Petri net modeling and analysis of an FMS cell

Petri nets have evolved into a powerful tool for the modeling, analysis and design of asynchronous, concurrent systems. This thesis presents the modeling and analysis of a flexible manufacturing system (FMS) cell using Petri nets. In order to improve the productivity of such systems, the building of mathematical models is a crucial step. In this thesis, the theory and application of Petri nets are presented with emphasis on their application to the modeling and analysis of practical automated manufacturing systems. The theory of Petri nets includes their basic notation and properties. In order to illustrate how a Petri net with desirable properties can be modeled, this thesis describes the detailed modeling process for an FMS cell. During the process, top-down refinement, system decomposition, and modular composition ideas are used to achieve the hierarchy and preservation of important system properties. These properties include liveness, boundedness, and reversibility. This thesis also presents two illustrations showing the method adopted to model any manufacturing systems using ordinary Petri nets. The first example deals with a typical resource sharing problem and the second the modeling of Fanuc Machining Center at New Jersey Institute of Technology. Furthermore, this thesis presents the analysis of a timed Petri net for cycle time, system throughput and equipment utilization. The timed (deterministic) Petri net is first converted into an equivalent timed marked graph. Then the standard procedure to find the cycle time for marked graphs is applied. Secondly, stochastic Petri net is analyzed using SPNP software package for obtaining the system throughput and equipment utilization. This thesis is of significance in the sense that it provides industrial engineers and academic researchers with a comprehensive real-life example of applying Petri net theory to modeling and analysis of FMS cells. This will help them develop their own applications

Scheduling and discrete event control of flexible manufacturing systems based on Petri nets

A flexible manufacturing system (FMS) is a computerized production system that can simultaneously manufacture multiple types of products using various resources such as robots and multi-purpose machines. The central problems associated with design of flexible manufacturing systems are related to process planning, scheduling, coordination control, and monitoring. Many methods exist for scheduling and control of flexible manufacturing systems, although very few methods have addressed the complexity of whole FMS operations. This thesis presents a Petri net based method for deadlock-free scheduling and discrete event control of flexible manufacturing systems. A significant advantage of Petri net based methods is their powerful modeling capability. Petri nets can explicitly and concisely model the concurrent and asynchronous activities, multi-layer resource sharing, routing flexibility, limited buffers and precedence constraints in FMSs. Petri nets can also provide an explicit way for considering deadlock situations in FMSs, and thus facilitate significantly the design of a deadlock-free scheduling and control system. The contributions of this work are multifold. First, it develops a methodology for discrete event controller synthesis for flexible manufacturing systems in a timed Petri net framework. The resulting Petri nets have the desired qualitative properties of liveness, boundedness (safeness), and reversibility, which imply freedom from deadlock, no capacity overflow, and cyclic behavior, respectively. This precludes the costly mathematical analysis for these properties and reduces on-line computation overhead to avoid deadlocks. The performance and sensitivity of resulting Petri nets, thus corresponding control systems, are evaluated. Second, it introduces a hybrid heuristic search algorithm based on Petri nets for deadlock-free scheduling of flexible manufacturing systems. The issues such as deadlock, routing flexibility, multiple lot size, limited buffer size and material handling (loading/unloading) are explored. Third, it proposes a way to employ fuzzy dispatching rules in a Petri net framework for multi-criterion scheduling. Finally, it shows the effectiveness of the developed methods through several manufacturing system examples compared with benchmark dispatching rules, integer programming and Lagrangian relaxation approaches

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

Property-preserving subnet reductions for designing manufacturing systems with shared resources

AbstractThis paper handles two problems in manufacturing system design: resource sharing and system abstraction. In a manufacturing system, resources such as robots, machines, etc. are shared by several processes. When the resources are switched from one process to another, they may need some modifications such as cleaning oil, adding equipments and so on. Previous designing methods assume that the resources have no intermediate modifications. Hence, they need to be extended to handle such kinds of resource-sharing problems. As for abstraction, modeling operations with single places in manufacturing system design is very popular. From the viewpoint of verification, the objective is to verify whether the reduced model has the same desirable properties as the original one. This paper presents three kinds of property-preserving subnet reduction methods. For each reduction method, conditions are presented for ensuring that the properties liveness, boundedness and reversibility are preserved. Applications of these reduction methods to handling the above resource sharing and system abstraction problems are illustrated with an example from the manufacturing system

On cost-effective reuse of components in the design of complex reconfigurable systems

Design strategies that benefit from the reuse of system components can reduce costs while maintaining or increasing dependability—we use the term dependability to tie together reliability and availability. D3H2 (aDaptive Dependable Design for systems with Homogeneous and Heterogeneous redundancies) is a methodology that supports the design of complex systems with a focus on reconfiguration and component reuse. D3H2 systematizes the identification of heterogeneous redundancies and optimizes the design of fault detection and reconfiguration mechanisms, by enabling the analysis of design alternatives with respect to dependability and cost. In this paper, we extend D3H2 for application to repairable systems. The method is extended with analysis capabilities allowing dependability assessment of complex reconfigurable systems. Analysed scenarios include time-dependencies between failure events and the corresponding reconfiguration actions. We demonstrate how D3H2 can support decisions about fault detection and reconfiguration that seek to improve dependability while reducing costs via application to a realistic railway case study

Recent advances in petri nets and concurrency

CEUR Workshop Proceeding

Artificial Intelligence and Systems Theory: Applied to Cooperative Robots

This paper describes an approach to the design of a population of cooperative robots based on concepts borrowed from Systems Theory and Artificial Intelligence. The research has been developed under the SocRob project, carried out by the Intelligent Systems Laboratory at the Institute for Systems and Robotics - Instituto Superior Tecnico (ISR/IST) in Lisbon. The acronym of the project stands both for "Society of Robots" and "Soccer Robots", the case study where we are testing our population of robots. Designing soccer robots is a very challenging problem, where the robots must act not only to shoot a ball towards the goal, but also to detect and avoid static (walls, stopped robots) and dynamic (moving robots) obstacles. Furthermore, they must cooperate to defeat an opposing team. Our past and current research in soccer robotics includes cooperative sensor fusion for world modeling, object recognition and tracking, robot navigation, multi-robot distributed task planning and coordination, including cooperative reinforcement learning in cooperative and adversarial environments, and behavior-based architectures for real time task execution of cooperating robot teams

The valuation of riparian fisheries in Southern and Eastern Africa

This paper reviews the published and gray literature concerning economic valuations of river fisheries in eastern and southern Africa, extracting the best available information on their direct economic values and on the impacts of changes in water management on this value. It then assesses the methods used and makes recommendations regarding approaches to be used in future. The review concentrates on rivers with their associated floodplains and major deltas. The values and issues associated with estuaries and lakes are not considered

ANALYSIS OF A TRANSPORT PROCESS USING HYBRID PETRI NETS

Purpose of the paper is to analyze the Petri net model, to describe the transport process, part of amanufacturing system and its dynamics.A hibrid Petri net model is built to describe the dinamics of the transport process manufacturingsystem. Mathematical formulation of the dinamycs processes a detailed description. Based on this model, theanalysis of the transport process is designed to be able to execute a production plan and resolve any conflictsthat may arise in the system.In the analysis dinamics known two stages: in the continuous variables are discrete hybrid system in thehibrid discrete variables are used as safety control with very well defined responsibilities.In terms of the chosen model, analyze transport process is designed to help execute a production planand resolve conflicts that may arise in the process, and then the ones in the syste