
Figure 1. Brake-By-Wire Functional View.
The BBW is composed of two mains functions. First a brake controller reads wheel speed sensors and a brake pedal sensor. The brake controller computes the desired brake torque to be applied to the wheels. In addition to this basic brake controller functionality, a second function ABS (Anti blocking System) adapts the brake force on each wheel if the speed of one wheel is significantly smaller than the estimated vehicle speed. In this case, the brake force is reduced on that wheel until it regains speed that is comparable with the estimated vehicle speed. The ABS takes as inputs the sensors values on each wheel and the estimated vehicle speed.
The hardware architecture and the allocation of BBW functions on this architecture are represented in Figure 2.
The hardware platform consists of sensors/actuators and computing parts (five electronic control units connected by a communication bus). Each ECU runs independently with their own temporal references (timebase), which is not necessarily (well) synchronized with the other one and the communication between them is still mainly asynchronous (despite the apparition of Time Triggered buses).

Figure 2. Hardware architecture and allocation of BBW functions
TC. 1: A Delay constraint XVL is bounded with a minimum value of 180ms and a maximum value of 200ms. This delay is measured from brake pedal stimulus to brakes response. Here, activation of the brake pedal sensor is the stimulus and brake actuation is the response.
TC. 3: The tolerated maximum Synchronization constraint between first and last wheel brake actuation is 5 ms.
In the design process based on EAST-ADL and AUTOSAR, the functional description is refined while passing different development levels.
Figure 3 shows a complemented view of BBW timing constraints that follows the functional decomposition through the levels.

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