Evaluating quality of service and behavioral reliability of steer-by-wire systems

Steer-by-wire systems must meet not only reliability but also real-time requirements. This paper presents an integrated approach for evaluating both the temporal performance and the behavioral reliability of steer-by-wire systems taking into account the delay variation introduced by network transmission errors. The considered temporal performance is the quality of service perceived by the user, i.e. the vehicle stability. Tests in vehicles and simulations have been realized to estimate the maximum tolerable response time of the system, and to evaluate the impact of this delay on the quality of service. We quantify then the worst case response time of the system for a generic architecture based on TDMA protocol but independent of the communication network (could actually be TTP/C or FlexRay), and apply these generic results to a case study. We further define the notion of "behavioral reliability" as the probability that "the worst case response time is less than a threshold". In our case study this behavioral reliability is evaluated and linked to the Safety Integrity Levels defined in IEC61508-1 standard. Based on this behavioral reliability concept, the final objective of our work is to propose a new dependability analysis method for X-by-Wire systems by taking into account both dynamic performance, fault-tolerance mechanisms and static redundancy of the system.