Braking With ABS

Braking with ABS




When a wheel threatens to lock during braking, the control module closes the inlet valve (2) to the brake circuit concerned, cutting the supply of brake fluid from the master cylinder. This prevents the hydraulic pressure from increasing any further, irrespective of the pressure in the master cylinder (maintaining pressure). The hydraulic pump starts up at the same time.
If the wheel is still threatening to lock, the outlet valve (3) opens so that brake fluid returns to the low pressure accumulators (4) with the result that the pressure in the brake circuit is reduced (pressure dumping). This reduces the hydraulic pressure and, thus, also the braking effect so that the wheel picks up speed. The hydraulic pump (5) returns brake fluid from the low pressure accumulators to the master cylinder.
If the acceleration is too great, the brake pressure must be increased (pressure buildup). To increase the pressure in the brake circuit, the outlet valve (6) closes and the inlet valve (7) opens so that the hydraulic pressure in the circuit increases. While pressure is increasing, any fluid required will be supplied by the master cylinder and by the pump if the low pressure accumulators contain brake fluid. As the volume of the fluid supplied to the brake circuit is greater on average than the volume returned to the low pressure accumulators from the brake circuits, the accumulators will only in principle compensate for peak flows on the pump inlet side. The pump returns brake fluid from the low pressure accumulators to the master cylinder or brake circuits, depending on the position of the inlet valves.
In accordance with the volume flow from the pump, the position of the master cylinder pistons and thus the brake pedal position will correspond to the actual fluid supply to the brakes. This means that, at low pressure in the brake circuits, the brake pedal will maintain a high position and at high pressure in the brake circuits, the brake pedal will maintain a low position. As the controlled pressure in the brake circuit increases and decreases, this causes the pedal to move, telling the driver that controlled braking has been activated.
The hydraulic pump cuts in approx. 1 second after ABS cuts out to ensure that the low pressure accumulators are empty.