Integrating model checking with HiP-HOPS in model-based safety analysis

The ability to perform an effective and robust safety analysis on the design of modern safety–critical systems is crucial. Model-based safety analysis (MBSA) has been introduced in recent years to support the assessment of complex system design by focusing on the system model as the central artefact, and by automating the synthesis and analysis of failure-extended models. Model checking and failure logic synthesis and analysis (FLSA) are two prominent MBSA paradigms. Extensive research has placed emphasis on the development of these techniques, but discussion on their integration remains limited. In this paper, we propose a technique in which model checking and Hierarchically Performed Hazard Origin and Propagation Studies (HiP-HOPS) – an advanced FLSA technique – can be applied synergistically with benefit for the MBSA process. The application of the technique is illustrated through an example of a brake-by-wire system

Integrated application of compositional and behavioural safety analysis

To address challenges arising in the safety assessment of critical engineering systems, research has recently focused on automating the synthesis of predictive models of system failure from design representations. In one approach, known as compositional safety analysis, system failure models such as fault trees and Failure Modes and Effects Analyses (FMEAs) are constructed from component failure models using a process of composition. Another approach has looked into automating system safety analysis via application of formal verification techniques such as model checking on behavioural models of the system represented as state automata. So far, compositional safety analysis and formal verification have been developed separately and seen as two competing paradigms to the problem of model-based safety analysis. This thesis shows that it is possible to move forward the terms of this debate and use the two paradigms synergistically in the context of an advanced safety assessment process. The thesis develops a systematic approach in which compositional safety analysis provides the basis for the systematic construction and refinement of state-automata that record the transition of a system from normal to degraded and failed states. These state automata can be further enhanced and then be model-checked to verify the satisfaction of safety properties. Note that the development of such models in current practice is ad hoc and relies only on expert knowledge, but it being rationalised and systematised in the proposed approach – a key contribution of this thesis. Overall the approach combines the advantages of compositional safety analysis such as simplicity, efficiency and scalability, with the benefits of formal verification such as the ability for automated verification of safety requirements on dynamic models of the system, and leads to an improved model-based safety analysis process. In the context of this process, a novel generic mechanism is also proposed for modelling the detectability of errors which typically arise as a result of component faults and then propagate through the architecture. This mechanism is used to derive analyses that can aid decisions on appropriate detection and recovery mechanisms in the system model. The thesis starts with an investigation of the potential for useful integration of compositional and formal safety analysis techniques. The approach is then developed in detail and guidelines for analysis and refinement of system models are given. Finally, the process is evaluated in three cases studies that were iteratively performed on increasingly refined and improved models of aircraft and automotive braking and cruise control systems. In the light of the results of these studies, the thesis concludes that integration of compositional and formal safety analysis techniques is feasible and potentially useful in the design of safety critical systems

Formale Verifikationsmethodiken für nichtlineare analoge Schaltungen

The objective of this thesis is to develop new methodologies for formal verification of nonlinear analog circuits. Therefore, new approaches to discrete modeling of analog circuits, specification of analog circuit properties and formal verification algorithms are introduced. Formal approaches to verification of analog circuits are not yet introduced into industrial design flows and still subject to research. Formal verification proves specification conformance for all possible input conditions and all possible internal states of a circuit. Automatically proving that a model of the circuit satisfies a declarative machine-readable property specification is referred to as model checking. Equivalence checking proves the equivalence of two circuit implementations. Starting from the state of the art in modeling analog circuits for simulation-based verification, discrete modeling of analog circuits for state space-based formal verification methodologies is motivated in this thesis. In order to improve the discrete modeling of analog circuits, a new trajectory-directed partitioning algorithm was developed in the scope of this thesis. This new approach determines the partitioning of the state space parallel or orthogonal to the trajectories of the state space dynamics. Therewith, a high accuracy of the successor relation is achieved in combination with a lower number of states necessary for a discrete model of equal accuracy compared to the state-of-the-art hyperbox-approach. The mapping of the partitioning to a discrete analog transition structure (DATS) enables the application of formal verification algorithms. By analyzing digital specification concepts and the existing approaches to analog property specification, the requirements for a new specification language for analog properties have been discussed in this thesis. On the one hand, it shall meet the requirements for formal specification of verification approaches applied to DATS models. On the other hand, the language syntax shall be oriented on natural language phrases. By synthesis of these requirements, the analog specification language (ASL) was developed in the scope of this thesis. The verification algorithms for model checking, that were developed in combination with ASL for application to DATS models generated with the new trajectory-directed approach, offer a significant enhancement compared to the state of the art. In order to prepare a transition of signal-based to state space-based verification methodologies, an approach to transfer transient simulation results from non-formal test bench simulation flows into a partial state space representation in form of a DATS has been developed in the scope of this thesis. As has been demonstrated by examples, the same ASL specification that was developed for formal model checking on complete discrete models could be evaluated without modifications on transient simulation waveforms. An approach to counterexample generation for the formal ASL model checking methodology offers to generate transition sequences from a defined starting state to a specification-violating state for inspection in transient simulation environments. Based on this counterexample generation, a new formal verification methodology using complete state space-covering input stimuli was developed. By conducting a transient simulation with these complete state space-covering input stimuli, the circuit adopts every state and transition that were visited during stimulus generation. An alternative formal verification methodology is given by retransferring the transient simulation responses to a DATS model and by applying the ASL verification algorithms in combination with an ASL property specification. Moreover, the complete state space-covering input stimuli can be applied to develop a formal equivalence checking methodology. Therewith, the equivalence of two implementations can be proven for every inner state of both systems by comparing the transient simulation responses to the complete-coverage stimuli of both circuits. In order to visually inspect the results of the newly introduced verification methodologies, an approach to dynamic state space visualization using multi-parallel particle simulation was developed. Due to the particles being randomly distributed over the complete state space and moving corresponding to the state space dynamics, another perspective to the system's behavior is provided that covers the state space and hence offers formal results. The prototypic implementations of the formal verification methodologies developed in the scope of this thesis have been applied to several example circuits. The acquired results for the new approaches to discrete modeling, specification and verification algorithms all demonstrate the capability of the new verification methodologies to be applied to complex circuit blocks and their properties.Gegenstand dieser Dissertation ist die Entwicklung neuer Methodiken zur formalen Verifikation nichtlinearer analoger elektronischer Schaltungen. Dazu werden im Rahmen dieser Arbeit entstandene neue Ansätze in den Bereichen verifikationsgerechte diskrete Modellierung analoger Schaltungen, Spezifikation analoger Schaltungseigenschaften und formale Verifikationsalgorithmen vorgestellt. Ausgehend vom Stand der Technik der Modellierung analoger Schaltungen für die simulationsbasierte Verifikation wird im Rahmen dieser Arbeit die diskrete Modellierung analoger Schaltungen für zustandsraumbasierte formale Verifikationsverfahren betrachtet. Dazu wurde ein neuer Ansatz zur diskreten Modellierung entwickelt, der die Aufteilungsstruktur anhand der Trajektorien der Vektorfelddynamik bestimmt. So wird eine hohe Genauigkeit der Nachfolgerrelation ermöglicht, woraus eine niedrigere Zahl an Zuständen für ein diskretes Modell gleicher Genauigkeit im Vergleich mit dem bisherigen Stand der Technik folgt. Die Abbildung der Trajektorien-gesteuerten Partitionierung auf eine diskrete analoge Transitionsstruktur (DATS) erlaubt die Anwendung von formalen Verifikationsalgorithmen. Die formale Spezifikation von Eigenschaften in ersten Ansätzen zum Model Checking analoger Schaltungen hat sich stark an den bestehenden temporallogischen Verfahren aus dem Bereich digitaler Hardware orientiert. Ausgehend von einer Analyse digitaler Spezifikationskonzepte und der bestehenden Ansätze für analoge Eigenschaften wurden Anforderungen an eine neue Spezifikationssprache in dieser Arbeit abgeleitet. Die aus diesen Anforderungen im Rahmen dieser Arbeit entwickelte analoge Spezifikationssprache "Analog Specification Language" (ASL) basiert auf einer natürlichsprachlichen Kapselung temporallogischer Operationen, die mit erweiterten Algorithmen zur Transitionspfadbestimmung, Durchführung von Berechnungen auf Zustandsparametern und Oszillationsbestimmung eine hohe Ausdrucksstärke analoger Eigenschaften mit einer anwenderfreundlichen Syntax kombinieren konnte. Die zusammen mit ASL entwickelten Model Checking-Verifikationsalgorithmen zur Auswertung von ASL-Spezifikationen auf einem mit dem Trajektorien-gesteuerten Diskretisierungsverfahren erzeugten DATS-Modell bilden eine wesentliche Erweiterung zum Stand der Technik. Um einen Übergang der Verifikation von signalbasierten zu zustandsraumbasierten Methodiken zu ermöglichen, wurde im Rahmen dieser Arbeit ein Ansatz entwickelt, der die Übertragung von transienten Simulationsergebnissen aus nicht-formalen Testbench-Simulationsumgebungen in eine partielle DATS-Zustandsraumdarstellung ermöglicht. Damit kann, wie anhand von Beispielen gezeigt werden konnte, die gleiche ASL-Spezifikation für Eigenschaften eines vollständigen diskreten Modells ohne Modifikation auch auf Simulationsergebnissen ausgewertet werden. Ein für das formale ASL-basierte Model Checking entwickelter Ansatz zur Erzeugung von Gegenbeispielen für als spezifikationsverletzend identifizierte Zustandsraumgebiete erlaubt es, Transitionsfolgen von einem definierten Startzustand zu einem spezifikationsverletzenden Zustand zu ermitteln. Auf Basis dieses Gegenbeispiel-Verfahrens wurde eine neue formale Eigenschaftsverifikationsmethodik mittels vollständig den Zustandsraum einer Schaltung abdeckenden Eingangsstimuli entwickelt. Die vollständig den Zustandsraum abdeckenden Eingangsstimuli bieten noch eine weitere Anwendungsmöglichkeit im Bereich des Äquivalenzvergleichs. Die im Rahmen dieser Arbeit entwickelte Methodik zum formalen Äquivalenzvergleich auf Basis der vollständig den Zustandsraum abdeckenden Eingangsstimuli ersetzt die anwenderdefinierten Eingangsstimuli durch die vollständig den Zustandsraum abdeckenden. So kann die Äquivalenz für jeden möglichen Zustand der zu vergleichenden Implementierungen anhand eines automatisierten Vergleichs der Simulationsergebnisse beider Implementierungen gezeigt werden. Um die Ergebnisse der neu eingeführten formalen Verifikationsmethodiken visuell zu untersuchen wurde ein Verfahren entwickelt, das den Zustandsraum und seine Dynamik mittels eines Partikel-Simulationsansatzes visualisiert. Da die Partikel über den gesamten Zustandsraum randomisiert verteilt werden und sich dann gemäß der Vektorfelddynamik fortbewegen, kann auch hier ein Einblick in das Systemverhalten gewonnen werden, der eine weitestgehend vollständige und somit formale Repräsentation des Zustandsraums bietet. Die prototypische Implementierung der im Rahmen dieser Arbeit entwickelten formalen Verifikationsmethodiken wurde auf zahlreiche Beispielschaltungen angewendet. Die Ergebnisse für die neuen Ansätze zur diskreten Modellierung, zur Spezifikation und zu Verifikationsalgorithmen analoger Schaltungen zeigen, dass die aus diesen Ansätzen erzeugten Verifikationsmethodiken erfolgreich auf komplexe Zustandsraumstrukturen angewendet werden können

The Symmetry Method for Coloured Petri Nets

This booklet is the author's PhD-dissertation

A Comparison of the Declarative Modelling Languages B, Dash, and TLA+

Declarative behavioural modelling is a powerful modelling paradigm that enables users to model system func- tionality abstractly and concisely. We compare two well-used formal declarative modelling languages, B and TLA+, with a new modelling language called Dash. Dash is an extension of Alloy with explicit syntactic constructs for modelling transition systems, and it includes control state hierarchy and events. Particular topics that we cover in our comparison are: differences in the datatypes and type systems; how the transitions/operations can be described; how the transition relation is a combination of the transitions; and the default choice each language makes regarding permitted variable changes in a transition. Our goal is to discuss the interesting differentiating characteristics of each language to aid users in determining which language is the most suitable for their system

A systems biology approach to multi-scale modelling and analysis of planar cell polarity in drosophila melanogaster wing

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Systems biology aims to describe and understand biology at a global scale where biological systems function as a result of complex mechanisms that happen at several scales. Modelling and simulation are computational tools that are invaluable for description, understanding and prediction these mechanisms in a quantitative and integrative way. Thus multi-scale methods that couple the design, simulation and analysis of models spanning several spatial and temporal scales is becoming a new emerging focus of systems biology. This thesis uses an exemplar – Planar cell polarity (PCP) signalling – to illustrate a generic approach to model biological systems at different spatial scales, using the new concept of Hierarchically Coloured Petri Nets (HCPN). PCP signalling refers to the coordinated polarisation of cells within the plane of various epithelial tissues to generate sub-cellular asymmetry along an axis orthogonal to their apical-basal axes. This polarisation is required for many developmental events in both vertebrates and non-vertebrates. Defects in PCP in vertebrates are responsible for developmental abnormalities in multiple tissues including the neural tube, the kidney and the inner ear. In Drosophila wing, PCP is seen in the parallel orientation of hairs that protrude from each of the approximately 30,000 epithelial cells to robustly point toward the wing tip. This work applies HCPN to model a tissue comprising multiple cells hexagonally packed in a honeycomb formation in order to describe the phenomenon of Planar Cell Polarity (PCP) in Drosophila wing. HCPN facilitate the construction of mathematically tractable, compact and parameterised large-scale models. Different levels of abstraction that can be used in order to simplify such a complex system are first illustrated. The PCP system is first represented at an abstract level without modelling details of the cell. Each cell is then sub-divided into seven virtual compartments with adjacent cells being coupled via the formation of intercellular complexes. A more detailed model is later developed, describing the intra- and inter-cellular signalling mechanisms involved in PCP signalling. The initial model is for a wild-type organism, and then a family of related models, permitting different hypotheses to be explored regarding the mechanisms underlying PCP, are constructed. Among them, the largest model consists of 800 cells which when unfolded yields 164,000 places (each of which is described by an ordinary differential equation). This thesis illustrates the power and validity of the approach by showing how the models can be easily adapted to describe well-documented genetic mutations in the Drosophila wing using the proposed approach including clustering and model checking over time series of primary and secondary data, which can be employed to analyse and check such multi-scale models similar to the case of PCP. The HCPN models support the interpretation of biological observations reported in literature and are able to make sensible predictions. As HCPN model multi-scale systems in a compact, parameterised and scalable way, this modelling approach can be applied to other large-scale or multi-scale systems.This study was funded by Brunel University

Qualitative Process Analysis : Theoretical Requirements and Practical Implementation in Naval Domain

Understanding complex behaviours is an essential component of everyday life, integrated into daily routines as well as specialised research. To handle the increasing amount of data available from (logistic) dynamic scenarios, analysis of the behaviour of agents in a given environment is becoming more automated and thus requires reliable new analytical methods. This thesis seeks to improve analysis of observed data in dynamic scenarios by developing a new model for transforming sparse behavioural observations into realistic explanations of agent behaviours, with the goal of testing that model in a real-world maritime navigation scenario