We demonstrate diagrammatic Event-B formal modelling of a hybrid, 'fixed virtual block' approach to train movement control for the emerging European Rail Traffic Management System (ERTMS) level 3.We perform a refinement-based formal development and verification of the no-collision safety requirement. The development reveals limitations in the specification and identifies assumptions on the environment. We reflect on our team-based approach to finding useful modelling abstractions and demonstrate a systematic modelling method using the UMLlike state and class diagrams of iUML-B. We suggest enhancements to the existing iUML-B method that would have benefitted this development.
Formal methods use abstraction and rigorously verified refinement to manage the design of complex systems, ensuring that they satisfy important invariant properties. However, formal verification is not sufficient: models must also be tested to ensure that they behave according to the informal requirements and validated by domain experts who may not be expert in formal modelling. This can be satisfied by scenarios that complement the requirements specification. The model can be animated to check that the scenario is feasible in the model and that the model reaches states expected in the scenario. However, there are two problems with this approach. 1) The provided scenarios are at the most concrete level corresponding to the full requirements and cannot be used until all the refinements have been completed in the model. 2) The natural language used to describe the scenarios is often verbose, ambiguous and therefore difficult to understand; especially if the modeller is not a domain expert. In this paper we propose a method of abstracting scenarios from concrete ones so that they can be used to test early refinements of the model. We also show by example how a precise and concise domain specific language can be used for writing these abstract scenarios in a style that can be easily understood by the domain expert (for validation purposes) as well as the modeller (for behavioural verification). We base our approach on the Cucumber framework for scenarios and the Event-B modelling language and tool set. We illustrate the proposed methods on the ERTMS/ETCS Hybrid Level 3 specification for railway controls.
Abstract. We wish to model railway control systems in a formally precise way so that product lines can be adapted to specific customer requirements. Typically a customer is a railway operator with national conventions leading to different variation points based on a common core principle. A formal model of the core product must be precise and manipulatable so that different feature variations can be specified and verified without disrupting important properties that have already been established in the core product. Cyber-physical systems such as railway interlocking, are characterised by the combination of device behaviours resulting in an overall safe system behaviour. Hence there is a strong need for correct sequential operation with safety "interlocks" making up a process. We utilise diagrammatic modelling tools to make the core product more accessible to systems engineers. The RailGround example used to discuss these techniques is an open source model of a railway control system that has been made available by Thales Austria GmbH for research purpose, which demonstrates some fundamental modelling challenges.
Abstract. The Event Refinement Structures (ERS) approach provides a graphical extension of the Event-B formal method to represent event decomposition and control-flow explicitly. In this paper we present an improved version of the ERS plug-in, which provides a graphical environment for the ERS approach within the Event-B tool, Rodin. The improved ERS plug-in is based on the available frameworks that are developed to support Event-B with an EMF framework, language extensions and generic diagram extensions.
Abstract. We propose to extend iUML-B class-diagrams to elaborate Abstract Data Types (ADTs) specified using Event-B theories. Classes are linked to data types, while attributes and associations correspond to operators of the data types. Axioms about the data types and operators are specified as constraints on the class. We illustrate our approach on a development of a control system in the railway domain.
We demonstrate refinement-based formal development of the hybrid, 'fixed virtual block' approach to train movement control for the emerging European Rail Traffic Management System (ERTMS) level 3. Our approach uses iUML-B diagrams as a front end to the Event-B modelling language. We use abstraction to verify the principle of movement authority before gradually developing the details of the Virtual Block Detector component in subsequent refinements, thus verifying that it preserves the safety properties. We animate the refined models to demonstrate their validity using the scenarios from the Hybrid ERTMS Level 3 (HLIII) specification. We reflect on our team-based approach to finding useful modelling abstractions and demonstrate a systematic modelling method based on the state and class diagrams of iUML-B. The component and control flow architectures of the application, its environment and interacting systems emerge through the layered refinement process. The runtime semantics of the specification's state-machine behaviour are modelled in the final refinements. We discuss how the model could be used to generate an implementation using code generation tools and techniques. Abbreviations ADT Abstract Data Type RodinRodin platform ERS
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