Anti-Lock Braking System

Operating principle

Braking efficiency is greatest when the grip of the tire with the road surface is maximum. During braking, the tire slides over the surface and the circumferential speed of the wheel becomes less than the speed of the car.

Slip minimum (0%), when the wheel rolls freely, and the maximum (100%), when the wheel is locked. Maximum braking performance is achieved when the slip is 15%. This value also allows you to maintain the stability and controllability of the car.

The role of the anti-lock braking system is to limit the pressure generated in the hydraulic brake system so that the amount of slip is kept near the optimum amount (15 %). 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.

Design

Anti-lock braking system (ABS) Teves contains four wheel speed sensors, a hydraulic control unit and an electronic control unit.

Four sensors (at each wheel of the car) transmit signals to the electronic control device informing about the speed of rotation of each wheel of the vehicle.

Electronic control device located on the right side under the front panel, based on the received information about the unequal speed of rotation of individual wheels (or about their blocking) sends a command to reduce the braking force to the hydraulic control unit (pressure applied to the brake caliper) wheel that has begun to experience excessive slip. The electronic control device also performs an auto-diagnostic function, warning the driver by illuminating a control lamp about the registration of a malfunction of electrical circuits or elements of the anti-lock braking system. If the ABS system is faulty, the brakes are still serviceable and function as if the system were not in the car. The only control light warns the driver that the ABS system does not correct his errors.


The hydraulic control unit comprises a tandem brake master cylinder that controls the front brakes and a hydraulic auxiliary that controls the rear brakes. The hydraulic auxiliary device contains six solenoid valves controlled by an electronic control device that regulates the pressure in the brakes of the individual wheels, and therefore the braking force. Three intake solenoid valves during normal braking (no slip) remain open and the three exhaust valves remain closed. They are located in the circuits of the right front brake, left front brake and rear brakes.

Functioning

When one of the four speed sensors transmits a lock signal to the corresponding wheel, immediately the electronic control device issues a close signal to the corresponding inlet solenoid valve, which immediately shuts off the flow of brake fluid through the brake pipeline to the brake of that wheel - the braking force remains constant. If the sliding still continues, then the release valve opens and the pressure in the hydraulic system of this brake decreases - the wheel does not brake (excess brake fluid returns to the reservoir). As soon as the wheel begins to rotate again, the intake valve immediately 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 car stops or until the brake pedal is released.

Anti-lock braking system (ABS) valid at speeds over 7 km/h.