by Nigel Holmes

Created on: December 24, 2008   Last Updated: March 27, 2012

The purpose of an anti-lock braking system (ABS) is to allow the driver of a vehicle to retain control while applying maximum braking force. It achieves this by preventing the wheels from locking completely, avoiding a skid. Without ABS, under emergency braking the wheels lock, and the tires slide. When this happens, momentum takes over, and the vehicle continues in a straight line, with the driver unable to steer around an obstacle.

An anti-lock braking system has three main components. These are: wheel speed sensors (one on each wheel), an electronic brake control module (EBCM), and a hydraulic modulator.

The wheel speed sensor consists of a toothed ring fitted to the inside of each wheel and a magnetic sensor mounted on the wheel hub. As each tooth passes the magnet, a voltage signal is generated and transmitted to the EBCM. The frequency of these pulses tells the EBCM how quickly the wheel is rotating.

When the brakes are applied, the EBCM checks the speed of each wheel. If it senses that a wheel is about to stop turning, it signals the modulator to reduce the braking force, allowing that wheel to speed up slightly.

ABS manufacturers use differing designs of modulator, but all have the same basic function: They reduce the hydraulic pressure in the brake system by slightly increasing the volume available for the brake fluid. This allows the brake calipers to pull the pads away from the surface of the brake disc, allowing the wheel to speed up.

The EBCM then detects that the wheel is turning faster and signals the modulator to increase pressure. The modulator reduces the volume available for the brake fluid, and the pads grip the brake disc with greater force, slowing the vehicle.

Most modulator designs use a number of solenoids (essentially, electromagnetic pistons) to rapidly increase and decrease the volume available for the brake fluid. This is what creates the "pulsing" the driver feels through the brake pedal during an ABS stop. However, GM has used an alternative design in its small cars. This consists of motors that drive pistons up and down inside bores to change the available volume. This design is termed a "low-pressure system," meaning that the ABS cannot apply a greater pressure than that generated by the driver's foot. In contrast, a "high-pressure system" uses an accumulator where brake fluid is held at a higher pressure until called upon by the EBCM. This means a high-pressure system can generate greater stopping power.

Note however that while ABS can reduce stopping distances, its primary purpose is to allow the driver to steer the vehicle while simultaneously applying maximum braking force.

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