Anti-lock Braking Systems

Basic Description

Anti-lock Braking Systems (ABS) prevent wheel lockup by modulating the braking pressure. These systems play a significant role in improving the safety of modern vehicles. In slippery road conditions (e.g. wet, icy or gravel), a driver may hit the brake so hard that one or more wheels locks and begins to skid over the surface. This results in longer stopping distances, a loss of steer-ability, and vehicle instability. ABS systems monitor the wheel speed in real time and regulate the brake pressure automatically in order to prevent wheel lockup and improve the driver's control of the vehicle. The main components of these systems include regular brake parts (such as the brake pedal, hydraulic cylinders and lines), wheel speed sensors, and a hydraulic modulator operated by an electronic control unit. The architecture of the ABS system (including the hydraulic modulator) is illustrated in the figure below.

When operating in normal conditions, the outlet valve (C) of the hydraulic modulator is closed and the inlet valve (A) stays open until the pressure reaches the desired value. Then both the inlet and outlet valves remain closed to hold this pressure and provide sufficient brake torque for wheel brake cylinders. Once the control unit detects any excessive wheel slip, the corresponding outlet valve is opened to release the pressure and prevent possible wheel lockup. After the wheel slip returns to normal, the hydraulic modulator will resume the regular braking process.

Sensors

Wheel speed sensor

Actuators

Hydraulic modulator, master cylinder, wheel brake cylinders, warning light

Data Communications

High-speed CAN bus.

Manufacturers

Bosch, Continental, TRW, Wabco

For More Information

[1] How Anti-Lock Brakes Work, Karim Nice, HowStuffWorks.com, Aug. 23, 2000.

[2] Anti-lock Braking System, Wikipedia.

[3] Antilock Brake Systems (ABS), National Highway Traffic Safety Administration (NHTSA) web site.

[4] Questions and Answers Regarding Antilock Brake Systems (ABS), National Highway Traffic Safety Administration (NHTSA) web site.

[5] ABS Brakes and the Facts, Matthew Wright, About.com.

[6] Anti-Lock Brake System (ABS) Safety Film, YouTube, Feb. 2007.