Extended Research On Software Safety Analysis Methodology Based On Safety Requirements

With the rapid development of computer technology, software has become the dominant factor to system safety. How to improve software safety, prevent catastrophic accidents, has become an important research topic in the field of software engineering. Currently, the software safety analysis is mainly concentrated in the software requirements specification and design stage. And in the actual software development process, safety requirements analysis and software design are usually two relatively independent processes. One hand, this has resulted the analysis result of saftey requirements is difficult to directly reflected in the software design, difficult to guide the establishment and modify of the design model. On the other hand safety requirements analysis work hard in the software design stage, it is difficult to carry on software safety analysis based on design model. The traditional fault tree analysis focuses on system failure and the relationship between the fault causes but can’t judge whether there is such a problem in the design of the system. Statecharts can effectively describe the functions of the system, but due to a lack of visual expression of system safety requirements, potential hazards are difficult to be found.In this thesis, we integrate the safety requirements described in the fault tree into the statechart. The extended model can also describe safety requirements and functions of the system. And we’ll model and validate the extended model by model checking method. The main contents of this thesis are as follows:Firstly, we propose a concept called fault statechart based on the fault tree and the statechart, it unifies the safty requirements analysis model and functional model of the system. And a software safety analysis method based on fault statechart is proposed.Secondly, we extract and describe the safety requirements from fault tree based on Backus-Naur Form. We transform the continuous time basic event and the logic gates of fault tree into statechart notations through conversion rules. A mapping table is given to map the safety requirements with the statechart elements. Then we design an algorithm based safety requirements mapping table and conversion rule to construct fault statechart automatically.Thirdly, an approach is presented to transform fault statechart to timed automata.It maps the semantic between the elements of statechart and timed automata, and provides conversion rules to transform the concurrent and branch structure of statechart to timed automata. And the reachability of X the fault state will be validated and analyzed through the model checking tool UPPAAL.Finally, a case study of gas burner control system is given. It establishs the fault statechart and timed automata model of the system, and explain the feasibility and effectiveness of our method through the verification and analysis of the reachability of the fault state accessibility. Our approach provides a new idea for the software safety analysis.