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Discrete Event Systems, 2008. WODES 2008. 9th International Workshop on

Date 28-30 May 2008

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Displaying Results 1 - 25 of 90
  • On control and cyber-physical systems: Challenges and opportunities for discrete event and hybrid systems

    Page(s): 1
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (86 KB)  

    Summary form only given. Networked digital devices interacting with physical and engineered systems are becoming ubiquitous and will be more so in the future. A deep understanding of the interactions between the cyber and physical worlds is essential in being able to build robust and safe systems, and this issue is receiving increased attention in the US by the networking and information technology community. Stronger cross-fertilization of ideas between control systems, networking and computer science is needed in both research and education to build these systems. In this talk, the main issues in cyber-physical systems will be discussed first in detail. A brief overview of a supervisory control approach in discrete event systems using Petri nets will be given with connections to computer science problems and a promising approach to design cyber-physical systems using passivity that include human interaction will be outlined. View full abstract»

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  • Covering-based supervisory control of partially observed discrete event systems for state avoidance

    Page(s): 2 - 8
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (270 KB) |  | HTML iconHTML  

    In this work we present a new polynomial complexity approach to state avoidance for nondeterministic and partially observed discrete event systems. Our approach generates control based on a covering of the system state space that identifies overlapping sets of indistinguishable states. This approach is shown to be more permissive than existing state-feedback control techniques. View full abstract»

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  • Coordination control of discrete-event systems

    Page(s): 9 - 15
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (168 KB) |  | HTML iconHTML  

    The concept of a coordinator is proposed for control of modular discrete-event systems. The coordinator makes all subsystems conditionally independent generators as defined in the paper. The coordinator receives part of the partial observations of the subsystems and its task is to satisfy the global part of the specification and of the nonblockingness. The complete supervisor then consists of the coordinator, its supervisor, and the local supervisors for the subsystems. An example of control of a distributed discrete-event system shows that a coordinator is necessary for achieving safety and nonblockingness. View full abstract»

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  • Yet another approach to compositional synthesis of discrete event systems

    Page(s): 16 - 21
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (204 KB) |  | HTML iconHTML  

    A two-pass algorithm for compositional synthesis of modular supervisors for large-scale systems of composed finite-state automata is proposed. The first pass provides an efficient method to determine whether a supervisory control problem has a solution, without explicitly constructing the synchronous composition of all components. If a solution exists, the second pass yields an over-approximation of the least restrictive solution which, if nonblocking, is a modular representation of the least restrictive supervisor. Using a new type of equivalence of nondeterministic processes, called synthesis equivalence, a wide range of abstractions can be employed to mitigate state-space explosion throughout the algorithm. View full abstract»

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  • Supervisory control of DES with distinguishing sensors

    Page(s): 22 - 27
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (234 KB) |  | HTML iconHTML  

    This paper deals with supervisory control of discrete event systems with the introduction of sensors that distinguish certain occurrences of observable events. Distinguishing sensors are formally represented by an entity named distinguisher that maps the original events into disjoint sets of refined events and specify which of them could actually occur according to the evolution of the system. The control problem is formulated to incorporate distinguishers in order to have specifications that are presumably more easily modeled in the refined alphabet. It is proved that minimally restrictive nonblocking supervisors can be obtained by incorporating the distinguisher in the plant model. Further it is shown that suboptimal solutions can be efficiently obtained from an external approximation of the refined plant. Conditions for optimality are provided together with a polynomial complexity algorithm to verify them. A manufacturing system example illustrates the results. View full abstract»

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  • Opacity enforcing control synthesis

    Page(s): 28 - 35
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (342 KB) |  | HTML iconHTML  

    Given a finite transition system and a regular predicate, we address the problem of computing a controller enforcing the opacity of the predicate against an attacker (that partially observes the system), supposedly trying to push the system to reveal the predicate. Assuming that the controller can only control a subset of the events it observes (possibly different from the ones of the attacker), we show that an optimal control always exists and provide sufficient conditions under which it is regular and effectively computable. These conditions rely on the inclusion relationships between the observable alphabets of the attacker and the controller and the controllable alphabet. View full abstract»

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  • Max-plus algebra and max-plus linear discrete event systems: An introduction

    Page(s): 36 - 42
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (190 KB) |  | HTML iconHTML  

    We provide an introduction to the max-plus algebra and explain how it can be used to model a specific class of discrete event systems with synchronization but no concurrency. Such systems are called max-plus linear discrete event systems because they can be described by a model that is ldquolinearrdquo in the max-plus algebra. We discuss some key properties of the max-plus algebra and indicate how these properties can be used to analyze the behavior of max-plus linear discrete event systems. We also briefly present some control approaches for max-plus linear discrete event systems, including model predictive control. Finally, we discuss some extensions of the max-plus algebra and of max-plus linear systems. View full abstract»

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  • Performance analysis of linear systems over semiring with additive inputs

    Page(s): 43 - 48
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (305 KB) |  | HTML iconHTML  

    This paper deals with the computation of a maximal flow in single input single output (max, +) linear systems. Assuming known a system composed of some subsystems - each one being described by a transfer function and some secondary inputs interfering with the principal flow on consecutive sub-systems, the computation of a maximal principal output is addressed. Transfer functions, inputs and outputs are represented by periodical series in a semiring of formal series, namely Nopfmindelta. Previously, it is shown that the Hadamard product of such series allows to compute the addition of inputs, and that this product is both residuated and dually residuated. These properties are used to compute the maximal principal output. An example concludes the paper and allows to illustrate the efficiency of the proposed approach. View full abstract»

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  • A fast approximation algorithm for the Lyapunov exponent of stochastic max-plus systems

    Page(s): 49 - 54
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (179 KB) |  | HTML iconHTML  

    This paper addresses the problem of approximately computing the Lyapunov exponent of stochastic maxplus linear systems. Our approach allows for an efficient simulation of bounds for the Lyapunov exponent. Depending on the simulation budget the bounds can be made arbitrarily close. We illustrate the effectiveness of our bounds with application to (real-life) railway systems. View full abstract»

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  • Control of (max,+) automata: Logical and timing aspects

    Page(s): 55 - 60
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (184 KB) |  | HTML iconHTML  

    A new framework for control of (max,+) automata is introduced. The tensor product of their linear representations used in this paper is an extension of parallel composition from Boolean to (max,+) automata and can be nicely applied to both logical and timing aspects of supervisory control. Case of uncontrollable events that can neither be disabled nor delayed is studied within a behavioral framework. Optimal (least restrictive) control of (max,+) automata is studied using residuation theory applied to Hadamard product of (multivariable) formal power series. View full abstract»

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  • SAT-solving in practice

    Page(s): 61 - 67
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (168 KB) |  | HTML iconHTML  

    Satisfiability solving, the problem of deciding whether the variables of a propositional formula can be assigned in such a way that the formula evaluates to true, is one of the classic problems in computer science. It is of theoretical interest because it is the canonical NP-complete problem. It is of practical interest because modern SAT-solvers can be used to solve many important and practical problems. In this tutorial paper, we show briefly how such SAT-solvers are implemented, and point to some typical applications of them. Our aim is to provide sufficient information (much of it through the reference list) to kick-start researchers from new fields wishing to apply SAT-solvers to their problems. View full abstract»

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  • Beyond Boolean SAT: Satisfiability modulo theories

    Page(s): 68 - 73
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (159 KB) |  | HTML iconHTML  

    Many systems can be naturally represented in some decidable fragments of first order logic. The expressive power provided by a background theory allows to describe important aspects such as real time, continuous dynamics, and data flow over integer variables. The corresponding verification problems can be tackled by means of Satisfiability Modulo Theory (SMT) solvers. SMT solvers are based on the tight integration of propositional SAT solvers with dedicated procedures to reason about the theory component. In this paper, we overview the techniques underlying SMT, we show how to represent dynamic systems in fragments of first order logic, and discuss the application of SMT solvers to their verification. View full abstract»

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  • Practical applications of Boolean Satisfiability

    Page(s): 74 - 80
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (207 KB) |  | HTML iconHTML  

    Boolean satisfiability (SAT) solvers have been the subject of remarkable improvements since the mid 90s. One of the main reasons for these improvements has been the wide range of practical applications of SAT. Indeed, examples of modern applications of SAT range from termination analysis in term-rewrite systems to circuit-level prediction of crosstalk noise. The success of SAT solvers motivated many practical applications, but many practical applications have also provided the examples and the challenges that allowed the development of more efficient SAT solvers. This paper provides an overview of some of the most well-known applications of SAT and outlines several other successful applications of SAT. Moreover, the improvements in SAT solvers motivated the development of new algorithms for strategic extensions of SAT. As a result, the paper also provides a brief survey of recent work on extensions of SAT, including pseudo-Boolean constraints, maximum satisfiability, model counting and quantified Boolean formulas. View full abstract»

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  • Supervisory control using satisfiability solvers

    Page(s): 81 - 86
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (150 KB) |  | HTML iconHTML  

    This paper discusses how satisfiability solvers may be used to verify and synthesize discrete event supervisors as defined in the supervisory control theory. By using the supervisory control theory it is possible to generate control functions that are correct by construction. However, the computations for verification and synthesis of the supervisors are NP-complete and in order to make the method applicable for industrial use it is necessary to use algorithms and tools that could solve problems of industrial size. Within the model checking community satisfiability solvers have become an important tool for verification of large hardware circuits. In this paper it is shown how to formulate some problems in the supervisory control theory as Boolean satisfiability problems. Formulations of satisfiability problems for synthesizing a path to a marked state, verification of controllability and verification of deadlock presence are presented. The method is evaluated on some examples of high complexity. View full abstract»

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  • IPA for flow-control fluid queues with delays

    Page(s): 87 - 92
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (354 KB) |  | HTML iconHTML  

    This paper concerns the application of Infinitesimal Perturbation Analysis (IPA) to a fluid queue with flow control. The control law restricts the inflow rate to the queue by an amount that is proportional to the loss rate, and the control signal incurs a delay. The performance measure consists of a weighted sum of the loss volume and cumulative workload, and it is treated as a function of the buffer limit. The IPA derivative is derived, and its use in optimization is demonstrated via a simulation example. Published results on IPA for flow-control systems assume that the control signal is instantaneous, and the main contribution of this paper is in a technique for analysis of a system with delayed control. The derivation of the IPA derivative for the loss volume is presented in detail, and analogous results for the cumulative workload are highlighted. View full abstract»

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  • Stochastic optimization for control of maintenance systems

    Page(s): 93 - 98
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (243 KB) |  | HTML iconHTML  

    This paper addresses stochastic optimization of a multi-component maintenance system. In particular, we present a novel phantom gradient estimator called CPhET. We compare the performance of the CPhET estimator with that of the general purpose method simultaneous perturbation (SP). Our results indicate that CPhET is more suitable than SP for sensitivity analysis of moderately sized systems. In addition, CPhET is preferable to SP for optimization of systems if the dimension of the vector of control parameters is small. View full abstract»

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  • Perturbation analysis of Markov chains

    Page(s): 99 - 104
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (218 KB) |  | HTML iconHTML  

    We present a new approach to perturbation analysis of Markov chains. Our analysis is based on bounding the distance of stationary distributions in a suitable functional space. View full abstract»

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  • Optimal node visitation in acyclic stochastic digraphs with multi-threaded traversals and internal visitation requirements

    Page(s): 105 - 112
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (237 KB) |  | HTML iconHTML  

    The original definition of the problem of optimal node visitation (ONV) in acyclic stochastic digraphs concerns the identification of a routing policy that will enable the visitation of each leaf node a requested number of times, while minimizing the expected number of the graph traversals. The original work of (T. Bountourelis et al., 2006) formulated this problem as a stochastic shortest path (SSP) problem, and since the state space of this SSP formulation is exponentially sized with respect to the number of the target nodes, it also proposed a suboptimal policy that is computationally tractable and asymptotically optimal. This paper extends the results of (T. Bountourelis et al., 2006) to the cases where (i) the tokens traversing the graph can ldquosplitrdquo during certain transitions to a number of (sub-)tokens, allowing, thus, the satisfaction of many visitation requirements during a single graph traversal, and (ii) there are additional visitation requirements attached to the internal graph nodes, which, however, can be served only when the visitation requirements of their successors have been fully met. In addition, the presented set of results establishes stronger convergence properties for the proposed suboptimal policies, and it provides a formal complexity analysis of the considered ONV formulations. From a practical standpoint, the extension of the original results performed in this paper enables their effective usage in the application domains that motivated the ONV problem, in the first place. View full abstract»

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  • Performance analysis of χ models using discrete-time probabilistic reward graphs

    Page(s): 113 - 118
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (629 KB) |  | HTML iconHTML  

    We propose the model of discrete-time probabilistic reward graphs (DTPRGs) for performance analysis of systems exhibiting discrete deterministic time delays and probabilistic behavior, via their interpretation as discrete-time Markov reward chains, full-fledged platform for qualitative and quantitative analysis of timed systems based on the modeling language chi. The extension proposed in this paper is based on timed branching bisimulation reduction followed by a tailored inclusion of probabilities and rewards. The approach is applied in an industrial case study of a turntable drill. The resulting performance measures are shown to be comparable to those obtained by existent methods of the chi environment, viz. simulation and continuous-time Markovian analysis. View full abstract»

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  • STSLib and its application to two benchmarks

    Page(s): 119 - 124
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (218 KB) |  | HTML iconHTML  

    State tree structures (STS) are an adaptation of statecharts to supervisory control theory. STSLib is a C++ library that we have developed to support the symbolic analysis and synthesis of STS. This paper presents a short introduction to the library, and then applies the library to two benchmarks: (1) cat and mouse tower (CMT), (2) dining philosophers (DP). We demonstrate that STSLib can design optimal nonblocking supervisors for systems of state size up to 10626, and the resulting controllers are tractable and readily comprehensible. View full abstract»

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  • libFAUDES — An open source C++ library for discrete event systems

    Page(s): 125 - 130
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (184 KB) |  | HTML iconHTML  

    The libFAUDES (Friedrich-Alexander University Discrete Event Systems) library is an open source C++ software library for discrete event systems (DES) that is developed at the University of Erlangen-Nuremberg. The core library supports methods for the DES analysis and supervisor synthesis, while a built-in plugin mechanism allows of specialized library extensions. In this paper, we evaluate libFAUDES according to the benchmark examples provided for the Workshop on Discrete Event Systems 2008. View full abstract»

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  • Solving two supervisory control benchmark problems using Supremica

    Page(s): 131 - 136
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (176 KB) |  | HTML iconHTML  

    Two supervisory control benchmark problems for WODESpsila08 are solved using the tool Supremica. Supremica is a tool for formal synthesis of discrete-event control functions based on discrete event models of the uncontrolled plant and specifications of the desired closed-loop behavior. By using formal synthesis of control functions the need for formal verification is reduced since the control functions are computed to automatically fulfill the given specifications, that is, they are ldquocorrect by constructionrdquo. The modeling framework in Supremica is based on finite automata. Supremica implements several techniques for being able to solve large scale problems. In this paper it is evaluated how the algorithms implemented in Supremica that are based on binary decision diagrams performs on the two benchmark problems. The two benchmark problems are generalization of two classical problems; cat and mouse, and the dining philosopherspsila problem. The benchmark problems are parameterized such that it is possible to create problem instances with huge state-spaces. The benchmark shows that supremica can efficiently solve rather large problem instances. View full abstract»

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  • Performing fault diagnosis for PNs using g-markings: A benchmark case

    Page(s): 137 - 143
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (300 KB) |  | HTML iconHTML  

    The authors have recently presented an approach to fault diagnosis based on the online computation of the set of possible fault events required to explain the last observed event. This is efficiently achieved by using Petri nets, since their mathematical representation allows us to formulate the fault diagnosis problems in terms of mathematical programming, which is a standard tool. Then, an interpreted diagnoser is obtained. In this paper the performance of such a diagnoser are evaluated on a benchmark example. View full abstract»

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  • A comparison between two diagnostic tools based on automata and Petri nets

    Page(s): 144 - 149
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (179 KB) |  | HTML iconHTML  

    In this paper we consider two diagnosis procedures for discrete event systems based respectively on automata and Petri nets. First we compare them in terms of applicability and generality. Secondly, we apply them to the WODES diagnosis benchmark and compare them in terms of computational complexity. As a result we conclude that the automata procedure is more general, but the Petri net approach presents significant advantages in terms of computational complexity. View full abstract»

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  • Scalable optimization algorithms for discrete event systems with real-time constraints: An overview of recent developments

    Page(s): 150 - 155
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (217 KB) |  | HTML iconHTML  

    Discrete event systems with real-time constraints involve tasks that must be completed within specified hard deadlines. Guaranteeing such constraints requires a control effort measured through a cost function, giving rise to a class of nonlinear optimization problems. We overview recent developments aimed at solving such problems in an efficient scalable fashion suitable for on-line applications with limited computational resources. We describe how structural properties of the optimal state trajectory in such problems can be exploited and present a critical task decomposition algorithm (CTDA) when tasks are processed by a single-stage system. When tasks are processed over multiple stages and are subject to end-to-end real-time constraints, a different set of structural properties is exploited leading to a virtual deadline algorithm (VDA). View full abstract»

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