An ABS is a system on motor vehicles which prevents the wheels from locking while braking stopping safely is one of the most important functions a motor vehicle can perform.
Fig. Anti-lock braking system
Explanation: Failure of the brake system will almost invariably results in property damage, personal injury or even death. An ABS allow the driver to maintain steering control under heavy braking by preventing a skid and allowing the wheel to continue to roll forward and create lateral control, as directed by driver steering inputs. A typical ABS is composed of a central electronic unit, four speed sensors (one for each wheel) and two or more hydraulic valves on the brake circuit. The electronic unit constantly monitors the rotation speed of each wheel. The pulsed output from the wheel speed sensors goes to an electronic controller which monitors each wheels speed relative to the speed of the other wheels. As long as the brakes are not being applied and all of the monitored wheels are rotating at roughly the same speed, the system takes no action. If however the brakes are being applied and one or more monitored wheels suddenly beings to reduce speed indicating a loss of traction with load the controller then activates the antilock system. When it senses that any one of the wheels is rotating slower than the others, it moves the valves to decrease the pressure on the braking circuit, effectively reducing the braking force on that wheel.