Abs Function
While driving, the brake control module (BCM) (1) registers the acceleration and deceleration of the wheels. The 4 wheel sensors (3) (one on each wheel) provide the brake control module (BCM) with information about the rotation speed of the wheels. The brake control module (BCM) uses this information to calculate the vehicle speed. The control module transmits the signal on the controller area network (CAN) where it can be accessed by other systems.
During braking, the brake control module (BCM) receives a CAN signal from the engine control module (ECM) and stop lamp switch (2) indicating braking. The brake control module (BCM) then goes into standby mode for ABS control. The signal from the stop lamp switch is not required for ABS control, but does give softer control at the beginning of ABS control.
If the brake control module (BCM) 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. This 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 increase) . 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 maintenance phase . If the brake control module (BCM) detects that one of the wheels is rotating too slowly, the brake control module (BCM) 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 release phase . The inlet valve remains closed. The brake control module (BCM) opens the outlet valve and starts the pump. This transfers brake fluid from the wheel, reducing 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.