immediate and separate for each wheel. The system must immediately respond to every surface change (adhesion coefficient) and vehicle loads.
The anti-lock braking system prevents the wheels from locking during heavy braking, thereby reducing the braking distance, especially in case of poor traction in rain or snow. The grip force between the wheels and the road in this case is greater when the wheels continue to rotate during braking. In addition, even with full braking, the car remains steerable. RPM sensors, one for each wheel, measure wheel speed. Based on the signals from the wheel speed sensors, the electronic control unit calculates an average speed that approximately corresponds to the speed of the vehicle. By comparing the rotation speed of each individual wheel with the average calculated speed, the electronic unit determines the slip state of the individual wheel and thereby determines which wheel is in the pre-lock state.
When one of the four speed sensors sends a lock signal to the respective wheel, the ECM immediately issues a close signal to the respective inlet solenoid valve, which immediately shuts off the brake fluid supply through the line to the wheel brake - braking force remains constant. If the sliding continues, the release valve opens, the pressure in the hydraulic system of this brake decreases - the wheel does not brake (excess brake fluid is returned to the reservoir). Once the wheel starts spinning again, the intake valve opens and the exhaust valve closes. The pressure in the circuit increases and the wheel is braked again.
This cycle of braking and free spinning of the wheel is very fast (several times per second) and continues until the vehicle stops or the brake pedal is released.
The process is repeated during hard braking separately for each wheel until the brake pedal is released or when the vehicle speed decreases to 2–3 km/h.
The emergency shutdown system ensures that the ABS is turned off in case of any malfunction or low voltage in the car's on-board network (below 10 V). The electronic control unit also performs a diagnostic function, warning the driver by lighting up a warning lamp about a malfunction in the electrical circuits or elements of the anti-lock braking system. ABS malfunction does not affect the operation of the brakes, they function as if this system is not in the car.
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