Fault isolation is the process of reasoning required to find the cause of a system failure. In a model-based approach, the available information is a model of the system and some observations. Using knowledge of how the system generally behaves, as given in the system model, together with partial observations of the events of the current situation the task is to deduce the failure causing event(s). In our setting, the observable events manifest themselves in a message log. We study post mortem fault isolation for moderately concurrent discrete event systems where the temporal order of logged messages contains little information. To carry out fault isolation one has to study the correlation between observed events and fault events of the system. In general, such study calls for exploration of the state space of the system, which is exponential in the number of system components. Since we are studying a restricted class of all possible systems we may apply aggressive specialized abstraction policies in order to allow fault isolation without ever considering the often intractably large state space of the system. In this thesis we describe a mathematical framework as well as a prototype implementation and an experimental evaluation of such abstraction techniques. The method is efficient enough to allow for not only post mortem fault isolation but also design time diagnosability analysis of the system, which can be seen as a non-trivial way of analyzing all possible observations of the system versus the corresponding fault isolation outcome. This work has been supported by VINNOVA’s Competence Center ISIS.

Este trabalho reflete os esforcos realizados no estudo das principais técnicas automaticas de verificacao de sistemas que podem ser modelados em Maquinas de Estados Finitas, em particular as que normalmente se enquadram dentro da denominacao de model checking (verificacao de modelos). De modo a permitir ao leitor uma compreensao das vantagens e desvantagens de cada tecnica, os fundamentos teoricos de cada uma delas sao apresentados e ilustrados atraves de exemplos. Alem de uma apresentacao da teoria associada a cada tecnica, esta dissertação ainda apresenta dois estudos de caso de interesse bastante pratico: a verificacao de propriedades de um sistema de manufatura originalmente modelado atraves de uma rede de Petri e a verificacao de propriedades do intertravamento de uma seção metroviaria. Os dois estudos de caso utilizam tecnicas denominadas simbolicas. No primeiro estudo de caso, propoe-se que as invariantes obtidas da equação de estado sejam acrescentadas ao modelo a ser verificado, o que permite a obtenção de ganhos de desempenho na verificacao. O segundo estudo de caso e resolvido a partir da utilizacao de um procedimento proposto nesta dissertacao. Este procedimento permite a verificacao de algumas propriedades de seguranca sem que a verificacao se inviabilize devido a explosao no numero de estados. A utilizacao deste procedimento permite a verificacao de propriedades de uma secao de intertravamento com cerca de 2000 variaveis digitais em questao de poucos segundos.A principal conclusao a que este trabalho chega e consequencia dos resultados positivos observados nos estudos de caso: o model checking simbólico parece possuir um amplo campo de aplicacoes ainda por ser mais bem explorado / This work is the result of the efforts oriented to the study of the main automatic verification techniques for systems that can be modeled as Finite State Machines, in particular of those techniques which are generally called as model checking. In order to make the reader able to understand the pros and cons of each technique, the theory associated to each one is presented, as well as some examples. This work also presents two case studies of practical interest, both of each were solved with techniques which are called symbolic. The first one is the verification of some properties of a manufacturing system originally modeled by a Petri net. In order to improve the verification performance, it is proposed that the model to be verified be enlarged with the inclusion of the invariants calculated with the help of the state equation. The second case study is the verification of some safety properties of an interlocking system of a subway section. The verification is carried out with the help of a procedure which is proposed in this work. The aim of such a procedure is to bypass the state explosion problem, in order to make the verification feasible. It was possible to verify an interlocking system with about 2000 digital variables in a matter of few seconds. The main conclusion of the work comes from the positive results reached by both case studies: it seems to be a large number of applications yet to be explored in which symbolic model checking may be considered.

Este trabalho reflete os esforcos realizados no estudo das principais técnicas automaticas de verificacao de sistemas que podem ser modelados em Maquinas de Estados Finitas, em particular as que normalmente se enquadram dentro da denominacao de model checking (verificacao de modelos). De modo a permitir ao leitor uma compreensao das vantagens e desvantagens de cada tecnica, os fundamentos teoricos de cada uma delas sao apresentados e ilustrados atraves de exemplos. Alem de uma apresentacao da teoria associada a cada tecnica, esta dissertação ainda apresenta dois estudos de caso de interesse bastante pratico: a verificacao de propriedades de um sistema de manufatura originalmente modelado atraves de uma rede de Petri e a verificacao de propriedades do intertravamento de uma seção metroviaria. Os dois estudos de caso utilizam tecnicas denominadas simbolicas. No primeiro estudo de caso, propoe-se que as invariantes obtidas da equação de estado sejam acrescentadas ao modelo a ser verificado, o que permite a obtenção de ganhos de desempenho na verificacao. O segundo estudo de caso e resolvido a partir da utilizacao de um procedimento proposto nesta dissertacao. Este procedimento permite a verificacao de algumas propriedades de seguranca sem que a verificacao se inviabilize devido a explosao no numero de estados. A utilizacao deste procedimento permite a verificacao de propriedades de uma secao de intertravamento com cerca de 2000 variaveis digitais em questao de poucos segundos.A principal conclusao a que este trabalho chega e consequencia dos resultados positivos observados nos estudos de caso: o model checking simbólico parece possuir um amplo campo de aplicacoes ainda por ser mais bem explorado / This work is the result of the efforts oriented to the study of the main automatic verification techniques for systems that can be modeled as Finite State Machines, in particular of those techniques which are generally called as model checking. In order to make the reader able to understand the pros and cons of each technique, the theory associated to each one is presented, as well as some examples. This work also presents two case studies of practical interest, both of each were solved with techniques which are called symbolic. The first one is the verification of some properties of a manufacturing system originally modeled by a Petri net. In order to improve the verification performance, it is proposed that the model to be verified be enlarged with the inclusion of the invariants calculated with the help of the state equation. The second case study is the verification of some safety properties of an interlocking system of a subway section. The verification is carried out with the help of a procedure which is proposed in this work. The aim of such a procedure is to bypass the state explosion problem, in order to make the verification feasible. It was possible to verify an interlocking system with about 2000 digital variables in a matter of few seconds. The main conclusion of the work comes from the positive results reached by both case studies: it seems to be a large number of applications yet to be explored in which symbolic model checking may be considered.