The anti-lock braking system limits the pressure generated in the hydraulic brake drive 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 respond immediately to each change in surface (coefficient of adhesion) and the load on the vehicle.
The anti-lock braking system prevents the wheels from locking during sudden braking, thereby reducing the braking distance, especially when there is poor traction with the road surface in rain or snow. The force of adhesion 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 controllable. Speed sensors, one on each wheel, measure the speed of rotation of the wheel. Based on signals from the speed sensors, the electronic control unit calculates the average speed, which approximately corresponds to the speed of the car. By comparing the speed of rotation of each individual wheel with the average calculated speed, the electronic unit determines the slippage state of an individual wheel and thus determines which wheel is in a pre-lock state.
When one of the four speed sensors transmits a signal about the locking of the corresponding wheel, the electronic control unit immediately issues a closing signal to the corresponding inlet solenoid valve, which immediately shuts off the supply of brake fluid through the pipeline to the wheel brake - the braking force remains constant. If the slip continues, the outlet valve opens, the pressure in the hydraulic system of this brake decreases - the wheel is not braked (excess brake fluid returns to the reservoir). As soon as the wheel starts to rotate again, the inlet valve opens and the outlet valve closes. The pressure in the circuit increases and the wheel is braked again.
This cycle of braking and free wheel rotation occurs very quickly (several times per second) and continues until the vehicle stops or until the brake pedal is released.
The process is repeated during sudden 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 ABS is switched off in the event of any malfunction or low voltage in the vehicle's on-board network (below 10 V). The electronic control unit also performs a diagnostic function, warning the driver by lighting a control lamp about the registration of a malfunction in the electrical circuits or elements of the anti-lock system. An ABS malfunction does not affect the operation of the brakes; they function as if this system were not in the vehicle.
[The original material is located on the website AudiManual]