Principle of ABS Control

PRINCIPLE OF ABS CONTROL




When the brake pedal is depressed during driving, the wheel speed decreases and the vehicle speed does as well. The decrease in the vehicle speed, however, is not always proportional to the decrease in the wheel speed. The non-correspondence between the wheel speed and vehicle speed is called "slip" and the magnitude of the slip is expressed by the "slip ratio" which is defined as follows:

Slip ratio = (Vehicle speed - Wheel speed)/Vehicle speed * 100%

When the slip ratio is 0%, the vehicle speed corresponds exactly to the wheel speed. When it is 100%, the wheels are completely locking while the vehicle is moving.
The braking effectiveness is represented by the "coefficient of friction" between the tire and road surface. The larger the coefficient, the higher the braking effectiveness. The diagram shows the relationship between the coefficient of friction and the slip ratio for two different road surface conditions (asphalt-paved road and icy road), assuming that the same tires are used for both the conditions and the vehicles are moving forward. Although the braking effectiveness (coefficient of friction) depends on the road surface condition as shown and also on the type of the tire, its peak range generally corresponds to the 8 - 30% range of the slip ratio.
The ABS controls the fluid pressure to each wheel to maintain the slip ratio within this range.