The demonstrator is a fully functional steer-by-wire driving control unit for a tractor. The project's goal is to demonstrate that steer-by-wire can fulfill all legal obligations for the use on the road. Fundamentally, this means that the steer-by-wire system will be able to tolerate any single failure. This kind of fault tolerance is required in order to obtain permission to operate the equipment on the road. Many conditions have to be taken into account, but the most important feature of a steer-by-wire system is the ability to tolerate any single fault in the system in order to avoid any dangerous situation.
In case of a failure the system must remain operational. As with all safety-critical applications, the system contains redundant electronic control units and redundant sensors and actuators. A TTP architecture is used for data communication. Single failures were injected into the system in order to verify that the redundancy level introduced was enough to satisfy the R79 regulation for steering systems.
The valves in the hydraulic circuit of the vehicle are controlled by the electronic system that receives the movements of the joysticks via sensors. As all movements are controlled by the electronics, the system is highly flexible, and it can manage an extensive range of functions. With such a system, the failure of one sensor, one actuator, one bus line, or even one whole ECU has no critical consequences; it does not influence the behavior of the control system at all.
http://www.ttcontrol.it/applications/tractors.htm