Anti-Lock Braking

ANTI-LOCK BRAKING
During normal braking and anti-lock braking, each wheel speed sensor sends an A/C signal to the anti-lock Electronic Control Unit (ECU). These signals are then processed into a digital speed signal for each wheel. The lower of the two rear wheel speeds is selected to control the rear axle modulation. The front wheels are independently controlled. An estimated vehicle speed (reference speed) is calculated from the highest of the four wheel speeds. The deceleration of the vehicle is calculated from the changes in the estimated vehicle speed.

The need to reduce brake pressure is determined by anti-lock ECU by comparing the wheel speed to the vehicle reference speed. Pressure decay is begun when the wheel speed is less than the reference speed by a preprogrammed amount. Based on the acceleration rate of the wheel and/or the relative speed of the wheel as compared to the vehicle reference speed, the decay cycle is ended and either the pressure is held or a pressure build cycle is begun.

The rate of pressure build is controlled by pulsing the build valves and by control orifices in the modulator. Pulsing occurs if the wheel acceleration exceeds a preprogrammed rate. Pulsing also occurs during wheel deceleration if the vehicle deceleration rate is lower than vehicle speed as determined by the anti-lock ECU.

If the driver releases the brake pedal during anti-lock cycling, the brake pressure is returned to the booster through the release check valves. This minimizes the possibility of supercharging the master cylinder. If the driver reapplies the brake pedal during this mode, the pedal will feel firm and brake pressure will rebuild from the boost source.

At the end of cycling, the build and isolation valves are momentarily held open to rebuild brake pressure from the boost source to minimize any possible pedal drop. During a long stop when additional cycling is not required, the isolation valve may turn off, restoring normal braking. This may result in a small pedal drop if the master cylinder pressure is greater than the boost pressure such as following a long stop on a low friction surface.