ABS Control
ABS controlNo braking
When the driver is not braking the car, the braking system is in stand-by mode: that is, the master cylinder is in its resting position and the connection between the brake fluid reservoir and the rest of the hydraulic system is open. The system is then completely unpressurized. In the hydraulic unit valve block, the hydraulic valves are in their resting position: that is, the inlet valves are open and the outlet valves are closed.
Braking without ABS
When the brake pedal is depressed, the force of the pedal is transmitted via the air hydraulic brake power assist unit (brake servo) to the master cylinder pistons and the brake light switch triggered. The center valve in the secondary piston and the seal on the primary piston close the connections to the brake fluid reservoir. In cars with TRACS the primary piston also has a center valve.
The hydraulic pressure in the two brake circuits increases and triggers the pistons in the brake calipers which press the brake pads against the brake discs.
When the brake pedal is released and returns to its resting position, the pistons in the master cylinder return to their resting position. The connection to the brake fluid reservoir opens as the center valve, the seals and the braking system are depressurized. The pistons in the brake calipers return to their resting position because of their seals.
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 it 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.
Electronic Brakeforce Distribution (EBD)
Depending on how much the rear wheels are slipping relative to the front wheels, the brake pressure in the rear wheel circuit will be controlled differently. The control system will try to keep rear wheel slip 0-2% higher than front wheel slip. It can therefore be said to be load- responsive.
Generally speaking, the difference in slip between the rear wheels and front wheels when braking mainly depends on how hard the braking is and how loaded the car is. Hard braking/no load can produce serious slipping, light braking/heavy load may give less slip.
EBD is integrated with the ABS system and controls the braking force to the brakes on the rear wheels.
EBD stands for Electronic Brakeforce Distribution. With EBD, the task of the control module is to control the hydraulic pressure in the rear wheel brake circuit (1) if the rear wheels are tending to slip more than the front wheels when braking. This ensures that the car brakes smoothly and safely without ABS coming on.
The control module controls the brake pressure by operating the inlet and outlet valves in the rear wheel brake circuit (1).
The EBD pressure sensor is connected to the master cylinder primary circuit and indicates if the primary circuit is under pressure. EBD can only be used when the EBD pressure sensor (2) is activated. EBD replaces the reducer valve which would otherwise reduce the pressure in the rear wheel brake circuit.
As EBD does not require as much brake fluid as ABS/TRACS, the hydraulic pump is not activated. Fluid returning from the brake circuits is taken up by the low pressure accumulators.