Anti-Lock Braking System (ABS)

Introduction

Assume that you are driving on a moderately busy highway in rain. You are in your own lane travelling at safe speed. Suddenly a car overtakes with a sharp turn and you steps on the brake paddle. What would happen next? There is chances of sliding and crashing into other lanes with other vehicles. You might lose control once your steering wheel is locked.

Such conditions might get worse and cause great losses of vehicles and in worst case even death.

To prevent this many attempts were made and in mid 1980s ABS was introduced that bring many changes and prevents from many accidents in such possible events.

ABS stands for Anti-lock Braking System. It was designed to help the driver maintain better steering control and avoid skidding while braking. Modern automobile is equipped with advanced ABS. This promotes directional stability and allows steering while maximizing braking by reducing stoppage distance. ABS allows driver to maintain control of vehicle. Since ABS allows driver to steer the vehicle and still maintain braking.  ABS operates by preventing the wheels from locking up during uncontrolled braking, thereby maintaining tractive contact with the road surface. On slippery surfaces, professional driver cannot stop the vehicle as quickly at safe distance without ABS as an average driver can with ABS. ABS operates at a very much faster rate and more effectively than most drivers could manage. On varieties of its application and capabilities, it is also known variously as Electronic Brake Force Distribution (EBD), Traction control system, emergency brake assist or electronic stability control (ESC).

Working Principle

It is simple to understand the working principle of ABS. A skidding wheel has less traction force than a non skidding wheel. If driver is stuck on a slippery surface like in rain or on ice, driver should that if the wheels are spinning then it has no traction. This is because the contact patch is sliding relative to the surface. 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.

Components of ABS

ABS has four major components:

a.       Speed Sensor

The speed sensor uses a magnet and a Hall effect sensor or a toothed wheel and an electromagnetic coil to generate a signal. The speed sensors, which are located at each wheel, or in some cases in the differential,  provide the information to ABS when a wheel is about to lock up. This sensor monitors the speed of each wheel and determines the necessary acceleration and deceleration of the wheels.

b.      Valves

There is a valve in the brake line of each brake controlled by the ABS. The valve regulates the air pressure to the brakes when ABS is active. On some systems, the valve has three positions:

Ø  In position one, the valve is open; pressure from the master cylinder is passed right through to the brake.

Ø  In second position; the valve blocks the line, isolating that brake from the master cylinder. This prevents the pressure from rising further should the driver push the brake pedal harder.

Ø  In third position; the valve releases some of the pressure from the brake.

c.       Electronic Control Unit (ECU)

ECU serves as the brain of ABS. It is an electronic control unit that receives, amplifies and filters the sensor signals for calculating the wheel rotational speed, acceleration or deceleration. It consists of pre-programmed advance algorithms which controls the brake pressure according to the data analyzed by the unit.

d.      Pump

When valve releases pressure from the brakes, for periodic use, pump put that pressure back. Pump restore the pressure to the hydraulic brakes after the valve have releases it. Pump is automatically controlled by ECU. When driver releases pedal, pressure is restored automatically with the help of pump.