Petri Net Formalism Research Articles

Perspectives in the use of coloured Petri nets for risk analysis and accident modelling

This paper will discuss the current uses and application perspectives of Petri Nets (PNs) in the fields of risk analysis and accident modelling. Severe time and combinatory limitations are encountered when trying to model complex events sequences with classical methods. Due to their large calculation capabilities and the development of recent tools, the PNs should be able to overcome these limitations. We will first review and briefly discuss the previous applications of PNs in the field of safety. The second aim of this paper is also to highlight the tool's capabilities through a “translation” catalogue in which common concepts in safety are expressed in the PNs formalism. Despite these promising examples and properties, safety oriented applications in the field of safety are still scarce. The lack of comprehensive tools available and PN inherent complexity may explain this situation. We can hope that the increasing attractiveness of PNs may somehow overcome these difficulties. As a matter of fact, due to the tool continuous development (i.e. recent stochastic reward net or object-oriented nets development), PNs may play a significant role in risk analysis or accident modelling in the future.

Fuzzy rule base systems verification using high-level petri nets

In this paper, we propose a Petri nets formalism for the verification of rule-based systems. Typical structural errors in a rule-based system are redundancy, inconsistency, incompleteness, and circularity. Since our verification is based on Petri nets and their incidence matrix, we need to transform rules into a Petri nets first, then derive an incidence matrix from the net. In order to let fuzzy rule-based systems detect above the structural errors, we are presenting a Petri-nets-based mechanism. This mechanism consists of three phases: rule normalization, rules transformation, and rule verification. Rules will be first normalized into Horn clauses, then transform the normalized rules into a high-level Petri net, and finally we verify these normalized rules. In addition, we are presenting our approach to simulate the truth conditions which still hold after a transition firing and negation in Petri nets for rule base modeling. In this paper, we refer to fuzzy rules as the rules with certainty factors, the degree of truth is computed in an algebraic form based on state equation which can be implemented in matrix computation in Petri nets. Therefore, the fuzzy reasoning problems can be transformed as the liner equation problems that can be solved in parallel. We have implemented a Petri nets tool to realize the mechanism presented fuzzy rules in this paper.

MODELING AND ANALYZING MULTI-AGENT BEHAVIORS USING PREDICATE/TRANSITION NETS

How agents accomplish a goal task in a multi-agent system is usually specified by multi-agent plans built from basic actions (e.g. operators) of which the agents are capable. The plan specification provides the agents with a shared mental model for how they are supposed to collaborate with each other to achieve the common goal. Making sure that the plans are reliable and fit for the purpose for which they are designed is a critical problem with this approach. To address this problem, this paper presents a formal approach to modeling and analyzing multi-agent behaviors using Predicate/Transition (PrT) nets, a high- level formalism of Petri nets. We model a multi-agent problem by representing agent capabilities as transitions in PrT nets. To analyze a multi-agent PrT model, we adapt the planning graphs as a compact structure for reachability analysis, which is coherent to the concurrent semantics. We also demonstrate that one can analyze whether parallel actions specified in multi-agent plans can be executed in parallel and whether the plans can achieve the goal by analyzing the dependency relations among the transitions in the PrT model.

Mending the Lag between Commercial Needs and Research Prototypes: A Logic-Based Workflow Verification Approach

The increasing complexity of business processes in recent years has heightened the need for workflow verification tools. However, there is a significant lag between current commercial needs and research prototypes. Our investigation indicates that existing workflow verification tools depend largely on the Petri net formalism, which is more complex than the modeling formalisms in commercial workflow systems. This makes it difficult to apply those workflow verification tools to commercial workflow systems. This lag between commerce and research motivated our study in developing an alternative approach to workflow verification. In this paper, we propose a logic-based verification method that is easier to implement in commercial workflow systems than Petri-net-based verification methods. We further demonstrate that our logic-based workflow verification approach is simple, yet powerful enough to handle complex, including cyclic, workflow models.

COSMOS: a discrete-event methodology for construction processes

Process modelling and simulation can be regarded as a framework within which discrete-event systems such as construction operations can be analysed, designed and improved. This paper proposes an alternative modelling methodology called COSMOS (construction oriented simulation modelling system). The approach is designed for the modelling of discrete-event processes for the purpose of simulation and is geared towards construction process orientation. While the basic COSMOS constructs are based on the basic formalism of Petri Nets, substantial extensions are made to the common Petri Net approach. COSMOS graphical network models are built by connecting appropriate nodes with appropriate arcs and assigning tokens to their places in a way that portrays the characteristics and logic of actual systems. If necessary, attributes and control statements can be attached to suitable modelling elements to enhance the capability of the models. The models are then capable of representing systems that are more complicated. COSMOS provides a means of reflecting reality and practicality in construction-process modelling.

Design and Implementation of Discrete Event Control Systems: A Petri Net Based Hardware Approach

This paper presents a methodology for modeling and hardware implementation of discrete event control systems based on the formalism of Petri nets (PN). The control algorithm is initially specified as an executable PN specification which is subsequently compiled directly into a compact machine code and stored in commercially available programmable read only memory (PROM). As an implementation platform, a programmable Petri net based dedicated discrete event controller is proposed. The scalable architecture of the controller is optimized to process PN constructs of all possible classes. A remarkable feature of the proposed architecture is its ability to handle explicit concurrency. The controller exploits hardware-level parallelism to track multiple tokens (control threads) through the net. The performance of the architecture has been verified and benchmarked on the fabricated integrated circuit (IC) prototype of the controller.

EXECUTION LEVEL CONTROL AND RECONFIGURATION FOR REMOTELY OPERATED VEHICLES

A method to design the control architecture for Remotely Operated Vehicles is presented that is based on the idea to use the Petri net formalism. The “safe” behaviour of the architecture is guaranteed by enforcing some place invariants on the marking of the net. An algorithm to find out feasible sequences of operations to switch between configurations is introduced. Finally, a method to automatically reconfigure the system is shown.

Developing virtual environments for industrial training

Virtual reality-based training systems (VRTSs) are advanced computer-assisted training systems using Virtual Reality (VR) technology. To have better structure and easier implementation, a virtual training system can be modeled as an integrated system consisting of a training task-planning module, an instruction module, a simulation module, a performance evaluation module, and an interface module. Presented in this paper are an architecture of VR-based training systems and a practical knowledge modeling approach to modeling the training scenarios of the systems by using Petri nets formalism. A Computer Numerical Control (CNC) milling operations virtual training prototype system was developed to illustrate the feasibility and effectiveness of this approach.

Knowledge intensive Petri net framework for concurrent intelligent design of automatic assembly systems

The integration of design and planning of flexible assembly system has been recognized as a tool for achieving efficient assembly in a production environment that demands assembly with a high degree of flexibility. This paper proposes a concurrent intelligent approach and framework for the design of robotic flexible assembly systems. The principle of the proposed approach is based on the knowledge Petri net formalisms, incorporating Petri nets with more general problem-solving strategies in AI using knowledge-based system techniques. The complex assembly systems are modeled and analyzed by adopting a formal representation of the system dynamic behaviors through knowledge Petri net modeling from the specifications and the analysis of those models. A template is first defined for a knowledge Petri net model, and then the models for assembly system individuals are established in the form of instances of the template. The design of assembly systems is implemented through a knowledge Petri net-based function–behavior–structure model. The research results show that the proposed knowledge Petri net approach is applicable for design, simulation, analysis and evaluation, and even layout optimization of the flexible assembly system in an integrated intelligent environment. The integration of assembly design and planning process can help reduce the development time of assembly systems.

Supervisor System for Detection and Treatment of Failures in Manufacturing Systems Using Distributed Petri Nets

This paper presents a framework for a supervisor in manufacturing systems operations. The supervisor not only considers normal process in the manufacturing system but also detection and treatment of failures. The framework is based on the formalism of Petri nets (PN). In particular, an extension of PN, Distributed Petri Net (DPN), is introduced, which permit to develop the supervisor framework. A case study of detection and treatment of failure in machining operations will be considered, in which, the normal process, the detection and treatment of failures in a manufacturing system are integrated through modules based on DPN.

Execution Control of Robotic Tasks for UUVs

The problem of designing an interface between the continuous-state and the discrete-state domains of intelligent control architectures is addressed in this paper, focusing on the case of a hierarchical navigation, guidance and control architecture for unmanned underwater vehicles. The proposed interface represents the underlying continuous-state execution level as a DES using a Petri net formalism. The correct behavior of the execution level is ensured, checking that no forbidden state is reached and that the proper task activation and deactivation order is respected. Methodologies for the off-line generation of the Petri net representation of the Execution level and realtime automatic system reconfiguration are presented.

A modeling and analysis methodology for modular logic controllers of machining systems using Petri net formalism

Logic controllers for machining systems typically have three control modes: auto, hand and manual. In this paper, a unified formal representation of logic controllers with three control modes is provided using Petri nets (PNs). A modular logic controller structure is introduced and formalized for high-volume transfer lines. The modular logic controller consists of one control module for the mode decision and other control modules for station logic controllers. Each station control module is represented by connecting together operation modules, which are designed with respect to the fault recovery processes of operations; their connection algorithm is also provided. In our formal representation, each control module is represented by a live, safe and reversible PN. A condition for the modular logic controller to generate a correct control logic is provided: the operation causality condition. Using the modular structure of a logic controller, the control logic can be easily reconfigured and automatic code generation is possible.

Mechanical systems and assemblies modeling using knowledge-intensive Petri nets formalisms

This paper presents a novel knowledge-based Petri net approach to mechanical systems and assemblies modeling within a design with objects environment. A new unified class of object-oriented knowledge Petri nets, which can incorporate a knowledge-based system with ordinary Petri nets, is defined and used for the unified representations of assembly design and modeling. The object knowledge Petri nets, as a graphical language and a new knowledge-based description scheme, can be used to express the qualitative and quantitative aspects of the assembly design and modeling process in an interactive and integrated way. The four-level hierarchy model is proposed and constructed in terms of function-behaviors, structures, geometries, and features. The function-behavior-structure description is built on more abstract concepts so that it can match well top-down design. The static and dynamic characteristics in the design of assembly can also be captured. With the help of fuzzy logic, the incomplete, imprecise knowledge and uncertainty in the design process can also be dealt with. Therefore, the hybrid design object model can incorporate product data model, top-down design process, and assembly process model using an object-oriented, knowledge-based, feature-based, parametric, and constraint-based modeling approach, and can provide a more accurate and more flexible representation. To verify and demonstrate the effective use of the proposed hybrid design object model, a prototype system has been developed. This research provides a knowledge-intensive framework for intelligent assembly design and modeling.

Inheritance of behavior

One of the key issues of object-oriented modeling and design is inheritance. It allows for the definition of subclasses that inherit features of some superclass. Inheritance is well defined for static properties of classes such as attributes and methods. However, there is no general agreement on the meaning of inheritance when considering the dynamic behavior of objects, captured by their life cycles. This paper studies inheritance of behavior both in a simple process-algebraic setting and in a Petri-net framework. Process algebra is chosen, because it concentrates on behavior, while abstracting from the internal states of processes. The result of the algebraic study is a clear conceptual understanding of inheritance of behavior. It can be expressed in terms of blocking and hiding method calls. The results in the algebraic framework inspire the development of the concept of inheritance of behavior in the Petri-net framework. The Petri-net formalism allows for a graphical representation of life cycles of objects with an explicit representation of object states. In the Petri-net framework, four inheritance rules are defined that can be used to construct life cycles of subclasses from the object life cycles of given (super-)classes. These inheritance rules can be used to structure a design process and they stimulate the reuse of life-cycle specifications. It turns out that the combination of blocking and hiding method calls captures a number of important operators for constructing life cycles of subclasses from life cycles of superclasses, namely choice, sequential composition, parallel composition, and iteration. A small case study validates our approach to inheritance of behavior.

Recursive Nested Petri Nets: Analysis of Semantic Properties and Expessibility

Recursive nested Petri nets may contain marked nets, including their own copies, as their tokens. It is proved that the termination problem is decidable for recursive nested nets with autonomous elements. The expessibility of such nets is compared with other extensions of the Petri net formalism.

INCOMPLETE INPUT INFERENCE ON FUZZY PRODUCTION SYSTEMS SUPPORTED BY PETRI NETS

This paper proposes a new reasoning technique on fuzzy production systems while given input knowledge is incomplete. Based on the fuzzy Petri net formalism, the proposed algorithm can infer all possible conclusions and their corresponding missing inputs. The most possible conclusion can also be determined based on the criteria of the minimum number of missing inputs as well as the degree of truth of the conclusion. In addition, finiteness and computational complexity of the algorithm is investigated. As real decisions are typically made under incomplete input knowledge, this reasoning technique provides more realistic applications for fuzzy production systems.

Analysis of discrete‐time stochastic petri nets

The Petri net formalism is widely applied in both theoretical and practical settings. For the sake of performance analysis, the original Petri net model has been extended with the notion of time. This paper addresses the different issues involved with this extension. Also, it provides a state‐of‐the‐art overview of different analysis techniques for timed Petri nets. A new analysis technique is presented, which combines the freedom of choosing arbitrary time distributions within a Petri net model on the one hand and efficient computation means on the other.

A formal specification framework for object-oriented distributed systems

In this paper, we present the Concurrent Object-Oriented Petri Nets (CO-OPN/2) formalism devised to support the specification of large distributed systems. Our approach is based on two underlying formalisms: order-sorted algebra and algebraic Petri nets. With respect to the lack of structuring capabilities of Petri nets, CO-OPN/2 has adopted the object-oriented paradigm. In this hybrid approach (model- and property-oriented), classes of objects are described by means of algebraic Petri nets, while data structures are expressed by order-sorted algebraic specifications. An original feature is the sophisticated synchronization mechanism. This mechanism allows to involve many partners in a synchronization and to describe the synchronization policy. A typical example of distributed systems, namely the Transit Node, is used throughout this paper to introduce our formalism and the concrete specification language associated with it. By successive refinements of the components of the example, we present, informally, most of the notions of CO-OPN/2. We also give some insights about the coordination layer, Context and Objects Interface Language (COIL), which is built on top of CO-OPN/2. This coordination layer is used for the description of the concrete distributed architecture of the system. Together, CO-OPN/2 and COIL provide a complete formal framework for the specification of distributed systems.

Testing and Debugging Message Passing Programs in Synergy with their Specifications

We outline Ensemble, a design and implementation methodology for composing message passing (MP) applications from program components directed by scripts. We define specification components corresponding to program components and we compose them, directed by the same scripts, obtaining formal specifications of the composed applications. We use the Petri net formalism to express component and application specifications. Petri Net composition is modelled by appropriate coupling of inscriptions on interface input and output arcs to and from a unified environment place. We elaborate on testing and debugging of MP applications based on the synergy of tools for Petri-Net simulations with tools for monitoring program executions.

SoftPM: a software process management system reconciling formalism with easiness

Various formal approaches to process modeling and analysis have been proposed. With the emerging importance of practicality in this field, easiness in adopting formal technology should be taken into account. In this paper, we propose a PSEE called SoftPM that is based on a high level Petri net formalism for process modeling. To overcome the difficulty in using this formalism, SoftPM exploits a multi-level modeling mechanism for the representation of software processes. SoftPM supports three different levels for process representation. They are cognitive, MAM net, and Pr/T net. Each level has different roles in SoftPM. The cognitive-level modeling provides the means of getting free from difficulties in manipulating the modeling formalism. MAM net takes the role of core modeling formalism in SoftPM. Finally, Pr/T nets support the low-level modeling formalism as an existing Petri-net class. Moreover, SoftPM offers various analysis techniques to aid managerial decision making, as well as conventional Petri-net analysis techniques. Using a Java-based thin client/server architecture, SoftPM smoothly supports execution at distributed heterogeneous platforms over the Internet.