Anti-lock braking system

Anti-lock braking system

1. Actual vehicle speed
2. Vehicle reference speed
3. Peripheral wheel speed
4. Wheel acceleration/deceleration
5. Braking system pressure
6. Permissible acceleration band
7. Permissible deceleration band

The signals (alternating or analogue) sent by the rpm sensors to the electronic control unit are converted by the input amplifier into square-wave (or digital) signals. The frequency of these signals provides the control unit with the corresponding speed (3) and acceleration/deceleration (4) of the individual wheels. From the combination of the individual peripheral speeds of the wheels, a reference speed (2) is worked out which, constantly updated, provides an indication of the vehicle's actual speed (1).

The electronic control unit also has stored in its memory the deceleration/acceleration thresholds (6) and (7) which each individual wheel must not exceed. So by means of a systematic, continuous and very rapid comparison between the wheel's deceleration/acceleration values and those of the memorized band, the rolling of the tyres during braking is monitored.

When the driver presses the brake pedal, the wheels may decelerate to different extents. The slowing down or total stoppage of the vehicle with deceleration within the memorized permissible band, does not lead to any intervention of the system in terms of control. However, at the moment when excessive braking force causes the wheel speed to decrease in relation to the vehicle's reference speed, the system starts the deceleration calculation cycle (point A).

If the deceleration threshold (7) is exceeded, the system intervenes by driving the solenoids to reduce the pressure (point B). After the pressure reduction, and after the first instants in which the deceleration increases further because of the inertia of the system, the wheel which is no longer braked reverses the tendency to lock, so regaining speed. When the deceleration returns within the threshold (7), the intervention of the control system is modified, starting the pressure maintenance phase (point C.)