Using Formal Notations to Augment a Hazard Analysis Method

Abstract

Safety-critical systems are those systems whose failure could result in loss of life. For that reason, it needs specific development activities in the software development life cycle to ensure that the system will operate safely. The overall objective of this research is to develop a theoretical framework that identifies unsafe functional behavior for each component in the system. Current safety analysis methods do not use formal methods to find hazardous conditions in a design. We provide guidance concerning mathematical notations to formalize an error ontology used in the architecture descriptions of systems represented in AADL (Architecture Analysis and Design Language) and to improve the rigor of STPA (Systems-Theoretic Process Analysis), a hazard analysis method. In this study, we investigated how a formalized error ontology could assist in identifying unsafe behavior. The ontology can aid in identifying mathematical expressions for each error flow in the canonical feedback control loop architecture. The results of our studies have shown that providing a formal notation for the feedback control loop and providing formal specification for the error ontology lead to finding hazards in the operational system context that other methods miss. By augmenting STPA with an error ontology described in a formal notation, we are able to find more hazards.

Keywords: ASAM, STPA, Error Propagation