EBD (Electronic Brake-Force Distribution)
EBD (Electronic Brake-force Distribution)
EBD, which is integrated with the control module, is a function which can be compared to a load sensing valve for the rear wheel brakes. For optimum braking and steering stability, the maximum braking force must be applied to both the front and rear wheels for all conditions/loads. The control module prevents the rear wheels from receiving excessive braking force by regulating brake pressure.
EBD modulation occurs sooner than the normal ABS mode and provides the occupants of the car with a more comfortable ride. The pump is not used during EBD modulation.
During braking, it is important for proper steering stability that the rear wheels do not lock first. In order to achieve this under different load conditions (e.g. a heavily loaded car requiring greater braking force in order to lock the wheels), the control module uses wheel speed to modulate the braking force on the rear wheels with help from the inlet solenoid valves. This prevents excessive permissible slip between the front and rear wheels.
When the criteria are met, the rear wheel inlet valves close and the EBD function is activated in order to regulate slip between front/rear wheels and provide stability. If any of the wheels locks during EBD modulation, the control module switches to normal ABS modulation.
CBC (Cornering Brake Control) in the ESP control module
CBC is a function that is used when the brakes show a tendency to lock. The function operates in combination with EBD and ABS in order to stabilize vehicle lateral and yaw rate movements in situations when the driver corners while braking. Modulation takes place by braking the four wheels individually.
CBC determines if the vehicle is cornering and approaching oversteer by comparing the four wheel speeds. If the vehicle shows a tendency towards oversteer at the same time as the driver depresses the brake pedal, CBC will begin adjustment.
The function is included in TCS and ESP and is activated before ABS modulation. This means that the function only operates with the brake pressure applied by the driver and regulates brake pressure with help from the inlet and outlet valves. The system limits rear braking force and primarily adjusts the front wheel brakes in order to achieve stability.
CBC includes functions which prevent activation on roads with uneven surfaces.
CBC requires information regarding wheel speed from all four wheels driver braking
CBC uses
The four inlet and outlet valves to regulate with varying brake pressure on one or more wheels.
CBC is activated in extreme situations only and prevents the car from oversteering.
ABS function
The ABS system provides optimum braking without any loss of directional stability.
Braking force/Tire slip
The diagram shows braking force as a function of tire slip.
The braking force is equivalent to the coefficient of adhesion or the friction between the tire and the road surface. Each application of braking force gives rise to a certain degree of slip. The slip of a freely rotating wheel is expressed as 0% and of a locked wheel as 100%.
When the brake is first applied at 0% slip, braking force increases sharply but the degree of slip increases only gradually up to a certain limit. Beyond that point, braking force decreases with increasing slip.
Lateral force/Tire slip
The diagram shows lateral force as a function of tire slip.
The maximum braking force is reached at a point known as the limit of optimum slip. The section of the curve between 0% slip and the limit of optimum slip is called the stable braking zone, and the section of curve between the limit of optimum slip and 100% slip is called the unstable braking zone, as stable braking cannot be achieved within this zone. This is because the wheel quickly becomes locked after the limit of optimum slip has been reached, unless the braking force is immediately reduced.
Slip also occurs when the tire is called upon to transmit a lateral force, e.g. during cornering. Diagram 2 shows how the lateral force falls away sharply with increasing slip. At 100% slip, i.e. when the wheels have locked up, no lateral force remains for steering and the driver will no longer be able to control the vehicle.
The diagram shows both curves with ABS modulation range above.
1. Braking/Lateral Force
2. Stable Position
3. Unstable Position
4. ABS Operating range
5. Limit of optimum slip
6. Tire slip
During braking, the braking force is allowed to increase to a point near the limit of optimum slip when ABS prevents further braking force. Hydraulic pressure is then adjusted so that the braking force is kept as close to the optimum level (limit of optimum slip) as possible, irrespective of how hard the driver depresses the brake pedal.
Thus, because the ABS system prevents the degree of slip from exceeding the limit of optimum slip, the car never enters the unstable braking zone. At the same time, some lateral force is preserved to ensure that steering control can be maintained (curve II).