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Antilock Brake System

Ongoing advance in car brake systems have resulted in reliable systems contributing to increased car safety.

In normal driving conditions, conventional brake systems (CBS) can provide effective and fast stopping for the vehicle. However, in the following driving circumstances, braking can result in locking of wheels which in turn leads to lack of steerability and slipping of the vehicle:


     1-      Slippery and wet roads


     2-      Frightened reaction of the driver


     3-      Errors of other drivers and pedestrians


The above are among the issues for addressing of which the Antilock Brake Systems have been invented.


History of ABS systems


The first instance of ABS system was designed for trains in 1900. After the Second World War, ABS systems were used for Jet planes. In 1960s, as brake systems of automotives were being revolutionized, brake manufacturers started to develop and use ABS systems for luxury cars such as Mercedes Benz.


Operational principles


When brake pedal is pressed with excessive force, there is the possibility that tires decelerate with a higher rate than the car itself. This leads to slipping of tires on the road surface. A way to prevent this is to prevent locking of brakes when the threshold of slipping is detected. This is exactly what the ABS does. At normal braking, ABS has no interference in the operation of the brake system. However, when brakes are applied with excessive force, ABS allows brakes to operate only until the threshold of locking of wheels. When possibility of slipping is detected for one wheel, ABS removes brakes pressure for that wheel. When this risk is removed, the brake fluid is pumped to that wheel again and this cycle is repeated so that deceleration is accomplished without slipping.

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Brake pipes containing hydraulic brake fluid coming from the two master cylinders are connected to the Hydraulic Modulator. This part is in turn connected via four brake pipes to all four wheels. Wheel speed sensors monitor the speed of all four wheels sending their signals to the Electronic Control Unit of the Hydraulic Modulator. The brake light switch also informs the ABS system that brake pedal has been pressed. When locking threshold is detected upon comparing the speed of the vehicle with the speed of each of the four wheels (with rapid drop of wheel speed), one or two solenoid valves are opened/closed repeatedly to sequentially apply and remove the brake force on the wheel which is on the verge of locking. This results in increased stopping distance, avoiding spinning of the vehicle on surfaces with different friction modules for left and right wheels, and maintaining the steerability of the vehicle at times of emergency braking.


Translated by: Ehsan Taghdiri


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