Safety concept requirements of steering systems with steer-by-wire functionality

Several suppliers and car makers are developing steer-by-wire (SbW) systems. Motivation are the functionalities which can be realised by SbW. SbW will mark the next quantum leap in the car functionality. In the conception of a SbW system that is ready for production, not only the functional aspects has to be considered but also the safety aspects. This is the reason why DaimlerChrysler makes already in an early concept phase of the SbW system investigations with non-expert drivers in a SbW test car and in the DaimlerChrysler driving simulator in Berlin to specify the safety concept requirements. The results show that by fail-silent designed SbW systems no functionalities with customer benefit can be realised. The reason is that already small changes in the steering ratio respectively small steering angle faults cannot be handled by the drivers and result in problems to keep the right lane. The steering angle fault is defined as differential angle between position of steering wheel and the front wheels after switching off the fail-silent SbW system. A mechanical backup guarantees in that context no transition to the safe state as it is required for safety relevant systems. Conclusion of DC investigations is that an appropriate SbW system has to be designed fail-tolerant. This is the only approach to make sure that the steering system will change directly to the safe state when failures occur. Thus Steer-by-Wire makes noticeable higher demands to the system concept as known driving dynamic systems (ABS, ESP) and even Brake-by-Wire. In consideration of the fact that the road is much longer as broad that can be easily understood.