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Anti-lock braking systems (ABS) have been with us for longer than you might think. Developed in 1929 for use on aircraft, motorists first experienced the benefits of ABS in the Jensen Ferguson Formula, a four-wheel-drive car unveiled in 1966. Further development was slow and most motorists had to wait until the mid-1980s to benefit, most notably when ABS was fitted as standard to the Ford Scorpio. ABS is now fitted to just about every new car and is used to help stability while cornering and as a crude aid to traction too as engineers start to think laterally about its benefits in situations other than braking.

How it works

An anti-lock system automatically applies a form of cadence braking by detecting when a wheel is about to lock, releasing the brake at that wheel and then immediately reapplying it. The system, therefore, needs three main parts: a means of telling when a wheel is about to lock; a means of releasing its brake; and a means of restoring the pressure to the brake line after release.

The basic theory behind anti-lock brakes is simple. It prevents the wheels from locking up, thus avoiding uncontrolled skidding. ABS generally offers improved vehicle control and decreases stopping distances on dry and slippery surfaces. With ABS, you get better stability and control over a car while braking .A skidding wheel (where the tyre contact patch is sliding relative to the road) has less traction (grip of the tyre on the road) than a non-skidding wheel. For example, if your car drives over a road covered in ice, it is unable to move forward and the wheels will keep spinning, since no traction is present. This is because the contact point of the wheel is sliding relative to the ice.

ABS modifies the brake fluid pressure, independent of the amount of pressure being applied on the brakes, to bring the speed of the wheel back to the minimum slip level that is mandatory for optimal braking performance.

ABS has four major components.

Speed Sensor

This sensor monitors the speed of each wheel and determines the necessary acceleration and deceleration of the wheels. It consists of an exciter (a ring with V-shaped teeth) and a wire coil/magnet assembly, which generates the pulses of electricity as the teeth of the exciter pass in front of it.


The valves regulate the air pressure to the brakes during the ABS action. There is a valve in the brake line of each brake that is controlled by the ABS.  In the first position, the brake valve is open and it allows the pressure from the master cylinder to be transferred to the brakes. In the second position, the brake valve remains closed and pressure from the master cylinder to the brakes is constrained.  In the third position, the valve releases some of the pressure on the brakes.

The third step is repeated until the car comes to a halt. The resistance that you feel when braking suddenly at high speeds is actually the brake valves controlling the pressure that is being transferred to the brakes from the master cylinder.

Electronic Control Unit (ECU)

The ECU is an electronic control unit that receives, amplifies and filters the sensor signals for calculating the wheel rotational speed and acceleration. The ECU receives a signal from the sensors in the circuit and controls the brake pressure, according to the data that is analyzed by the unit.

Hydraulic Control Unit

The Hydraulic Control Unit receives signals from the ECU to apply or release the brakes under the anti-lock conditions. The Hydraulic Control Unit controls the brakes by increasing the hydraulic pressure or bypassing the pedal force to reduce the braking power.

ABS in operation

While braking, if a wheel-locking situation is detected or anticipated, the ECU alerts the HCU by sending a current and commands it to release the brake pressure, allowing the wheel velocity to increase and the wheel slip to decrease. When the wheel velocity increases, the ECU reapplies the brake pressure and restricts the wheel slip to a certain degree (Note: When the braking action is initiated, a slippage between the tyre and the road surface in contact will occur, which makes the speed of the vehicle different from that of the tyre). The Hydraulic Control Unit controls the brake pressure in each wheel cylinder based on the inputs from the system sensor. As a result, this controls the wheel speed. This process is repeated for the next braking operation.

ABS is classified based on the number of sensors and the types of brakes used.  Brakes can also be differentiated by the number of channels, i.e how many valves are individually controlled and the number of speed sensors.

ABS is considered one of the most important safety features in cars. Current research shows that cars equipped with ABS are far less likely to be involved in multi-car accidents, because they still have access to steering capabilities. ABS has completely revolutionized the automobile industry.

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