In early days motor vehicles, a solid linkage system was used to transmit foot pressure to brake shoes. Later on, this mechanical system was replaced by a hydraulic system consist with a master pump, wheel cylinders, brake lines etc.
Next development was to introduce power assisted hydraulic brakes in which efficient braking was possible with less foot pressure. In this case, the hydraulic master pump was assisted by a servo unit which is a cylinder containing compressed or vacuum air. Incoming or outgoing air flow was controlled by valves with less effort and the corresponding diaphragm motion was transmitted to the master pump.
A moving vehicle gains kinetic energy due to its motion which will be converted into another form of energy when braking. The energy conversion can be taken place in many ways such as,
1. Absorption by the vehicle suspension system.
2. Heat energy in brake shoes, drums, pads and discs.
3. Stresses in body parts.
It is understandable that the more the number of brake applications, the more the energy conversion and the more the braking effect.
When the momentum of the moving vehicle exceeds the friction power(F.P.) between tires and the road, slipping starts and no matter whether the wheels are locked or not. The more the vehicle speed, the more the F.P. needed to stop the vehicle. A single braking stroke may be insufficient to convert the total momentum of the moving vehicle. More braking strokes provide more energy conversions. Hence, more efficient braking effect can be obtained when more braking strokes occur within a short time period. This is the principle behind ABS brakes.
Now a days, almost all the modern vehicles are fitted with ABS brakes. ABS stands for anti-locking brakes system which is employed to support conventional hydraulic servo assisted brakes system, in order to overcome its inherent shortcomings.
When brakes are applied on a slippery surface which may be wet or sandy, the wheels get locked and goes out of driver's control. This type of wheel lock is a potential danger to the vehicle and it is a key shortcoming of the conventional hydraulic braking system.
In order to stop a vehicle with good steering control, all the wheels should rotate until the vehicle speed becomes zero, i.e. no wheel should be slipping. For the braking to be stable and safe, brakes has to be applied only to reduce the rotating speed of wheels.
The conventional hydraulic brakes system is designed to apply equal braking effect to all four wheels. When braking, those wheels which are on slippery surfaces may get locked and start slipping while others remain rotating. Applying more braking effect to rotating wheels and no brakes to locked wheels is impossible in conventional hydraulic brakes system. Therefore the need of the ABS brakes system was arisen.
In addition to the conventional hydraulic brakes system ABS brakes system consists of ABS computer, speed sensors for each wheel, brake actuator etc. ABS computer receives wheel speeds from each wheel speed sensor. Vehicle speed is taken from the engine sensor. If the ABS computer get to know that the wheel speed is zero when vehicle speed is high it understands that the respective wheel is slipping. Then the computer immediately sends signals to hydraulic modulator to actuate solenoid valves to reduce hydraulic pressure to respective wheel to reduce brakes, without the consent of the driver. Within mili-seconds again ABS system applies brakes. This cycle continues until brake pedal remains depressed and the corresponding ABS activation pulsation can be felt to the driver. However, the system manages to maintain slip at a minimum. Scientific experiments have proved that ABS brakes helps to have a more stable, controlled, safe and efficient stop.
As a safety measure, ABS brakes system is designed to turn it into conventional hydraulic brakes system whenever the ABS system goes inoperative.