Anti lock Braking System (ABS)

What is ABS?

ABS is the short form for Anti lock Braking System which is a safety device installed in vehicles.

Why is ABS used?

ABS is used to prevent wheel locking while applying brakes to the wheels. In certain scenarios owing to the surrounding conditions (such as wet roads, dry surface, snowy surface, loose gravel, etc.), wheels might skid when brakes are applied suddenly.

In other words, wheels might get locked which increases the stopping distance of the vehicle and also defies the driver the control on the steering, leading to fatal injuries and in certain cases can lead to death. ABS helps in maintaining the traction between the wheels and the surface, thereby preventing the wheels from locking. It also allows the driver to steer the vehicle and avoid obstacles.

When was ABS introduced?

ABS was developed in the year 1929 by Gabriel Voisin, a French national who designed it only for aircraft. It was in the year 1958 that ABS was experimented on a Royal Enfield Super Meteor Motorcycle and it greatly reduced the stopping distance of the motorcycle. The system was however not used for commercial purpose.

Later in the year 1970, Ford added an electronic ABS system called 'sure-track' on the rear wheels of the Lincoln Continental model as optional for the customers. A year later, Ford made it standard in Lincoln models. Chrysler, along with the Bendix corporation, developed a computerized three channel, four sensor all wheels ABS called 'Sure Brake' for its Imperial model launched in 1971.

In the year 1978, Mercedes Benz introduced the second generation ABS with the help of Bosch. Mercedes Benz was the first company to introduce commercial cars with ABS and also ensured large scale production of ABS. This was all possible due to the revolution in electronics. Integrated circuits and computers could be built which with the help of computer codes could detect the speed of the wheels and actuating the valves in no time and made the process simpler and accurate.

ABS Components:

ABS has 4 main components:

1.Speed sensors: ABS has to know when the wheel is about to lock up. So sensors are located over the wheels to provide this information.

2. Valves: Valves are located in the brake line of each brake which can be controlled by the ABS. Valve has 3 positions in some systems:

• In position one, the valve is open and the pressure from the master cylinder is passed right through it to the brake.
• In position two, the valve is closed, thereby blocking the pressure from the master cylinder to the brake.
• In position three, the valve releases some amount of pressure from the brake. The controller signals the valve to release the pressure once the sensor senses the wheel is about to slip.

3. Pump: Pump is used to restore the pressure that was released from the brakes. The controller will control the pump and makes it pump the required amount of pressure to the brake.

4. Controller: Controller is the brain of the ABS, also called the Electronic Control Unit (ECU) which receives the signals from the wheel speed sensors and modulates the ABS system by controlling the braking valves. If the vehicle is about to slip, controller will release the pressure from the brakes through the valve.

ABS Working:

The controller (ECU) continuously monitors the speed of each wheel with the help of speed sensors. If the ECU detects that one wheel is rotating slower (for a 4 wheeled vehicle) than the other wheels, then it actuates the valve to reduce the braking pressure on that particular wheel so that it can start rotating faster and match the speed of the other wheels.

Conversely, for a wheel rotating faster than the other wheels, the controller actuates the valve to increase the braking pressure on that particular wheel to slow it down and match other wheels' speed.

The driver can feel the ABS working through his or her foot through the brake pedals which will be pulsating. In some cases, ABS can release or apply the brake pressure 15 times per second.

Types of ABS:

ABS can be classified into various types based on the number of speed sensors and valves:

1.        Three channel, three sensor ABS: This system has a speed sensor and a valve for each of the front wheels, and a common speed sensor and a valve for both the rear wheels. For rear wheels, it is located on the rear axle. This system provides maximum efficiency for the front wheels. In the case of rear wheels, both the wheels have to lock up for the ABS to function. There is always a possibility of one of the rear wheels to lock up, reducing the effectiveness of ABS.

2.        One channel, one sensor ABS: This system is commonly used only for rear wheels which will have a common valve and a speed sensor. Both the rear wheels have to lock up so that the ABS can work. This is a rather ineffective system.

3.        Three Channel, four sensor ABS: This system employs separate speed sensors for each wheel and separate valves for each of the front wheels. A common valve is used for the rear wheels. If the controller detects lock up of one of the rear wheels, then brake pressure will be applied on both the rear wheels.

4.        Four channel, four sensor ABS: Separate speed sensors and valves are used for each wheel. This system provides the maximum brake effectiveness.

5.        Two channel, two sensor ABS: Separate speed sensors are used on each wheel and separate valves are used for both front and rear axle. If there is a wheel lock up on any wheel (say front left wheel), then braking pressure will be applied to both the front wheels.