The Miles Before Formal Methods - A Case Study on Modeling and Analyzing a Passenger Lift System
Cyber-Physical Systems (CPS) pervade our everyday lives. As users, we need assurances that such systems satisfy requirements on safety, reliability, security and interoperability. CPS presents a major challenge for formal analysis because of their complexity, physical dependencies and non-linearity, and for smart CPS - the ability to improve their behavior over time. Existing approaches on analyzing CPS (e.g., model checking and model-based testing) often assume the existence of a system model. Such approaches have limited application in practice as the models often do not exist. In this work, we report our experience on applying a three-step approach to analyzing a practical CPS: a passenger lift system in a commercial building. The three steps are (1) determining the right level of system abstraction, (2) building the model automatically using grammatical inference, and (3) analyzing the model. The inferred model is in the form of a probabilistic deterministic real time automaton, which allows us to verify the system against properties demanded by the lift requirement. The resulting models form the basis of formal analysis and potentially other approaches. We believe that our approach and experience are applicable to other CPSs.