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The System

How do wheels lock?

During braking, wheels lock if the brake force applied is more than the friction between the road and tyre. This often happens in a panic braking situation, especially on slippery roads. When the front wheels lock, the vehicle slides in the direction of motion and the vehicle loses steerability. When the rear wheels lock, the vehicle loses stability and often swings around. It is impossible to steer around an obstacle with wheels locked. Locked wheels can thus result in accidents. Skidding also results in flat spots on tyre and reduces tyre life. 

What does ABS do?

 The system detects an impending wheel lock situation and momentarily releases the brake pressure on the locking wheel. The brakes are reapplied as soon as the wheels have recovered.

       A toothed wheel (polewheel) is fitted to the rotating wheel hub. A magnetic sensor mounted on each wheel in close proximity to the polewheel, generates electrical pulses when the wheel rotates. The rate at which the pulses are generated (frequency) is a measure of the wheel speed. This signal is read and processed by the Electronic Control Unit (ECU). When a wheel is about to lock, the ECU sends an electrical signal to the modulator valve solenoid, which releases pressure from the brake chamber. When the wheel recovers sufficiently, the brake pressure is reapplied by the ECU by switching off the signal to the modulator valve.

The modulator valve has an additional ‘hold’ state which maintains pressure in the brake chamber, thus optimizing the braking process. The cycling of the modulator valves (5 to 6 times per second) is continued till the vehicle comes to a controlled stop.

       With ABS, the vehicle remains completely stable even when the driver continues to press the brake pedal during panic brake situations. The vehicle also remains steerable during braking, thus avoiding accidents.