Showing papers on "Petri net published in 1997"

Verification of Workflow Nets

Abstract: Workflow management systems will change the architecture of future information systems dramatically. The explicit representation of business procedures is one of the main issues when introducing a workflow management system. In this paper we focus on a class of Petri nets suitable for the representation, validation and verification of these procedures. We will show that the correctness of a procedure represented by such a Petri net can be verified by using standard Petri-net-based techniques. Based on this result we provide a comprehensive set of transformation rules which can be used to construct and modify correct procedures.

Deadlock analysis of Petri nets using siphons and mathematical programming

Abstract: This paper exploits the potential of siphons for the analysis of Petri nets, It generalizes the well-known Commoner condition and is based on the notion of potential deadlocks which are siphons that eventually become empty. A linear programming based sufficient condition under which a siphon is not a potential deadlock is obtained. Based on the new sufficient condition, a mathematical programming approach and a mixed-integer programming approach are proposed for checking general Petri nets and structurally bounded Petri nets respectively without explicitly generating siphons. Stronger results are obtained for asymmetric choice nets and augmented marked graphs. In particular, we show that an asymmetric choice net is live iff it is potential-deadlock-free and an augmented marked graph is live and reversible iff it is potential-deadlock-free.

Petrify: a tool for manipulating concurrent specifications and synthesis of asynchronous controllers

Abstract: Petrify is a tool for (1) manipulating concurrent specifications and (2) synthesis and optimization of asynchronous control circuits. Given a Petri Net (PN), a Signal Transition Graph (STG), or a Transition System (TS) it (1) generates another PN or STG which is simpler than the original description and (2) produces an optimized net-list of an asynchronous controller in the target gate library while preserving the specified input-output behavior. An ability of back-annotating to the specification level helps the designer to control the design process. For transforming a specification petrify performs a token flow analysis of the initial PN and produces a transition system (TS). In the initial TS, all transitions with the same label are considered as one event. The TS is then transformed and transitions relabeled to fulfill the conditions required to obtain a safe irredundant PN. For synthesis of an asynchronous circuit petrify performs state assignment by solving the Complete State Coding problem. State assignment is coupled with logic minimization and speed-independent technology mapping to a target library. The final net-list is guaranteed to be speed-independent, i.e., hazard-free under any distribution of gate delays and multiple input changes satisfying the initial specification. The tool has been used for synthesis of PNs and PNs composition, synthesis and re-synthesis of asynchronous controllers and can be also applied in areas related with the analysis of concurrent programs. This paper provides an overview of petrify and the theory behind its main functions.

A Survey of Petri Net Methods for Controlled Discrete EventSystems

Abstract: This paper surveys recent research on the application of Petri net models to the analysis and synthesis of controllers for discrete event systems. Petri nets have been used extensively in applications such as automated manufacturing, and there exists a large body of tools for qualitative and quantitative analysis of Petri nets. The goal of Petri net research in discrete event systems is to exploit the structural properties of Petri net models in computationally efficient algorithms for computing controls. We present an overview of the various models and problems formulated in the literature focusing on two particular models, the controlled Petri nets and the labeled nets. We describe two basic approaches for controller synthesis, based on state feedback and event feedback. We also discuss two efficient techniques for the on-line computation of the control law, namely the linear integer programming approach which takes advantage of the linear structure of the Petri net state transition equation, and path-based algorithms which take advantage of the graphical structure of Petri net models. Extensions to timed models are briefly described. The paper concludes with a discussion of directions for future research.

Decidability of model checking for infinite-state concurrent systems

Abstract: We study the decidability of the model checking problem for linear and branching time logics, and two models of concurrent computation, namely Petri nets and Basic Parallel Processes.

Petri Nets, Commutative Context-Free Grammars, and Basic Parallel Processes

Abstract: The paper provides a structural characterisation of the reachable markings of Petri nets in which every transition has exactly one input place. As a corollary, the reachability problem for this class is proved to be NP-complete. Further consequences are: the uniform word problem for commutative context-free grammars is NP-complete; weak-bisimilarity is semidecidable for Basic Parallel Processes.

A Brief Introduction to Coloured Petri Nets

Abstract: Coloured Petri Nets (CP-nets or CPN) is a graphical oriented language for design, specification, simulation and verification of systems. It is in particular well-suited for systems in which communication, synchronisation and resource sharing are important. Typical examples of application areas are communication protocols, distributed systems, imbedded systems, automated production systems, work flow analysis and VLSI chips.

Supervisory control of discrete event systems based on structure theory of Petri nets

Abstract: The present work is related to the use of Petri nets structural techniques in the supervisory control of discrete event systems. A relevant property of the system behaviour under supervision is to be non-blocking, i.e. from any state reachable from initial state, it is always possible to reach a desirable (or marked) state. Recent works had shown that the synthesis of proper supervisors based on Petri net modelling of DES is an interesting approach. In this paper, we present a proper supervisor synthesis method based on a purely structural reasoning. This parametrized method is especially well-suited for a large class of discrete event systems, called G-Task, for modelling concurrent automated manufacturing systems with flexibility on routings and on synchronization patterns with shared resources. Also, it can be exploited for enforcing constraints on the reachability set of any bounded uncontrolled net.

A new matrix model for discrete event systems: application to simulation

Abstract: Simulation schemes for discrete event (DE) systems based on a new DE matrix formulation are presented. This new formulation is a hybrid system with logical and algebraic components that allows fast, direct design and reconfiguration of rule-based controllers for manufacturing systems. It applies to general DE systems that include shared resources, dispatching, circular waits, and variable part routing. A certain DE matrix state equation together with the familiar Petri net marking transition equation yield a complete dynamical description of a DE system. Our goal in this article is to show that this provides a simplified computer tool that allows efficient simulation and modeling for DE systems.

Power systems fault diagnosis using Petri nets

Abstract: A new method of detecting fault locations in power networks is proposed. The diagnostic process uses Petri net models. This can reduce processing time and increase accuracy when compared with the traditional pattern recognition and hypotheses testing methods. The method is more able than previous methods to adapt to changing network configurations. The methodology was tested using the IEEE 118-bus standard test system. Fast and accurate results were obtained. The method is very suitable for online applications.

On the existence of supervisory policies that enforce liveness in discrete-event dynamic systems modeled by controlled Petri nets

Abstract: We consider discrete-state plants represented by controlled Petri nets (CtlPNs), where a subset of transitions can be prevented from firing by a supervisor. A transition in a CtlPN can fire at a marking if there are sufficient tokens in its input places and it is permitted to fire by the supervisor. A CtlPN is live if it is possible to fire any transition from every marking that is reachable under supervision. In this paper we derive a necessary and sufficient condition for the existence of a supervisory policy that enforces liveness in CtlPNs. We show this condition cannot be tested for an arbitrary CtlPN. However, for bounded CtlPNs or CtlPNs, where each transition is individually controllable, we show the existence of a supervisory policy which enforces that liveness is decidable. We also show the existence of a supervisory policy that enforces liveness is necessary and sufficient for the existence of a minimally restrictive supervisory policy.

Life-Cycle Inheritance: A Petri-Net-Based Approach

Abstract: Inheritance is one of the key issues of object-orientation. The inheritance mechanism allows for the definition of a subclass which inherits the features of a specific superclass. This means that methods and attributes defined for the superclass are also available for objects of the subclass. Existing methods for object-oriented modeling and design abstract from the dynamic behavior of objects when defining inheritance. Nevertheless, it would be useful to have a mechanism which allows for the inheritance of dynamic behavior. This paper describes a Petri-net-based approach to the formal specification and verification of this type of inheritance. We use Petri nets to specify the dynamics of an object class. The Petri-net formalism allows for a graphical representation of the life cycle of objects which belong to a specific object class. Four possible inheritance relations are defined. These inheritance relations can be verified automatically. Moreover, four powerful transformation rules which preserve specific inheritance relations are given. To illustrate the relevance of these results, the application to workflow management is demonstrated.

A Compositional Partial Order Semantics for Petri Net Components

Abstract: In this paper we introduce the concept of a Petri net component and show how systems can be composed from components. A component communicates with its environment via distinguished input and output places, which formalizes communication by message passing. Then, we present a compositional semantics for components. The semantics is an extension of processes for place/transition systems (partial order semantics). We show that the semantics is fully abstract with respect to the behaviour of closed components (essentially, processes of place/transition systems). A main feature of the compositional semantics is that composition of components corresponds to conjunction. This feature makes the compositional semantics applicable in combination with a temporal logic, which then allows to reason about systems in a compositional way. This is demonstrated by help of a simple temporal logic.

Multi-Agent-Systems Based on Coloured Petri Nets

Abstract: Based on Y. Shoham's paradigm, called Agent-Oriented Programming (AOP), multi-agent-systems are presented as a specialization of distributed, Object-Oriented systems. Equipped with knowledge, general concurrent inference mechanisms dealing with this knowledge, and a declarative agent program, these multi-agent-systems are intended to be a foundation of a new approach uniting advantages of many contributing areas: The precise semantics of Petri nets, the abstraction and encapsulation proposed in Object-Oriented approaches, and the power of logic programming, making it easy to adopt well-known AI-methods. As an example, an urban traffic information system will be designed which solves path searching problems in a distributed graph.

A Petri net synthesis theory for modeling flexible manufacturing systems

Abstract: A theory that synthesizes Petri nets for modeling flexible manufacturing systems is presented. The theory adopts a bottom-up or modular-composition approach to construct net models. Each module is modeled as a resource control net (RCN), which represents a subsystem that controls a resource type in a flexible manufacturing system. Interactions among the modules are described as the common transition and transition subnets. The net obtained by merging the modules with two minimal restrictions is shown to be conservative and thus bounded. An algorithm is developed to detect two sufficient conditions for structural liveness of the net. The algorithm examines only the net's structure and the initial marking, and appears to be more efficient than state enumeration techniques such as the reachability tree method. In this paper, the sufficient conditions for liveness are shown to be related to some structural objects called siphons. To demonstrate the applicability of the theory, a flexible manufacturing system of a moderate size is modeled and analyzed using the proposed theory.

Deadlock Checking Using Net Unfoldings

Abstract: McMillan presented a deadlock detection technique based on unfoldings of Petri net systems. It is realized by means of a backtracking algorithm that has its drawback for unfoldings that increase widely. We present an approach that exploits precisely this property. Moreover, we introduce a fast implementation of McMillan's algorithm and compare it with our new technique.

Choice-free Petri nets: a model for deterministic concurrent systems with bulk services and arrivals

Abstract: Among discrete event systems, those exhibiting concurrency are especially challenging, requiring the use of formal methods to deal with them. Petri nets are a well-established such formalism. The structure theory aims at overcoming the state space explosion problem, inherent to the analysis of concurrent systems, by bridging structural and behavioral properties. To date, this has been successfully achieved mainly for some subclasses of ordinary nets. However weights are a modeling convenience in many situations. In this paper we study a formal model for a subclass of concurrent systems with bulk services and arrivals which structurally avoids conflicts. Structural results and techniques for dealing with them are introduced. These include structural conditions on properties of correct behavior and a unified framework for checking general behavioral properties by reasoning solely on the structure.

Symbolic Methods and Tools for Discrete Event Dynamic Systems

Abstract: The interest in Discrete Event Dynamic Systems (DEDS) has increased during the last years, due to the lack of methods and tools that are capable of handling the complexity of problems and tasks present in industry today. In this thesis we will consider a framework based on relations over finite domains. The framework is used for modeling, analysis, and synthesis of DEDS.Binary Decision Diagrams (BDDs) are used to represent relations, as well as the operations for modeling, analysis and synthesis of DEDS. To utilized the structure of integers and arithmetic operation, Integer Decision Diagrams (IDDs) are developed and implemented. Polynomials over finite fields are another type of representation that is used for the relational framework. Here Grobner bases, and Integrated Monomial Diagrams (IMDs) are the tools that are used. IDDs and IMDs are both developed, by the author, to represent integer structures and arithmetic operations efficiently.With tools for efficient relational representation, it possible to improve scalability of DEDS computations, as shown in this thesis by the modeling and analysis of the landing gear controller of the Swedish fighter aircraft JAS 39 Gripen. A relational model, represented by a BDD, is automatically generated from a 1200 lines Pascal implementation, which contains 105 binary variables of which 26 are state variables. Function specifications expressed with temporal algebra, are verified using tools for dynamic analysis, which we also use to compute a polynomial representing the set of all reachable states in the model. The landing gear controller serves as a benchmark test of BDDs and IDDs. The IDDs reduced the computation time by 50%.To explore the ability and applicability of using a polynomial relational representation when doing synthesis, we use a tank system containing actuators (pump and valves) and sensors (the tank level and measurable disturbances). We propose a synthesis method that uses actuator priority, weighting of states, and Grobner bases to compute explicit control laws for the actuators, fulfilling the control objectives even if one of the actuators (the pump) is defective.Modeling aspects are emphasized further, by comparing the polynomial approach which we have used, with Boolean expressions and established DEDS approaches in the community of automatic control like Ramadge-Wonham, Petri nets, and COCOLOG. We discuss how to handle transformation between signals and events for DEDS and how to modularize DEDS to gain complexity advantages. Model description languages are discussed and desirable features are stated, using the experiences achieved from the modeling of the tank system and the landing gear controller.

Model Checking Coloured Petri Nets - Exploiting Strongly Connected Components

Abstract: In this paper we present a CTL-like logic which is interpreted over the state spaces of Coloured Petri Nets. The logic has been designed to express properties of both state and transition information. This is possible because the state spaces are labelled transition systems. We compare the expressiveness of our logic with CTL's. Then, we present a model checking algorithm which for efficiency reasons utilises strongly connected components and formula reduction rules. We present empirical results for non-trivial examples and compare the performance of our algorithm with that of Clarke, Emerson, and Sistla.

Partial order semantics and read arcs

Abstract: We study a new partial order semantics of Petri nets with read arcs, where read arcs model reading without consuming, which is often more adequate than the destructive-read-and-rewrite modelled in ordinary nets without read arcs. As basic observations we take ST-traces, which are sequences of transition starts and ends. We define processes of our nets and derive two partial orders modelling causality and start precedence. These partial orders are related to basic observations and their system states just as in the ordinary approach the single partial order of a process is related to firing sequences and reachable markings. Our approach also supports a new view of concurrency as captured by steps.

Petri net state estimators based on event observation

Abstract: We discuss the problem of estimating the marking of a place/transition net based on event observation. We assume that the net structure is known while the initial marking is totally or partially unknown. We give two algorithms to compute a marking estimate that is a lower bound of the actual marking. The error between actual marking and estimate is a monotonically nonincreasing function of the observed word length. The special structure of Petri nets allows us to use a simple linear algebraic formalism for estimate and error computation. In particular, the set of markings consistent with an observed word, i.e., the set of marking in which the system may actually be given the observed word, can be easily characterized. Finally, we show how the estimate generated by the observer may be used to design a state feedback controller.

The PEP Tool

Abstract: The PEP tool embeds sophisticated programming and verification components in a user-friendly graphical interface. The basic idea is that the programming component allows the user to design concurrent algorithms in an imperative language, and that the PEP system then generates Petri nets from such programs in order to use Petri net theory for simulation and verification purposes. A key feature is flexibility; its modular design eases the task of adding new interfaces to other verification packages, such as ‘INA’, ‘PROD’ or ‘SMV’.

Ad-hoc workflow: problems and solutions

Abstract: The paper introduces ad hoc workflow, adding flexibility to traditional workflow. A problem that stems from the added flexibility is the need to support end users in the selection and modification of the process for a specific case. We propose a class of Petri nets to describe workflow processes, featuring safeness and guaranteed termination. A set of transformation rules with sufficient power for this class is given that can be implemented in a graphical editor. A second problem is monitoring the work being done. The solution is to approximate the states of the cases being treated by the states of a few standard cases.

Storage Alternatives for Large Structured State Spaces

Abstract: We consider the problem of storing and searching a large state space obtained from a high-level model such as a queueing network or a Petri net. After reviewing the traditional technique based on a single search tree, we demonstrate how an approach based on multiple levels of search trees offers advantages in both memory and execution complexity. Further execution time improvements are obtained by exploiting the concept of “event locality”. We apply our technique to three large parametric models, and give detailed experimental results.