Multiplexed Networks for Embedded Systems: CAN, LIN, FlexRay, Safe-by-Wire...

9

Safe-by-Wire

Let us now examine a particularly good example of a communication bus, namely the one used to control airbags.

9.1 A Little History

The airbag system of a vehicle is highly complex, and a detailed description would require more space than this book can provide. For anybody unfamiliar with the conventional structure of this system, it can be divided into three major parts:

• everything relating to the sensors (accelerometers, inertial unit, etc.) and the information that they deliver;
• everything relating to the triggering of the actuators, igniters (squibs), safety belt pre-tensioning devices, etc.
• the intelligent system that controls everything else.

For ordinary vehicles, because of the large number of interconnections required between these three units, the members of the system are generally all connected to each other by a ‘point-to-point’ architecture. Figure 9.1 provides an idea of this kind of structure.

However, it should be borne in mind that security architectures can be complicated to suit any requirements, and top-range vehicles can have up to 20 airbags at least (in front, at the sides, for the head, for the knees, under the knees, to prevent sliding in a collision, etc.) and a large number of sensors. In this case, the processing unit must be centralized in order to limit the lengths of the connecting cables; indeed, we can soon reach the mechanical limits on the size of the connector(s) which may include up to 100 interconnection points and which end ...