Addressing design and control issues of AGV-based FMSs with Petri net aided simulation

Automated Guided Vehicle Systems (AGVS) are widely used in Flexible Manufacturing Systems (FMS) as they provide modularity and flexibility. These systems are highly complex and expensive, hence careful design and operational planning are essential. Realizing the limitations of the available analytical and simulation methods, this paper proposes extensions to the Timed Petri nets method, and presents an hierarchical modelling and simulation methodology. It addresses design and control issues such as the number of vehicles required, machine scheduling, AGV dispatching, etc., of an AGV-based FMS.     

Petri net-based workflow modelling and analysis of the integrated manufacturing business processes

The well-behaved properties in workflow systems are very important for business processes. In this paper, a structured business process modelling approach by using free-choice Petri nets and workflow analysis techniques is proposed. This approach can be used to verify the soundness of workflow process specification for finding control-flow errors in the designs. For example, deadlock and dangling tasks in process models can be avoided by using this approach. However, for arbitrary Petri nets, the checking of sound properties is a NP-hard problem. This paper presents a structured business process modelling algorithm. This algorithm employs six well-behaved control structures, chaining rule and modified nesting rule to construct well-behaved process models. The resulting process models from this algorithm are free-choice workflow-nets, for which sound property can be decided in polynomial time. The proposed method is an extension of the workflow-net. The basic transformation rules in the workflow-net are subsets of the proposed nesting rule in this paper. The process models resulted from this structured business process modelling algorithm also have the advantages of modularity, readability and maintainability.     

The development of an AGVS model by the union of the modulised floor-path nets

The purpose of this paper is to propose an easy and quick method for the development of an automated guided vehicle system (AGVS) model. It suggests that the AGVS model can be developed directly by the union of modulised floor-path nets without any modification. Several modulised floor-path nets for both uni- and bi-directional systems are established in this paper. To make sure that the complete model obtained by this method is robust, the modulised floor-path nets are required to possess several essential properties — safeness, boundedness, conservation, reachability and liveness. To keep these properties in the complete model after the union, three union rules and an union procedure are proposed.     

Using a Petri-Net-Based Approach for the Real-Time Supervisory Control of an Experimental Manufacturing System

A new Petri-net-based design technique, called the inhibitor are method, for the synthesis of compiled supervisors for discrete event systems is used to solve a forbidden state problem in an experimental manufacturing system. The technique used offers the following advantages: 1. The closed-loop (i.e. controlled) behaviours of the systems are non-blocking and do not contradict the forbidden state specifications. 2. The closed-loop behaviours of the systems are maximally permissive within the specifications. The supervisors to be synthesised consist of a controlled automation Petri net (APN) model of the syetem. Automation Petri nets include the following extensions to the ordinary Petri-net framework; sensor readings as firing conditions at transitions and actions assigned to places. Ladder logic diagram (LLD) code is used to implement the superviosors on programmable logic controllers (PLC). It is important to note that the supervisors obtained are correct by construction, therefore there is no need for verification. This paper particularly shows the applicability of previous results [1] to low-level real-time control where the role of the supervisor is to arrange low-level interaction between the control devices, such as motors, actuators, etc. This is done by considering an experimental manufacturing system.     

Modelling and Evaluation of Manufacturing Systems Using Dedicated Petri Nets

The design of a manufacturing system requires modelling and performance evaluation techniques. To support this process, a modelling method based on Petri nets is proposed in this paper. A new class of coloured Petri nets is introduced, which is well suited to the modelling of manufacturing systems. Using this net class, the structure and the work plans of a manufacturing system can both be modelled separately. A library of model templates helps to create large models. The different model parts are merged automatically to create a complete model of the manufacturing system. Measures of interest can be obtained from the model by numerical analysis or simulation, showing its performance and dependability. The usefulness of the approach is shown by applying the proposed techniques to a real-life manufacturing system.     

Performance Modelling and Evaluation of Dynamic Tool Allocation in Flexible Manufacturing Systems using Coloured Petri Nets: An Object-Oriented Approach

Manufacturing systems are faced with ever-increasing customisation and unstable demand. The traditional hierarchical control structures for shop floor (pre-release planning, scheduling, dispatching and activity control) are often inflexible in responding to unexpected scenario changes and are thus not robust to system disturbances. In this paper, an object-oriented approach to modelling of FMS dynamic tool allocation and control under a non-hierarchical shop floor control scheme using coloured Petri nets is presented. A client–server paradigm is used in the proposed modelling method. The complete FMS model is partitioned into individual classes (machines, magazines, tool transport system, SGVs, tool storage, etc.) thereby significantly reducing the complexity of the model to a tractable size. The system performance under different tool request selection rules is also evaluated using coloured Petri net simulation. The proposed method can provide the designer of a tool management system with a high-level and structured representation of the tool-sharing control. It also provides an effective method for prototyping and evaluating performance of object-oriented shop floor control software.     

Application of Petri Nets and a Genetic Algorithm to Multi-Mode Multi-Resource Constrained Project Scheduling

Multi-mode and multi-resource constrained scheduling of a project is a complex task. This paper addresses the use of a Petri net as a modelling and scheduling tool in this context. The benefits of Petri nets in project scheduling are discussed. We propose extensions to Petri nets to suit scheduling of activities in a decision CPM. We also propose the use of a P-matrix for token movements in Petri nets. A genetic algorithm is used to find a better solution. Petri-net-aided software including genetic-algorithm-based search and heuristics is described to deal with a multi-mode, multi-constrained scheduling problem with pre-emption of activities.     

冗余信标及其在Petri网结构控制中的应用

提出了冗余严格极小信标的概念 ,冗余信标是网拓扑的一种特殊结构。在保证网系统的所有信标不被清空时 ,只需要考虑那些非冗余的严格极小信标即可。这样便降低无死锁Petri控制器设计的复杂性 ,大大增强了Petri网处理复杂系统的能力。同时提出了一种死锁控制的迭代算法 ,使用冗余信标的概念可以大大简化Petri网的设计。以自动制造系统为例 ,说明了这种概念的应用。     

Deadlock avoidance control for manufacturing systems with multiple capacity resources

This paper addresses the problem of designing a control scheme capable of avoiding deadlock conditions for a class of manufacturing systems. The considered model for the system under control is based on Petri nets, and is formulated according to well-known modelling rules. A deadlock avoidance policy is proposed which is an extension of a previously presented one. The proofs of its main properties are briefly sketched. Finally, an example of an automated manufacturing system for which such a deadlock avoidance policy has been applied is discussed.     

Design of deadlock prevention supervisors using Petri nets

This paper proposes a methodology to synthesize supervisors for a class of sequential resource allocation system for flexible manufacturing systems. The type of Petri nets are called S3PR, where deadlocks are related to emptied siphons. In a former paper (Huang et al., IEEE Trans Syst Man Cybern, 2007), a deadlock prevention policy was proposed based on Petri nets siphons for the type of Petri nets. Since all minimal siphons should be controlled, the deadlock prevention policy is very time-consuming when the system is large. In this research, a concept of the elementary siphon is used to reduce the number of control places. A new siphon-based policy of deadlock prevention for the type of Petri nets is presented. This policy consists of two main stages: The first stage, called elementary siphons control, adds control places to the original net model to prevent elementary siphons from being emptied. The second stage, called generalized siphons control, adds control places that adopt a conservative policy of controlling only the release of parts into the system are used. Compared to the existing approaches, the new deadlock prevention policy can obtain a structurally compact deadlock prevention supervisor by adding only a few control places. Finally, numerical experiments under reachable states illustrate that the proposed algorithm appears to be more permissive than the closely related approaches.     

Discrete event control system design using automation Petri nets and their ladder diagram implementation

As automated manufacturing systems become more complex, the need for an effective design tool to produce both high-level discrete event control systems (DECS) and low-level implementations becomes more important. Petri nets represent the most effective method for both the design and implementation of DECSs. In this paper, automation Petri nets (APN) are introduced to provide a new method for the design and implementation of DECSs. The APN is particularly well suited to multiproduct systems and provides a more effective solution than Grafcet in this context. Since ordinary Petri nets do not deal with sensors and actuators of DECSs, the Petri net concepts are extended, by including actions and sensor readings as formal structures within the APN. Moreover, enabling and inhibitor arcs, which can enable or disable transitions through the use of leading-edge, falling-edge and level of markings, are also introduced. In this paper, the methodology is explained by considering a fundamental APN structure. The conversion of APNs into the IEC1131-3 ladder diagrams (LD) for implementation on a PLC is also explained by using the token passing logic (TPL) concept. Finally, an illustrative example of how APNs can be applied to a discrete manufacturing problem is described in detail.     

Modelling,simulation and scheduling of flexible assembly systems with coloured Petri nets

Scheduling in flexible assembly systems is a complicated phenomenon owing to the large variability in the operating parameters. In this paper, coloured Petri nets are shown to be useful in modelling, simulating and scheduling flexible assembly systems. Case studies from published literature are chosen to show its superiority over some of the heuristic and analytical tools. In addition, the tool is also shown to model and analyse systems with some realistic constraints.     

An intelligent modeling system to improve the machining process quality in CNC machine tools using adaptive fuzzy Petri nets

The paper first presents an AND/OR nets approach for planning of a computer numerical control (CNC) machining operation and then describes how an adaptive fuzzy Petri nets (AFPNs) can be used to model and control all activities and events within CNC machine tools. It also demonstrates how product quality specification such as surface roughness and machining process quality can be controlled by utilizing AFPNs. The paper presents an intelligent control architecture based on AFPNs with learning capability for modeling a CNC machining operation and control of machining process quality. In this paper it will be shown that several ideas and approaches proposed in the field of robotic assembly are applicable to the planning procedure modeling with minor modifications. Graph theories, Petri nets, and fuzzy logic are powerful tools which are employed in this research to model different feasible states for performing a process and to obtain the best process performance path using exertion of the process designer’s criteria.     

On Petri net implementation of proactive resilient holistic supply chain networks

Holistic supply chain network (H-SCN) systems are a set of supply chains that have inter-dependence relations. Design and management of H-SCN systems are largely ignored in the literature. Modeling of H-SCN systems will provide a computational facility for design and management of H-SCN systems. A model for disruption of the H-SCN system was proposed by us before, which consists of two parts: a model for disruption of a single firm and a model for propagation of disruption. The present paper mainly discusses how these models can be implemented especially with Petri nets. Guidelines with templates of how the Petri net models for H-SCN systems can be constructed in industrial setting are also given to facilitate the use of our model. Since H-SCN systems are complex network systems, this paper may also contribute to the field of the Petri net technique for network systems modeling in the aspect of reduction of complexity arising from vast amounts of construct instances—a common problem that existed in a Petri net model. Indeed, the proposed Petri net model is suitable for any type of supply chains or supply chain networks.     

A unified modeling of Kanban systems using petri nets

The success of some Just In Time (JIT) systems has led to a growing interest in Kanban systems, which provide a way to implement a JIT control policy. Much work has recently been devoted to this problem, and especially many models have been developed to evaluate the performance of such systems. In this article, we focus our attention of these existing models. Each author uses his/her own representation, which is not formal in most cases, and so it is often difficult to understand the proposed model and to compare it with others. In this article, we show that Petri nets are well suited to provide a unified modeling of Kanban systems. We first propose a basic model, then show that most models encountered in the literature can easily be represented by a Petri net model. Once such a formal model is obtained, it can then be used to analyze the behavior of the system, both qualitatively and quantitatively. Some preliminary results pertaining to the quantitative analysis are presented at the end of the article.     

基于设计结构矩阵和着色Petri网的产品设计过程建模与仿真

将设计结构矩阵与着色Petri网结合后,进行了产品设计过程建模与仿真的研究。首先利用设计结构矩阵对产品设计过程进行建模,从产品设计过程特点出发,为支持基本过程逻辑的描述定义了着色Petri网基本网模型结构,将设计结构矩阵映射为着色Petri网,建立了产品设计过程着色Petri网模型。在此基础上建立了对设计资源进行有效分配的动态分配模型,通过定量仿真分析,实现了产品开发过程资源冲突的检测及资源的重新分配和调整。桥式起重机小车设计的实例说明了该方法的有效性。     

自动制造系统的一种死锁避免策略

基于Petri网的结构分析理论，提出了自动制造系统Petri网模型的一种死锁控制方法，在这种策略的控制下，避免了系统中死锁的产生，从而许多制造系统的Petri网模型具有活性，提出了一种保证所有严格极小信标至少含有一个托肯的方法，对冗余严格极小信标的研究，提高了Petri网复杂自动制造系统的建模能力。结果表明，在设计无死锁的Petri网格型时，不是所有的严格极小信标都要考虑，从而简化了设计结果和控制算法。     

An FMS scheduling and control decision support system based on generalised stochastic Petri nets

This paper presents an FMS scheduling and control DSS (decision support system) using generalised stochastic Petri nets (GSPNs) for model representation and analysis. The function of the DSS is to assist the FMS operator to search for suitable control strategies that will achieve efficient performance. It can also be called upon in the event of unforeseen changes in production requirements, component failures, or other related contingencies. The GSPN modelling approach is employed because GSPNs can characterise random processing times, machine breakdown and repair rates. The GSPN employed in the proposed DSS also incorporates dispatching control at conflicting immediate transitions and can be used to determine various performance measures pertaining to scheduling and control. An application example of the DSS for the processing of two part types in a three-machine FMS is also described.     

Modeling FMS with decision Petri nets

Decision point extended timed Petri nets or decision Petri nets (DPN) are introduced as an extended modeling framework for FMS performance evaluation. The decision point extension allows the explicit modeling of the control of the flow of tokens in timed Petri nets and hence represents the control of the flow of material, resources, and information in FMS. Further, the concept of a bounded transition is proposed to conveniently model the blocking logic in an FMS with limited buffer capacities. The motivation to present these conventions is to develop a user-friendly graphic model to represent FMS designs for analysis by discrete event simulation. DPN affords concise models that can be conveniently developed and easily transformed into discrete event simulation models. With the help of a simple FMS example, which includes a number of part types, loading rules, dispatching rules, and probabilistic branching (at an inspection station), we illustrate the DPN model development. As an illustration of the ease with which it can be tranformed into a simulation model, we have developed a generalized simulator called ROBSIM and outline here its methodological basis. The proposed concepts should be of interest to users of discrete event simulation in FMS design or elsewhere to tap the potential of basic Petri net concepts for graphic representation and specification purposes. In particular, our work should encourage other researchers to develop extensions relevant to their own areas of interest.     

A production order-driven AGV control model with object-oriented implementation

To effectively manage the material handling in Flexible Manufacturing Systems (FMS), where a large amount of data is required in the dynamic decision-making, integrated control is needed to consider the overall production schedule. The focus of this research is on the development of an integrated Automated Guided Vehicle System (AGVS) control model that includes essential features like dynamic vehicle path determination and conflict-free routing. An object-oriented implementation of the AGVS model is proposed that forms the basis of systems integration with a production planning module such as MRP. Static and dynamic informational and functional models of the AGVS are developed. The system incorporates: (i) conflict-free shortest path routing procedures, and (ii) vehicle assignment rules or scheduling strategies. A prototype version of each of these has been developed for demonstrative purposes. This object-oriented modelling methodology provides the capability of rapid development and change. The approach has been demonstrated for a real manufactured product through simulation studies which confirm the superior performance of anticipatory AGVS control rules, even in a production orderdriven environment.