Anti-Lock Braking System

The anti-lock braking system(ABS) is a safety system that allows the wheels on a motor vehicle to continue interacting with the road surface while braking, preventing the wheels from locking up and therefore avoiding skidding.

Recent versions not only prevent wheel lock under braking, but also electronically control the front-to-rear brake bias. This function is known as electronic brake force distribution (EBD), the traction control system(TCS), emergency brake assist or electronic stability control (ESC).

The Traction Control System(TCS), also known as Anti-Slip Regulation (ASR), is typically a secondary function of the anti-lock braking system and it is designed to prevent loss of traction of the driven road wheels, and therefore maintain the control of the vehicle when the condition of the road surface is unable to cope with the torque applied.

A typical ABS includes a centralelectronic control unit (ECU), four wheel speed sensors, and at least two hydraulic valves within the brake hydraulics. The ECU constantly monitors the rotational speed of each wheel; if it detects a wheel rotating much slower than the others, it actuates the valves to reduce hydraulic pressure to the brake at the wheel, thus reducing the braking force on that wheel; the wheel then turns faster.

Conversely, if the ECU detects a wheel turning much faster than the others, brake hydraulic pressure to the wheel is increased so the braking force is reapplied, slowing down the wheel. Some anti-lock systems can apply or release braking pressure 16 times per second.

The modern ABS applies individual brake pressure to all four wheels through a control system of hub-mounted sensors and a dedicated micro-controller.