Courtesy of Bosch
Electronic Brake Force Distribution Components
The safety and regulatory hardware used by an electronic brake force distribution system is much the same as the hardware used by an antilock braking system without EBD. It's just programmed differently. For the purposes of EBD, three different pieces of hardware are necessary: sensors that can determine the slip ratio of each wheel, valves that can modulate the amount of brake force that goes to each wheel and an electronic control unit that can calculate the amount of force required.
Speed sensors: To determine the slip ratio of a wheel, the EBD system needs two pieces of information: the speed at which the wheel is rotating and the speed of the car. If the speed at which the wheel is rotating is slower than the speed at which the car is moving, then the wheel is slipping and a skid can result. A sensor is placed at each wheel to determine wheel speed. There is no specific sensor to measure the forward motion of the car. Instead, the speed measurements from all four wheels are averaged to create an estimate of the vehicle's overall speed.
Brake force modulators: Brake force is applied to the wheels hydraulically, with brake fluid pumped into brake lines in such a way as to pneumatically activate the brake cylinders. The EBD system can modulate the amount of brake fluid going to each wheel through electrically actuated valves.
Electronic Control Unit (ECU): The ECU is a small computer embedded in the antilock braking system. It receives input from the speed sensors, calculates the slip ratio of the wheels, and uses the brake force modulators to apply an appropriate amount of force to keep the slip ratio of each wheel within a reasonable range.
Most EBD systems also include a yaw sensor, which detects the rotation of the vehicle as it turns. This can be compared with the angle of the steering wheel by using a steering wheel angle sensor to detect oversteer (too much rotation relative to the angle of the wheel) or understeer (not enough rotation relative to the angle of the wheel). EBD can then correct the steering by activating one of the rear brakes. For instance, if the car begins to understeer, the inner rear brake is activated to increase the car's rotation. If the car begins to oversteer, the outer rear brake is activated to decrease the car's rotation. The yaw sensor can also be used in conjunction with electronic stability control (ESC) to prevent rollover accidents.
How important is it for you to have a car with electronic brake force distribution? On the next page we'll look at how EBD helps you to drive safely.