Strict Minimal Siphon-Based Colored Petri Net Supervisor Synthesis for Automated Manufacturing Systems With Unreliable Resources

Various deadlock control policies for automated manufacturing systems with reliable and shared resources have been developed, based on Petri nets. In practical applications, a resource may be unreliable. Thus, the deadlock control policies proposed in previous studies are not applicable to such applications. This paper proposes a two-step robust deadlock control strategy for systems with unreliable and shared resources. In the first step, a live (deadlock-free) controlled system that does not consider the failure of resources is derived by using strict minimal siphon control. The second step deals with deadlock control issues caused by the failures of the resources. Considering all resource failures, a common recovery subnet based on colored Petri nets is proposed for all resource failures in the Petri net model. The recovery subnet is added to the derived system at the first step to make the system reliable. The proposed method has been tested using an automated manufacturing system deployed at King Saud University.publishedVersio

Comparison and Evaluation of Deadlock Prevention Methods for Different Size Automated Manufacturing Systems

In automated manufacturing systems (AMSs), deadlocks problems can arise due to limited shared resources. Petri nets are an effective tool to prevent deadlocks in AMSs. In this paper, a simulation based on existing deadlock prevention policies and different Petri net models are considered to explore whether a permissive liveness-enforcing Petri net supervisor can provide better time performance. The work of simulation is implemented as follows. (1) Assign the time to the controlled Petri net models, which leads to timed Petri nets. (2) Build the Petri net model using MATLAB software. (3) Run and simulate the model, and simulation results are analyzed to determine which existing policies are suitable for different systems. Siphons and iterative methods are used for deadlocks prevention. Finally, the computational results show that the selected deadlock policies may not imply high resource utilization and plant productivity, which have been shown theoretically in previous publications. However, for all selected AMSs, the iterative methods always lead to structurally and computationally complex liveness-enforcing net supervisors compared to the siphons methods. Moreover, they can provide better behavioral permissiveness than siphons methods for small systems. For large systems, a strict minimal siphon method leads to better behavioral permissiveness than the other methods

A Synthesis Method for Designing Shared-Resource Systems

In system synthesis, one needs to derive from a given set of processes a system design which reflects exactly the functionalities of the processes and is free from erroneous situations such as deadlock and capacity overflow. This is especially important for shared-resource systems, in which errors are easily induced because of the sharing of common resources among different competing processes. In this paper, a synthesis method is proposed for designing shared-resource systems. It begins with specifying the given processes as augmented marked graphs. These augmented marked graphs are then synthesized through the fusion of commonplaces which represents the shared resources. The net so obtained serves to represent the integrated system which reflects exactly the functionalities of the processes in the sense that the event sequences as well as the pre-conditions and post-conditions of each event occurrence are preserved. Based on the known properties of augmented marked graphs, the system properties such as liveness, boundedness and reversibility can be analysed effectively. The method is applied to manufacturing system design. Promising results are obtained

Design and Management of Manufacturing Systems

Although the design and management of manufacturing systems have been explored in the literature for many years now, they still remain topical problems in the current scientific research. The changing market trends, globalization, the constant pressure to reduce production costs, and technical and technological progress make it necessary to search for new manufacturing methods and ways of organizing them, and to modify manufacturing system design paradigms. This book presents current research in different areas connected with the design and management of manufacturing systems and covers such subject areas as: methods supporting the design of manufacturing systems, methods of improving maintenance processes in companies, the design and improvement of manufacturing processes, the control of production processes in modern manufacturing systems production methods and techniques used in modern manufacturing systems and environmental aspects of production and their impact on the design and management of manufacturing systems. The wide range of research findings reported in this book confirms that the design of manufacturing systems is a complex problem and that the achievement of goals set for modern manufacturing systems requires interdisciplinary knowledge and the simultaneous design of the product, process and system, as well as the knowledge of modern manufacturing and organizational methods and techniques

Recent advances in petri nets and concurrency

CEUR Workshop Proceeding

Petri Nets at Modelling and Control of Discrete-Event Systems with Nondeterminism - Part 2

Discrete-Event Systems (DES) are discrete in nature. Petri Nets (PN) are one of the most widespread tools for DES modelling, analyzing and control. Different kinds of PN can be used for such purposes. Some of them were described in [3], being the first part of this paper. Here, the applicability of Labelled PN (LbPN) and Interpreted PN (IPN) for modelling and control of nondeterministic DES, especially with uncontrollable and/or unobservable transitions in the models, will be pointed out. Moreover, another kinds of nondeterminism in DES (errors, failures) will be modelled, and the possibilities of the error recovery of failed system will be presented

A methodology for workflow modeling : From business process modeling towards sound workflow specification

Der Einsatz von Workflow Management Systemen (WFMS) in Unternehmen oder Verwaltungen mit einfach strukturierten und automatisierbaren Prozessen bietet ein hohes Potenzial für die Optimierung der Geschäftsprozesse. Für die Koordinierung von Geschäftsprozessen zur Laufzeit benötigen WFMS Workflow-Spezifikationen, die den automatisierbaren Anteil der Geschäftsprozesse in einer maschinenlesbaren Form beschreiben. In der Praxis werden Workflow-Spezifikationen bislang oft unabhängig von bereits existierenden Geschäftsprozessmodellen erstellt. Es existiert kein methodisch fundiertes Vorgehensmodell, dass die Modellierung von Gechäftsprozessen und die Weiterverwendung der erstellten Modelle für die Workflow-Spezifikation unterstützt [GHS95,AaHe02]. Diese Arbeit schlägt ein durchgehendes Vorgehensmodell für die Spezifikation von Workflows in Form von Petrinetzen vor. In dem fünfstufigen Vorgehensmodell wird der Schwerpunkt auf die Modellierung der Kontrollflussaspekte gelegt. Im Rahmen der Modellierung werden die folgenden Schritte unterstützt: 1. Modellierung der Geschäftsprozesse 2. Formalisierung durch Petrinetze 3. Korrektheitstest und Fehlerkorrektur 4. Festlegung und Integration einer Ausführungsstrategie 5. Kontrollverfeinerung. Das Ergebnis ist ein Prozessmodell mit formal fundierter und operationaler Semantik, das zudem sound [Aal98] ist. Ein solches Modell entspricht den Anforderungen an eine Workflow-Spezifikation, deren Verwendung für ein WFMS eine zuverlässige Ausführung der Geschäftsprozesse zur Laufzeit garantiert. In dem ersten Schritt "Modellierung der Geschäftsprozesse" wird die Verwendung semiformaler Modellierungstechniken unterstützt. Diese räumen dem Modellierer Spielraum in der Beschreibung der Prozesse ein. Im nächsten Schritt wird das erstellte Modell intern formalisiert. Die Formalisierung basiert auf einer Abbildung in Petrinetze. Dabei werden Mehrdeutigkeiten nicht eliminiert sondern explizit gemacht. Im dritten Schritt wird das Modell auf Korrektheit überprüft. Dafür werden neue, pragmatische Kriterien eingeführt. Es werden präzise Fehlermeldungen zurückgegeben, die ein iteratives Verbessern der Geschäftsprozessmodelle ermöglichen. In Schritt vier und fünf wird das erstellte Modell auf eine Workflow-Spezifikation abgebildet. Dazu wird auf die bereits erstellte Petrinetz-Formalisierung zurückgegriffen. Die Petrinetze werden zunächst so erweitert, dass eine Ausführungsstrategie festgelegt wird. Durch die Integration der Strategie werden alle vorher noch enthaltenen Mehrdeutigkeiten beseitigt. Abschließend werden Aktivitäten verfeinert. Das vorgeschlagene Vorgehensmodell bindet in der Praxis bewährte Techniken ein und stellt angemessene Kriterien für die Fehlerkorrektur zur Verfügung. Das gesamte Vorgehensmodells ist methodisch unterlegt und greift auf Ergebnisse der Petrinetztheorie, der Spieltheorie und der Controller Synthesis zurück.Supporting business processes with the help of workflow management systems is a necessary prerequisite for many companies to stay competitive. An important task is the specification of workflow, i.e. these parts of a business process that can be supported by a computer system. A workflow specification mainly refines a business process description, incorporating details of the implementation. Despite the close relation between the two process descriptions there is still no satisfactory link between their modeling. This fact mainly relies on the assignment to different peolpe (IT- vs. domain experts) having a different modeling culture. The thesis provides a methodically well-founded approach for the specification of functional workflow requirements. It supports domain experts in their modeling of business processes in a semiformal manner and guides them stepwise towards a formal workflow specification, i.e. helping to bridge the gap between business process modeling and workflow specification. The proposed approach acknowledges the need to describe business processes at different levels of abstraction and combines the advantages of different modeling languages that proved to fit the respective requirements. A semiformal modeling language is proposed to be used by the domain expert. As a prominent example, widely accepted in practice, are Event-driven Process Chains (EPCs). For the definition of the workflow specification we use a particular type of Petri nets. The strength of Petri-nets is their formally founded, operational semantics which enables their use as input format for workflow management systems. The key concept for the proposed process model is the use of pragmatic correctness criteria, namely relaxed soundness and robustness. They fit the correctness requirements within this first abstraction level and make it possible to provide a feedback to the modeler. To support the execution of the business process at run time, the resulting process description must be refined to fit the requirements of a workflow specification. The proposed process model supports this refinement step, applying methods from controller synthesis. A sound WF-system is automatically generated on the basis of a relaxed sound and robust process description. Only within this step do performance issues become relevant. Information that is incorporated relates to a certain scheduling strategy. The late determination of performance issues is especially desirable as corresponding information (the occurrence probability of a certain failure, costs of failure compensation, or priorities) will often only become available at run-time. Their incorporation towards the end of the proposed process model extends the possibility to reuse modeling results under changing priorities. The resulting process description is sound. Using it as a basis for the execution support during run-time reliable processing can be guaranteed

An agile and adaptive holonic architecture for manufacturing control

Tese de doutoramento. Engenharia Electrotécnica e de Computadores. 2004. Faculdade de Engenharia. Universidade do Port

Supervisory Control and Analysis of Partially-observed Discrete Event Systems

Nowadays, a variety of real-world systems fall into discrete event systems (DES). In practical scenarios, due to facts like limited sensor technique, sensor failure, unstable network and even the intrusion of malicious agents, it might occur that some events are unobservable, multiple events are indistinguishable in observations, and observations of some events are nondeterministic. By considering various practical scenarios, increasing attention in the DES community has been paid to partially-observed DES, which in this thesis refer broadly to those DES with partial and/or unreliable observations. In this thesis, we focus on two topics of partially-observed DES, namely, supervisory control and analysis. The first topic includes two research directions in terms of system models. One is the supervisory control of DES with both unobservable and uncontrollable events, focusing on the forbidden state problem; the other is the supervisory control of DES vulnerable to sensor-reading disguising attacks (SD-attacks), which is also interpreted as DES with nondeterministic observations, addressing both the forbidden state problem and the liveness-enforcing problem. Petri nets (PN) are used as a reference formalism in this topic. First, we study the forbidden state problem in the framework of PN with both unobservable and uncontrollable transitions, assuming that unobservable transitions are uncontrollable. For ordinary PN subject to an admissible Generalized Mutual Exclusion Constraint (GMEC), an optimal on-line control policy with polynomial complexity is proposed provided that a particular subnet, called observation subnet, satisfies certain conditions in structure. It is then discussed how to obtain an optimal on-line control policy for PN subject to an arbitrary GMEC. Next, we still consider the forbidden state problem but in PN vulnerable to SD-attacks. Assuming the control specification in terms of a GMEC, we propose three methods to derive on-line control policies. The first two lead to an optimal policy but are computationally inefficient for large-size systems, while the third method computes a policy with timely response even for large-size systems but at the expense of optimality. Finally, we investigate the liveness-enforcing problem still assuming that the system is vulnerable to SD-attacks. In this problem, the plant is modelled as a bounded PN, which allows us to off-line compute a supervisor starting from constructing the reachability graph of the PN. Then, based on repeatedly computing a more restrictive liveness-enforcing supervisor under no attack and constructing a basic supervisor, an off-line method that synthesizes a liveness-enforcing supervisor tolerant to an SD-attack is proposed. In the second topic, we care about the verification of properties related to system security. Two properties are considered, i.e., fault-predictability and event-based opacity. The former is a property in the literature, characterizing the situation that the occurrence of any fault in a system is predictable, while the latter is a newly proposed property in the thesis, which describes the fact that secret events of a system cannot be revealed to an external observer within their critical horizons. In the case of fault-predictability, DES are modeled by labeled PN. A necessary and sufficient condition for fault-predictability is derived by characterizing the structure of the Predictor Graph. Furthermore, two rules are proposed to reduce the size of a PN, which allow us to analyze the fault-predictability of the original net by verifying that of the reduced net. When studying event-based opacity, we use deterministic finite-state automata as the reference formalism. Considering different scenarios, we propose four notions, namely, K-observation event-opacity, infinite-observation event-opacity, event-opacity and combinational event-opacity. Moreover, verifiers are proposed to analyze these properties