Showing papers on "Petri net published in 2014"

Optimal Supervisory Control of Flexible Manufacturing Systems by Petri Nets: A Set Classification Approach

Abstract: Supervisory control is usually considered as an external control mechanism to a system by controlling the occurrences of its controllable events. There exist Petri net models whose legal reachability spaces are nonconvex. In this case, they cannot be optimally controlled by the conjunctions of linear constraints. For Petri net models of flexible manufacturing systems, this work proposes a method to classify the legal markings into several subsets. Each subset is associated with a linear constraint that can forbid all first-met bad markings. Then, the disjunctions of the obtained constraints can make all legal markings reachable and forbid all first-met bad markings, i.e., the controlled net is live and maximally permissive. An integer linear programming model is formulated to minimize the number of the constraints. A supervisory structure is also proposed to implement the disjunctions of the constraints. Finally, examples are provided to illustrate the proposed method.

Memory Efficient Data Structures for Explicit Verification of Timed Systems

Abstract: Timed analysis of real-time systems can be performed using continuous symbolic or discrete explicit techniques. The explicit state-space exploration can be considerably faster for models with moderately small constants, however, at the expense of high memory consumption. In the setting of timed-arc Petri nets, we explore new data structures for lowering the used memory: PTries for efficient storing of configurations and time darts for semi-symbolic description of the state-space. Both methods are implemented as a part of the tool TAPAAL and the experiments document at least one order of magnitude of memory savings while preserving comparable verification times.

New Petri Net Structure and Its Application to Optimal Supervisory Control: Interval Inhibitor Arcs

Abstract: This paper presents a new Petri net structure, namely, an interval inhibitor arc, and its application to the optimal supervisory control of Petri nets. An interval inhibitor arc is an arc from a place to a transition labeled with an integer interval. The transition is disabled by the place if the number of tokens in the place is between the labeled interval. The formal definition and the firing rules of Petri nets with interval inhibitor arcs are developed. Then, an optimal Petri net supervisor based on the interval inhibitor arcs is designed to prevent a system from reaching illegal markings. Two techniques are developed to simplify the supervisory structure by compressing the number of control places. The proposed approaches are general since they can be applied to any bounded Petri net models. A marking reduction approach is also introduced if they are applied to Petri net models of flexible manufacturing systems. Finally, a number of examples are provided to demonstrate the proposed approaches and the experimental results show that they can obtain optimal Petri net supervisors for some net models that cannot be optimally controlled by pure net supervisors. Furthermore, the obtained supervisor is structurally simple.

Diagnosability of Discrete-Event Systems Using Labeled Petri Nets

Abstract: In this paper, we focus on labeled Petri nets with silent transitions that may either correspond to fault events or to regular unobservable events. We address the problem of deriving a procedure to determine if a given net system is diagnosable, i.e., the occurrence of a fault event may be detected for sure after a finite observation. The proposed procedure is based on our previous results on the diagnosis of discrete-event systems modeled with labeled Petri nets, whose key notions are those of basis markings and minimal explanations, and is inspired by the diagnosability approach for finite state automata proposed by Sampath in 1995. In particular, we first give necessary and sufficient conditions for diagnosability. Then, we present a method to test diagnosability that is based on the analysis of two graphs that depend on the structure of the net, including the faults model, and the initial marking.

Timed Compatibility Analysis of Web Service Composition: A Modular Approach Based on Petri Nets

Abstract: Recently, the temporal constraint satisfiability is regarded as an important criterion in Web service composition to guarantee its timely completion. This leads to a new challenge in analyzing the compatibility of Web services under temporal constraints. The existing methods either do not consider message mismatches between services in a composition or suffer from state-space explosion by verifying a service composition model as a whole; or lack the ability to generate execution paths of each participating service. In this paper, we present a Petri net-based method to address these three issues in a holistic manner, and also in a modular way. Compared with the existing work, the proposed approach not only composes Web services by adding a mediation net to deal with message mismatches, but also checks the compatibility w.r.t. temporal constraints by generating modular timed state graphs. Furthermore, the reliable and usable execution paths that satisfy the timed compatibility can be derived to guide service execution and avoid any temporal exception.

A robot on the shoulder: Coordinated human-wearable robot control using Coloured Petri Nets and Partial Least Squares predictions

Abstract: A wearable robot secured on the shoulder of a human is developed for assisting its wearer in the execution of tasks in the overhead workspace. Installing a ceiling panel is an example of such a task. During this task the robot can hold the panel or collaborate actively with the human in order to fix the panel with the appropriate equipment. This wearable robot, termed "Supernumerary Robotic Limbs", works closely with the human to streamline the operation and reduce the human workload. First, the design concept of the Robot-on-the-Shoulder system is described, and a new approach to the coordinated control between the wearable robot and the human is presented. A graphical task process representation based on Coloured Petri Nets (CPN) is used to model the concurrent and distributed nature of the human-robot system, which comprises two human hands and two robot hands. The CPN framework is then extended to a type of hybrid control system by imbedding local dynamic controllers in the Transition nodes of the CPN model. Each local dynamic controller collects sensor signals relevant to the target transition and makes a predictive control decision. This allows the robot on the shoulder to take a proactive and preemptive action as well as to confirm a successful Transition. The control parameters for these algorithms are tuned based on "teaching-by-showing" techniques using human demonstration data. Partial Least Squares is used for extracting significant sensor signals from high-dimensional sensor data in order to do real time predictions and control. A prototype robot-on-the-shoulder system is built, and the CPN hybrid control is implemented and tested for a ceiling panel installation task.

Determining Process Model Precision and Generalization with Weighted Artificial Negative Events

Abstract: Process mining encompasses the research area which is concerned with knowledge discovery from event logs. One common process mining task focuses on conformance checking, comparing discovered or designed process models with actual real-life behavior as captured in event logs in order to assess the “goodness” of the process model. This paper introduces a novel conformance checking method to measure how well a process model performs in terms of precision and generalization with respect to the actual executions of a process as recorded in an event log. Our approach differs from related work in the sense that we apply the concept of so-called weighted artificial negative events toward conformance checking, leading to more robust results, especially when dealing with less complete event logs that only contain a subset of all possible process execution behavior. In addition, our technique offers a novel way to estimate a process model’s ability to generalize. Existing literature has focused mainly on the fitness (recall) and precision (appropriateness) of process models, whereas generalization has been much more difficult to estimate. The described algorithms are implemented in a number of ProM plugins, and a Petri net conformance checking tool was developed to inspect process model conformance in a visual manner.

Scheduling of Dual-Arm Cluster Tools With Wafer Revisiting and Residency Time Constraints

Abstract: In its fabrication, a wafer must meet its residency time constraints, i.e., it must leave its process chamber within a certain time after its process is done. It may need to visit some processing steps for a number of times, called wafer revisiting. For the typical wafer revisiting process of atomic layer deposition (ALD), this paper studies the scheduling problem of dual-arm cluster tools with both residency time constraints and wafer revisiting. To do so, a Petri net model is developed for the system. Then, with the Petri net model and based on a one-wafer cyclic scheduling method previously developed by the authors of this paper, schedulability conditions and scheduling algorithms are derived. The schedulability can be checked by analytical expressions. If schedulable, an optimal one-wafer cyclic schedule can be found by simple calculation. Thus, the proposed approach is very efficient. In addition, with the Petri net model, a simple way is presented to implement the obtained schedule. Illustrative examples are given to show the application of the proposed approach.

Modular Design of Urban Traffic-Light Control Systems Based on Synchronized Timed Petri Nets

Abstract: Timed Petri nets (TPNs) have been utilized as visual formalism for the modeling of complex discrete-event dynamic systems. They illuminate the features in describing the properties of causality and concurrency. Moreover, it is well known that a synchronized TPN (STPN) allows us to present all of the concurrent states in a complex TPN. In this paper, we propose a new methodology to design and analyze an urban traffic network control system by using the STPN. In addition, the applications of the STPN to eight-phase, six-phase, and two-phase traffic-light control systems are modularized. The advantage of the proposed approach is the clear presentation of the behaviors of traffic lights in terms of the conditions and events that cause phase alternations. Moreover, the size of the urban traffic network control system can be easily extended with our proposed modular technique. An analysis of the control models is performed via a reachability graph method to demonstrate how the models enforce the transitions of the traffic lights.

Fault Diagnosis and Prognosis With Partially Observed Petri Nets

Abstract: This paper concerns the prevention of faults in discrete event systems modeled with partially observed Petri nets (POPNs) that include the definition of sensors used to measure the events and markings. Observation sequences result from this modeling, and the firing sequences and initial marking consistent with a given observation sequence are systematically obtained. The degree of confidence of past and future states and events are computed: state estimation fault diagnosis and fault prediction result from this computation. Finally, diagnosability, detectability, and predictability are defined for observation sequences and POPNs and are discussed with respect to the sensor configuration.

An SMT-Based Approach to Coverability Analysis

Abstract: Model checkers based on Petri net coverability have been used successfully in recent years to verify safety properties of concurrent shared-memory or asynchronous message-passing software. We revisit a constraint approach to coverability based on classical Petri net analysis techniques. We show how to utilize an SMT solver to implement the constraint approach, and additionally, to generate an inductive invariant from a safety proof. We empirically evaluate our procedure on a large set of existing Petri net benchmarks. Even though our technique is incomplete, it can quickly discharge most of the safe instances. Additionally, the inductive invariants computed are usually orders of magnitude smaller than those produced by existing solvers.

Transition Cover-Based Design of Petri Net Controllers for Automated Manufacturing Systems

Abstract: In automated manufacturing systems (AMSs), deadlock problems must be well solved. Many deadlock control policies, which are based on siphons or Resource-Transition Circuits (RTCs) of Petri net models of AMSs, have been proposed. To obtain a live Petri net controller of small size, this paper proposes for the first time the concept of transition covers in Petri net models. A transition cover is a set of Maximal Perfect RTCs (MPCs), and the transition set of its MPCs can cover the set of transitions of all MPCs. By adding a control place with the proper control variable to each MPC in an effective transition cover to make sure that it is not saturated, it is proved that deadlocks can be prevented, whereas the control variables can be obtained by linear integer programming. Since the number of MPCs in an effective transition cover is less than twice that of transition vertices, the obtained controller is of small size. The effectiveness of a transition cover is checked, and ineffective transition covers can be transformed into effective ones. Some examples are used to illustrate the proposed methods and show the advantage over the previous ones.

A Component Model for Model Transformations

Abstract: Model-driven engineering promotes an active use of models to conduct the software development process. In this way, models are used to specify, simulate, verify, test and generate code for the final systems. Model transformations are key enablers for this approach, being used to manipulate instance models of a certain modelling language. However, while other development paradigms make available techniques to increase productivity through reutilization, there are few proposals for the reuse of model transformations across different modelling languages. As a result, transformations have to be developed from scratch even if other similar ones exist. In this paper, we propose a technique for the flexible reutilization of model transformations. Our proposal is based on generic programming for the definition and instantiation of transformation templates, and on component-based development for the encapsulation and composition of transformations. We have designed a component model for model transformations, supported by an implementation currently targeting the Atlas Transformation Language (ATL). To evaluate its reusability potential, we report on a generic transformation component to analyse workflow models through their transformation into Petri nets, which we have reused for eight workflow languages, including UML Activity Diagrams, YAWL and two versions of BPMN.

Supervisor Simplification for AMS Based on Petri Nets and Inequality Analysis

Abstract: In the framework of automated manufacturing systems (AMS), Petri nets are widely used to model, analyze, and control them. Resolving deadlocks is of paramount significance because their emergence may likely zero a systems throughput, if not necessarily. Supervisory control technique is the most widely adopted method to resolve them. A control policy can be converted into satisfying a set of inequalities, each of which corresponds to a siphon in a Petri net structure. The number of siphons can be exponential in the worst case, so does the number of inequalities. Taking into account the independent and dependent inequalities, this paper proposes a method to remove all the dependent inequalities, while preserving only the independent ones. This method can significantly reduce the size of a supervisory controller. Examples are presented to illustrate the effectiveness and efficiency of this method.

Hierarchical modelling of Last Mile logistic distribution system

Abstract: Last Mile logistic distribution system is the final step in business-to-customer supply chain which needs careful investigation in order to efficiently and economically deliver goods to customers. This study is aimed at providing a conceptual planning approach of modelling a Last Mile system based on hierarchy which is particularly useful in routing planning of the system. The hierarchical modelling is implemented using the Petri net method which is suitable to the needs of the system being a discrete event dynamical system.

Modeling and Validating E-Commerce Business Process Based on Petri Nets

Abstract: E-commerce and online shopping with a third-party payment platform have rapidly developed recently, and encountered many fault tolerance and security problems concerned by users. The causes of these problems include malicious behavior and imperfect business processes. The latter lead to the emergence of security vulnerabilities and loss of user funds which become more and more serious these years. We focus on the business process of e-commerce, and propose a formal model for constructing an e-commerce business process called an E-commerce Business Process Net. It integrates both data and control flows based on Petri nets. Rationality and transaction consistency are defined and validated to guarantee the transaction properties of an e-commerce business process. This paper offers a complete methodology for modeling and validating an e-commerce system with a third-party payment platform from the view point of a business process. Its use enables a designer to identify errors early in the design process and correct them before the deployment phase. In order to demonstrate the applicability and feasibility of the methodology, we have modeled and validated a real-world e-commerce business process and discovered the problems that cause the violation of transaction properties.

Determining process model precision and generalization with weighted artificial negative events

Abstract: Process mining encompasses the research area which is concerned with knowledge discovery from event logs. One common process mining task focuses on conformance checking, comparing discovered or designed process models with actual real-life behavior as captured in event logs in order to assess the “goodness” of the process model. This paper introduces a novel conformance checking method to measure how well a process model performs in terms of precision and generalization with respect to the actual executions of a process as recorded in an event log. Our approach differs from related work in the sense that we apply the concept of so-called weighted artificial negative events towards conformance checking, leading to more robust results, especially when dealing with less complete event logs that only contain a subset of all possible process execution behavior. In addition, our technique offers a novel way to estimate a process model’s ability to generalize. Existing literature has focused mainly on the fitness (recall) and precision (appropriateness) of process models, whereas generalization has been much more difficult to estimate. The described algorithms are implemented in a number of ProM plugins, and a Petri net conformance checking tool was developed to inspect process model conformance in a visual manner.

Petri net-based process monitoring: a workflow management system for process modelling and monitoring

Abstract: Nowadays business process management is becoming a fundamental piece of many industrial processes. To manage the evolution and interactions between the business actions it is important to accurately model the steps to follow and the resources needed by a process. Workflows provide a way of describing the order of execution and the dependencies between the constituting activities of business processes. Workflow monitoring can help to improve and avoid delays in industrial environments where concurrent processes are carried out. In this article a new Petri net extension for modelling workflow activities together with their required resources is presented: resource-aware Petri nets (RAPN). An intelligent workflow management system for process monitoring and delay prediction is also introduced. Resource aware-Petri nets include time and resources within the classical Petri net workflow representation, facilitating the task of modelling and monitoring workflows. The workflow management system monitors the execution of workflows and detects possible delays using RAPN. In order to test this new approach, different services from a medical maintenance environment have been modelled and simulated.

Petri Net-Based Optimal One-Wafer Cyclic Scheduling of Hybrid Multi-Cluster Tools in Wafer Fabrication

Abstract: In a multi-cluster tool, there may be both single and dual-arm cluster tools. Such a multi-cluster tool is called hybrid multi-cluster tool. To operate such a multi-cluster tool, one needs to coordinate different types of robots for accessing the shared buffering modules. Aiming at finding a one-wafer periodic schedule such that the lower bound of cycle time can be reached, this paper conducts a study on scheduling a hybrid multi-cluster tool with its bottleneck tool being process-bound. The tool is modeled by a kind of timed Petri net model. With this model, the scheduling problem is reduced to determining the robots' waiting time. Then, the conditions under which a one-wafer periodic schedule exists such that the lower bound of cycle time can be reached are presented. Based on them, a closed-form algorithm is given to check whether such a one-wafer periodic schedule exists. If so, it is found via simple calculation. Examples are given to show the application of the proposed method.

On-Line Fault Diagnosis With Partially Observed Petri Nets

Abstract: This technical note concerns fault detection and diagnosis for discrete event systems modeled with partially observed Petri nets. The proposed method provides diagnosis decisions via the analysis of observation sequences that include some observable events and the partial measurement of the successive states visited by the system. To this end, the observation sequences are decomposed into elementary observation sequences, linear matrix inequalities are used to compute the firing sequences consistent with each elementary observation sequence and an algorithm of linear complexity with respect to the length of the observation sequences is proposed to provide on-line diagnosis decisions.

Design Verification of Instrumentation and Control Systems of Nuclear Power Plants

Abstract: Instrumentation and Control systems are the nervous system of a nuclear power plant. They monitor all facets of the plant's health and help respond with care and adjustments needed, thus ensuring goals of efficient power production and safety. Due to safety significance of I&C, it becomes increasingly important to have a design verification methodology which ensures I&C systems fully functional. The strategy discussed the system modeling for design verification using Petri Net, converting it into Markov Chain and solving the linear system mathematically. It also exploits the best attribute of the created Markov model. The approach has been validated on seven sets of operation profile data of reactor control system of seven Nuclear Power Plants. The singular & plural of an acronym are always spelled the same.

Petri Net-Based Polynomially Complex Approach to Optimal One-Wafer Cyclic Scheduling of Hybrid Multi-Cluster Tools in Semiconductor Manufacturing

Abstract: Due to the different behavior of single-arm and dual-arm cluster tools, it is challenging to schedule a hybrid multi-cluster tool containing both of them. This paper aims to find an optimal one-wafer cyclic schedule for such a multi-cluster tool. It is assumed that the bottleneck individual cluster tool in it is process-bound, thereby making it process-dominant. To do so, this paper models a hybrid multi-cluster tool with Petri nets. With this model, it derives the conditions under which individual cluster tools can operate in a paced way. Based on these conditions, this paper shows that for any process-dominant hybrid multi-cluster tool there is always a one-wafer cyclic schedule. Then, it develops the algorithms to find the minimal cycle time and the optimal one-wafer cyclic schedule. It is computationally efficient and easy-to-implement in practice. Examples are given to show the application and effectiveness of the proposed method.

Context-aware workflow management for virtual enterprises based on coordination of agents

Abstract: Although virtual enterprises (VE) make it possible for small flexible enterprises to form a collaborative network to respond to business opportunities through dynamic coalition and sharing of the core competencies and resources, they also pose new challenges and issues. Creation of VE involves dynamically established partnerships between the partners and relies on a flexible coordination scheme. The dynamic organizations formed in VE present a challenge in the development of a new methodology to dynamically allocate re-sources and deliver the relevant information to the right people at the right time. A key issue is the development of an effective workflow management scheme for VE. Multi-agent systems (MAS) provide a flexible architecture to deal with changes based on dynamic organization and collaboration of autonomous agents. Despite the extensive studies and research results on MAS, development of a design methodology to support coordination and operations is critical to the success and adoption of VE. The objectives of this research are to propose a design methodology to facilitate coordination and development of context-aware workflow management systems and achieve effective resource allocation for VE based on MAS architecture. To achieve these objectives, a scheme for coordination of agents is proposed. Petri net models are used in the coordination scheme to describe workflows and capture resource activities in VE. The interactions between agents lead to a dynamic workflow model for VE. Based on the aforementioned model, we propose architecture to dynamically generate context-aware graphical user interface to guide the users and control resource allocation based on the state of VE. An order management example is used throughout this paper to illustrate the proposed design methodology.

A Modeling and Simulation Framework for Health Care Systems

Abstract: In this paper, we propose a new modeling methodology named MedPRO for addressing organization problems of health care systems. It is based on a metamodel with three different views: process view (care pathways of patients), resource view (activities of relevant resources), and organization view (dependence and organization of resources). The resulting metamodel can be instantiated for a specific health care system and be converted into an executable model for simulation by means of a special class of Petri nets (PNs), called Health Care Petri Nets (HCPNs). HCPN models also serve as a basis for short-term planning and scheduling of health care activities. As a result, the MedPRO methodology leads to a fast-prototyping tool for easy and rigorous modeling and simulation of health care systems. A case study is presented to show the benefits of the MedPRO methodology.

Analysis and Application of Logical Petri Nets to E-Commerce Systems

Abstract: Logical Petri Nets (LPNs) can well describe and analyze batch processing functions and passing value indeterminacy in cooperative systems. Their structures are simpler than the equivalent inhibition Petri nets. To analyze them, a vector matching method has been given. In this paper a subclass of LPNs is proposed and its properties are analyzed. It can be used to model and simulate e-commerce systems well. Its special structures and relationships among its logical transitions are discussed. Based on them, a reachability tree is constructed and the state equations are used to analyze its properties, that is, reachability, liveness, conservativeness, and reversibility. The proposed analysis method significantly lowers the computational efforts in comparison with the existing method for LPNs.

Modeling self-adaptive software systems with learning petri nets

Abstract: Traditional models have limitation to model adaptive software systems since they build only for fixed requirements, and cannot model the behaviors that change at run-time in response to environmental changes. In this paper, an adaptive Petri net is proposed to model a self-adaptive software system. It is an extension of hybrid Petri nets by embedding a neural network algorithm into them at some special transitions. The proposed net has the following advantages: 1) It can model a runtime environment; 2) The components in the model can collaborate to make adaption decisions; and 3) The computing is done at the local, while the adaption is for the whole system. We illustrate the proposed adaptive Petri net by modeling a manufacturing system.

Transformation challenges: from software models to performance models

Abstract: A software model can be analysed for non-functional requirements by extending it with suitable annotations and transforming it into analysis models for the corresponding non-functional properties. For quantitative performance evaluation, suitable annotations are standardized in the "UML Profile for Modeling and Analysis of Real-Time Embedded systems" (MARTE) and its predecessor, the "UML Profile for Schedulability, Performance and Time". A range of different performance model types (such as queueing networks, Petri nets, stochastic process algebra) may be used for analysis. In this work, an intermediate "Core Scenario Model" (CSM) is used in the transformation from the source software model to the target performance model. CSM focuses on how the system behaviour uses the system resources. The semantic gap between the software model and the performance model must be bridged by (1) information supplied in the performance annotations, (2) in interpretation of the global behaviour expressed in the CSM and (3) in the process of constructing the performance model. Flexibility is required for specifying sets of alternative cases, for choosing where this bridging information is supplied, and for overriding values. It is also essential to be able to trace the source of values used in a particular performance estimate. The performance model in turn can be used to verify responsiveness and scalability of a software system, to discover architectural limitations at an early stage of development, and to develop efficient performance tests. This paper describes how the semantic gap between software models in UML+MARTE and performance models (based on queueing or Petri nets) can be bridged using transformations based on CSMs, and how the transformation challenges are addressed.

Towards an Object-Oriented Programming Language for Physarum Polycephalum Computing: A Petri Net Model Approach

Abstract: Our research is focused on creation of a new object-oriented programming language for Physarum polycephalum computing. Physarum polycephalum is a one-cell organism that can be used for developing a biological architecture of different abstract devices, among others, the digital ones. In the paper, we use an abstract graphical language in the form of Petri nets to describe the Physarum polycephalum behavior. Petri nets are a good formalism to assist designers and support hardware design tools, especially in developing concurrent systems. At the beginning stage considered in this paper, we show how to build Petri net models, and next implement them as Physarum polycephalum machines, of basic logic gates AND, OR, NOT, and simple combinational circuits on the example of the 1-to-2 demultiplexer.