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Electronic Brake Force Distribution (EBD)
Electronic Brake Force Distribution (EBD) became standard on the Crown Victoria Police Interceptor in 2003. A brake system engineer’s ultimate goal is to design a system that optimizes braking power by maximizing friction with the pavement while preventing wheel lock-up. This is the primary purpose of the Anti-lock Braking System (ABS).
To further refine braking performance under all conditions, EBD complements the performance of ABS. EBD utilizes existing ABS hardware with additional software to control rear axle brake pressure during partial braking events. This eliminates the need for a conventional rear brake proportioning valve.
EBD optimizes rear brake performance and vehicle stability despite variations in vehicle load, lateral forces, road grades, or drive train modes, and provides more consistent brake force distribution throughout the vehicle’s lifetime. The Police Interceptor’s EBD “Keep Alive” safety function continuously monitors the ABS system and maintains brake proportioning even in the event of certain ABS component failures.
During braking, weight transfers from the rear axle to the front axle. This causes the front end to move downward as the vehicle’s weight moves forward. Known as “brake dive”, this weight shift forward causes a loss of vertical weight on the rear axle and contributes to rear wheel lockup. Rear wheel lockup can result in vehicle instability and a tendency to over steer. The brake force distribution between the front and rear axles must compensate for the dynamic weight transfer to optimize braking performance.
High braking power without the wheels locking requires that the rear axle brakes achieve maximum friction to the road surface. However, for reasons of driving stability, front brake power must always be higher than the rear. The ideal brake force distribution system provides front and rear wheel lock simultaneously. A vehicle’s dynamic properties (weight distribution, road surface friction, weather conditions, etc.) determine how that ideal brake force distributions is to be applied.
In a vehicle without ABS and EBD, the percentage of braking force was divided between the front and rear and controlled by the hydraulic brake proportioning valve. The main criteria governing the operation of the proportioning valve was the front to rear weight distribution of the vehicle.
A proportioning valve mechanically limits rear brake forces below the ideal brake force distribution curve. This maintains vehicle stability at the expense of some lost rear brake force utilization. EBD electronically limits rear brake forces on or near the ideal brake force distribution curve. The system continually optimizes both vehicle stability and rear brake force utilization.
During normal operation, EBD monitors lateral forces due to curves in the road, performs wheel slip evaluations to determine optimal brake forces for each axle, checks vehicle deceleration for effects on dynamic weight distribution, and controls rear wheel braking pressure to help maintain vehicle stability.
In the curve detection function, EBD estimates vehicle lateral acceleration and curve direction based on the side-to-side differential in wheel speeds. Based on these estimates, the EBD algorithm modifies both the wheel slip evaluation and the vehicle deceleration check.
During each ABS computing cycle (10 times per second), the EBD algorithm determines the relative wheel slip between the fastest front wheel and the slowest rear wheel. The EBD also determines the relative wheel slip between the fastest front wheel and each of the rear wheels. During straight line partial braking, the EBD algorithm utilizes the wheel slip between the fastest front wheel and the slowest rear wheel to control both rear wheel pressures together in a function known as “select low.”
During partial braking while negotiating a curve, the EBD algorithm may consider wheel slip values between the fastest front and each of the rear wheels. Under these conditions, the EBD algorithm may control the inner rear wheel pressure more sensitively than the outer rear wheel in a function known as “independent rear control.”
With detected rear wheel slip and vehicle deceleration criteria satisfied, the EBD algorithm executes a series of pressure hold, pressure decrease, and/or pressure increase steps to optimize rear brake performance while maintaining vehicle stability. If excessive rear wheel slip (over-brake condition) is detected, the controller executes a rear wheel brake pressure decrease. If a specified amount of rear wheel slip is achieved, the controller holds the rear wheel brake pressure. If an insufficient amount of rear wheel slip (under-brake condition) is detected, the controller adds rear wheel brake pressure.
EBD continuously monitors the ABS system and maintains brake proportioning even in the event of certain ABS component failures. Although wheel lock becomes possible when ABS fails, the EBD “Keep Alive” safety function continues to proportion rear brake pressures so that the lock-up sequence, front axle then rear axle, is maintained.
Michael Blackmer is the Police Interceptor Technical Specialist in Product Development at Ford Motor Company. He can be reached at email@example.com.
Published in Police Fleet Manager, Jan/Feb 2006
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