Marco Bozzano

Many safety critical systems guarantee fault-tolerance by using several redundant copies of their components. When designing such redundancy architectures, it is crucial to analyze their fault trees, which describe combinations of faults of individual components that may cause malfunction of the system. State-of-the-art techniques for fault tree computation use first-order formulas with uninterpreted functions to model the transformations of signals performed by the redundancy system and an AllSMT query for computation of the fault tree from this encoding. Scalability of the analysis can be further improved by techniques such as predicate abstraction, which reduces the problem to Boolean case.In this paper, we show that as far as fault trees of redundancy architectures are concerned, signal transformation can be equivalently viewed in a purely Boolean way as fault propagation. This alternative view has important practical consequences. First, it applies also to general redundancy architectures with cyclic dependencies among components, to which the current state-of-the-art methods based on AllSMT are not applicable, and which currently require expensive sequential reasoning. Second, it allows for a simpler encoding of the problem and usage of efficient algorithms for analysis of fault propagation, which can significantly improve the runtime of the analyses. A thorough experimental evaluation demonstrates the superiority of the proposed techniques.

Diagnosability is a fundamental problem of partial observable systems in safety-critical design. Diagnosability verification checks if the observable part of system is sufficient to detect some faults. A counterexample to diagnosability may consist of infinitely many indistinguishable traces that differ in the occurrence of the fault. When the system under analysis is modeled as a Büchi automaton or finite-state Fair Transition System, this problem reduces to look for ribbon-shaped paths, i.e., fair paths with a loop in the middle.In this paper, we propose to solve the problem by extending the liveness-to-safety approach to look for lasso-shaped paths. The algorithm can be applied to various diagnosability conditions in a uniform way by changing the conditions on the loops. We implemented and evaluated the approach on various diagnosability benchmarks.