Validation of Requirements for Hybrid Systems: a Formal Approach

Flaws in requirements may have unacceptable consequences in the development of safety-critical applications. Formal approaches may help with a deep analysis that takes care of the precise semantics of the requirements. However, the proposed solutions often disregard the problem of integrating the formalization with the analysis, and the underlying logical framework lacks either expressive power, or automation. We propose a new, comprehensive approach for the validation of functional requirements of hybrid systems, where discrete components and continuous components are tightly intertwined. The proposed solution allows to tackle problems of conversion from informal to formal, traceability, automation, user acceptance, and scalability. We build on a new language, OTHELLO which is expressive enough to represent various domains of interest, yet allowing efficient procedures for checking the satisfiability. Around this, we propose a structured methodology where: informal requirements are fragmented and categorized according to their role; each fragment is formalized based on its category; specialized formal analysis techniques, optimized for requirements analysis, are finally applied. The approach was the basis of an industrial project aiming at the validation of the European Train Control System (ETCS) requirements specification. During the project a realistic subset of the ETCS specification was formalized and analyzed. The approach was positively assessed by domain experts.