by James Walker, Jr. of scR motorsports for STOPTECH
Anti-lock Brakes, Electronic Brakeforce Distribution, Vehicle Dynamics Control, Dynamic Rear Proportioning, Electronic Stability Program – the list goes on and on. With all of these integrated chassis control technologies now sprouting up on not-just-luxury-cars, maybe it’s time we step back for a minute and have a look at what these systems consist of and what their sensitivities are…before we make changes to our cars which could potentially impact their performance.
That being said, a little definition rollout is necessary so that we are all on the same page as we go forward with these discussions. This may be a little bit dry, but the good stuff is only a page or two away.
ABS – Anti-lock Braking System – An electro-mechanical control system designed to monitor and influence wheel dynamics, and ultimately vehicle dynamics during braking maneuvers. In order of priority, these systems are intended to enhance vehicle 1) stability, or the prevention of oversteer 2) steerability, or the prevention of understeer and 3) stopping distance. Typical systems consist of 3-4 wheel speed sensors, an ECU containing the algorithm processing the wheel speed information, a series of solenoid-driven valves, and a pump-motor subsystem which can be actuated to interrupt and release brake fluid pressure from the wheel-end brake components (calipers and such).
TCS – Traction Control System – An electro-mechanical control system designed to monitor and influence wheel dynamics, and ultimately vehicle dynamics during acceleration maneuvers. In order of priority, these systems are intended to enhance vehicle 1) stability (RWD applications) 2) steerability (FWD applications) and 3) launch performance (all applications). Typical systems consist of 3-4 wheel speed sensors, an ECU containing the algorithm processing the wheel speed information, a series of solenoid-driven valves, and a pump-motor subsystem which can be actuated to build, hold, and release brake fluid pressure from the wheel-end brake components.
ESP – Electronic Stability Program (also referred to as VDC, IVD, TRAXXAR, and others not listed here) – An electro-mechanical control system designed to monitor and influence wheel dynamics, and ultimately vehicle dynamics during any vehicle state (braking, accelerating, or coasting). In addition to the sensors and hardware used during ABS and TCS, ESP typically utilizes the additional input from a steering angle sensor, a yaw rate sensor, and a lateral/longitudinal accelerometer when determining both 1) the driver's intended heading and 2) the vehicle’s actual heading. Once the system has determined a significant difference (error) between (1) and (2) above, the solenoid-driven valves and pump-motor subsystem can be actuated to build, hold, and release brake fluid pressure at individual wheel-end brake components, creating asymmetric (cross-vehicle) brake forces in an attempt to create yaw moments, turning the vehicle toward the driver's intended path.
EBD – Electronic Brakeforce Distribution (also referred to as DRP, dynamic rear proportioning) – An electro-mechanical control system designed to monitor and influence rear wheel dynamics, and ultimately foundation brake (front-rear) balance. In so many words, the EBD utilizes the ABS hardware to function as an "intelligent brake proportioning valve." Unlike a traditional mechanical proportioning valve which is limited by design to one kneepoint and slope, the EBD algorithm relies on closed-loop feedback to continuously monitor rear wheel slip, adjusting brake line pressure to the rear wheels as appropriate.
Technology Dependency – ABS/TCS/ESP
The calibration of modern ABS/TCS/ESP is a complex and time consuming processes entailing the setting, or "tuning," of literally thousands of algorithm variables. These variables define the base vehicle characteristics (braking system included), control limits, and expected vehicle reaction to control activity. For this reason, one control algorithm may be utilized on several applications, but each vehicle requires its own unique variable set, or table.
Base vehicle dynamic response is of primary importance when tuning these variables, from a brake system, suspension system, and tire perspective. For example, the tuned and calibrated ABS expects certain vehicle reactions to its control signals. In simple terms, if the ABS control algorithm determines that a given wheel of the vehicle is in need of brake pressure reduction, it will calculate the amount of time required to actuate the pressure release solenoid based on the pressure-torque and/or pressure-volume characteristic of the wheel-end brake components. When calibrating the system, the Development Engineer essentially has to "teach" these characteristics to the ABS, one tedious variable at a time. Press repeat for TCS and ESP.
Because the ABS/TCS/ESP implements "learning" logic to modify the next control cycle based on the activity in the current control cycle, any time spent "re-learning" these characteristics will ripple through the control cycles, with possible impacts to stability, steerability, and/or stopping distance. In short, changes in the base brake system characteristics (hardware) may impact ABS/TCS/ESP performance in any or all of these three areas.
Technology Dependency – EBD
Of the four technologies described herein, EBD may be the simplest to define, yet may carry the most wide-reaching impacts to base vehicle braking performance. While not nearly as variable-intensive as its ABS, TCS, or ESP counterparts, any time spent "re-learning" vehicle characteristics due to a change in the base brake hardware could possibly impact the vehicle brake system balance, or bias, during partial braking maneuvers.
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