Anti-lock braking system
An anti-lock braking system or ABS prevents skidding. It is used in trucks, cars, vans, buses, motorcycles, and aircraft. The ABS works by maintaining tractive contact with the road surface thus preventing the wheels from locking up when the brakes are applied.
Driving a car when the road is slick or wet can be tricky especially if one is driving at a high speed when applying the brakes under these conditions without the anti-lock brake system, the car skids. ABS is an automated system that uses the principles of threshold braking and cadence braking, techniques which were once practised by skilful drivers before the ABS braking system became widespread.
The modern system of the ABS was introduced in the 1950s but its origin can be traced to the 1920s when engineers were looking for an automatic override braking system for aircraft. This formed the basis for the modern ABS as it prevents the wheels from locking up on sleek surfaces or during rapid deceleration.
It was not until the 1950s when ABS was used in motorcycles.
In the late 1960s, the first fully electronic anti-lock system was developed for the Concorde aircraft.
Components of the Anti-Lock Braking System
There are four major components of the ABS:
- Speed sensors: This is used to determine the acceleration or deceleration of the wheel. This is done by monitoring the speed of each wheel. The sensors generate a signal by using a toothed wheel and an electromagnetic coil or Hall effect sensor and a magnet. When the wheel rotates, it induces a magnetic field around the sensor. The sensors comprise a wire/coil magnet assembly that generates pulses of electricity as the teeth of the exciter passes in front of it and an exciter (a ring with V-shaped teeth).
- Valves: The valves regulate air pressure to the brakes when the ABS is in action. There is a valve in the brake line of each brake that is controlled by the ABS, the valve can take up to three positions on some systems:
- In the first position, the valve is opened, and it allows the transfer of pressure to the brake from the master cylinder.
- 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 pressure from the brake.
Clogged valves form a majority of the problems with the valve system because when a valve is clogged it cannot change position, open, or close.
- Pump: After the valves have released pressure, the pump is used to restore it to the hydraulic brakes. The controller gives a signal which releases the valve at the detection of wheel slip, the pump is used to regulate the amount of pressure supplied to the brake system after a valve releases pressure supplied from the user. After a valve releases pressure supplied from the user, the pump is used to restore the desired amount of pressure to the braking system. The controller will modulate the pump’s status to provide the desired amount of pressure and reduce slipping.
- Controller: This is an electronic control unit (ECU) that receives, amplifies and filters the sensor signals for calculating the wheel’s rotational speed and acceleration. It is a computer in the car that receives a signal if a wheel loses traction. It then limits the brake force and activates the ABS modulator which actuates the braking valves on and off.
How the Anti-Lock Braking System Works
The ABS includes at least two hydraulic valves within the brake hydraulics, a central electronic control unit (ECU) and four wheel speed sensors. The speed of each wheel is constantly monitored by the ECU. If it notices the wheel rotating considerably slower than the speed of the vehicle which is a signal of an impending wheel lock, it actuates the valves to reduce hydraulic pressure to the brake at the affected wheel, reducing the braking force on that wheel which then makes the wheel turn faster.
If the ECU senses a wheel turning significantly faster than the others, the wheel is gradually slowed down by increasing the brake hydraulic pressure to the wheel so the braking force is reapplied. The brake pedal pulsation is how the driver can detect this process, as it is repeated continuously.
A control system of hub-mounted sensors and a dedicated microcontroller are used in the modern ABS system to apply brake pressure individually to all four wheels. Most vehicles produced recently such as trucks, cars, refrigeration van come with the ABS and it is the foundation for electronic stability control systems which are fast becoming popular.
The speed sensors are being monitored at all times by the controller. When a wheel is about to lock up, it will experience quick deceleration and if left unchecked, the wheel will stop quicker than any vehicle can. The ABS controller, therefore, reduces pressure until it notices an acceleration in the brake and increases pressure until it notices a deceleration. It does this quickly before the tyre changes speed significantly; this makes the tyre slow down at the same rate as the car, with the brakes keeping the tyres near the point at which they will lock up. This gives the system maximum braking power.