ABS function
While driving, the ABS control module (1) continuously registers the acceleration and deceleration of the wheels. The wheel sensors (3) on each wheel send this information to the ABS control module. The ABS control module calculates the reference speed of the car using these signals. The ABS control module transmits a signal with the speed of the car via the Control area network (CAN), which is used by other control modules.
When braking, the ABS control module (1) receives a signal from the stop (brake) lamp switch (2) indicating that braking is occurring. The ABS control module goes into standby mode for ABS-control. The signal from the stop (brake) lamp switch is not required for ABS control, but does give softer control at the beginning of ABS control.
If the ABS control module detects that there is a risk of wheel lock-up, the ABS hydraulic modulator (4) is affected and the hydraulic pressure for the relevant wheel is adapted so that wheel lock up is avoided.
The pressure in the circuit is controlled so that the maximum possible braking force is transferred to the road surface, which means that the wheel rotates with 12-20% less slippage on the surface.
ABS is first activated when the speed of the car exceeds approximately 7 km/h, which means that at speeds below approximately 7 km/h the wheels can lock up. This has little practical effect on the function of the system or from the driver's point of view.
ABS control can be divided into three stages:
- Open position. (Pressure buildup). This is the normal condition during braking. The inlet valve is open and the outlet valve is closed. This permits the brake pressure to pass from the master cylinder to the wheel
- Pressure retention phase. (Constant pressure). If the ABS control module detects that one of the wheels is rotating too slowly, the control module will close the inlet valve. This stops the brake pressure to the wheel from increasing, even if the driver increases the pressure on the brake pedal. If the rotation speed of the wheel continues to fall the control moves into the pressure reduction phase
- Pressure reduction phase. (Pressure release). The inlet valve remains closed. The ABS control module opens the outlet valve and starts the pump which transfers brake fluid from the wheel, this reduces the brake pressure. When the rotation speed has increased sufficiently the control module closes the outlet valve, opens the inlet valve and control returns to the open position.
Steps 1, 2 or 3 are repeated until braking finishes or the ABS system stops.