PN PLC: A methodology for designing,simulating and coding PLC based control systems using Petri nets

Flexible manufacturing cells (FMCs) are well established as a means of improving manufacturing efficiency in many batch production industries. They consist of an integrated group of computer numerically controlled (CNC) machine tools, programmable logic controller (PLC) based work/tool handling equipment and a control system to synchronize their operation. The safe and reliable operation of FMCs is clearly essential for their efficient use. However, FMCs are complex systems with elements that operate concurrently and interact at irregular times, depending upon the components to be produced. This inherent complexity demands intricate and sophisticated control systems which require development methodologies that are both comprehensive and rigorous. In this paper the authors introduce a methodology (PN PLC) which uses Petri nets for producing correct programmable logic controller (PLC) programs directly from the specifications. The methodology is specifically contrasted with the benefits, and problems, encountered in using tools such as 'ladder diagrams' (LDs) and 'sequential function charts' (SFCs) for designing logical control systems. PN PLC has advantages over ladder logic in clarity, which allows the program to be checked, maintained and updated reliably. Furthermore, the Petri net graph is a one-to-one equivalent of a ladder logic diagram, and thus either representation may be algorithmically translated to the other without loss of information. Finally, the rules for creating Petri net graphs, and subsequently translating them, are both simple and robust in use.     

Critical scenarios derivation methodology for mechatronic systems

This paper deals with safety in design of mechatronic systems. We propose a method based on a qualitative analysis of a Petri net model of the system. It allows deriving feared scenarios by determining the sequences of actions and state changes leading to the feared state in which the passenger's safety is no longer guaranteed. The Petri net model of the system takes into account normal behaviour, failures and reconfiguration mechanisms. Our approach uses linear logic as formal framework and is based on a backward and a forward reasoning. It derives feared scenarios as causal relationships between normal states and the feared one.     

Implementation of Petri nets using a field‐programmable gate array

Although Petri nets have various capabilities, the Petri net approach is done on paper. A field‐programmable gate array (FPGA) is implemented in this study so as to realize basic Petri net symbols, logic structures in Petri nets, and specific functions for Petri nets by logic circuits. As an example, a Petri net for an early failure detection and isolation arrangement (EFDIA) is implemented as an application‐specific integrated circuit (ASIC) on a Xilinx Demonstration Board. This ASIC is verified by three simulations dealing with three different failure scenarios of a system, and the ASIC functions identically to the EFDIA Petri net. Accordingly, not only the EFDIA Petri net but also any specific function Petri nets can be implemented by FPGA circuits. Copyright © 2000 John Wiley & Sons, Ltd.     

An integrated IDEF0-3/CTPN/SFC approach for design and analysis of discrete event control systems

Due to its low cost and increased reliability, the programmable logical controller (PLC) plays an important role in industry automation. However, as systems become larger and more complex, efficient and systematic analysis and design of PLCs become a more important issue. In this research, an integrated IDEF0-3/CTPN/SFC (IPS) for developing discrete event control systems (DECSs) is presented. The proposed integrated methodology provides a complete and systematic development process for the DECSs. The development process is divided into four stages: functional analysis, system behaviour analysis, system design, and system implementation. The IDEF0-3, coloured timed Petri net (CTPN) and sequential function chart (SFC) are integrated into the corresponding stages based on their characteristics. In addition, the transformation rules between the IDEF0-3 and CTPN and between CTPN and SFC are also conducted for developing DECSs. The transformation rules provide a straightforward mapping from the IDEF0-3 to CTPN and from the CTPN to SFC. The result of the integrated IPS systematically leads to SFC for PLC implementation. Finally, a ball assorting system is given to illustrate how the integrated IPS approach is implemented for developing the DECSs.     

FAILURE ANALYSIS FOR AN AIRBAG INFLATOR BY PETRI NETS

Petri nets are useful for modelling a variety of asynchronous and concurrent systems, such as automated manufacturing, computer fault tolerant systems, and communication networks. This study employs an airbag inflator system as an example to demonstrate a Petri net approach to failure analysis. This paper uses Petri nets to study minimum cut sets finding, marking transfer, and dynamic behaviour of system failure. For Petri net models incorporating sensors, fault detection and higher-level fault avoidance is dealt with. Compared with fault trees that present only static logic relations between events, Petri nets indeed offer more capabilities in the scope of failure analysis. © 1997 John Wiley & Sons, Ltd.     

A transformation from a Boolean equation control specification to a Petri net

In this article we present an algorithm to transform a set(s) of Boolean equations into a Petri net(s). Boolean logic provides a mathematical basis for switching control and many other areas. In the case of programmable logic controllers used for the sequential control in manufacturing applications, Boolean equations are fundamental both in programming and in understanding their operation. A major difficulty with Boolean equations is the lack of any type of mathematical theory for the analysis of the systems that are being represented by these equations. Petri nets, on the other hand, possess a mathematical framework to analyze the properties of the system being modeled. Furthermore, Petri nets have been proved to be appropriate models for sequential control. The popularity as well as the generality of Boolean logic, on one hand, and the modeling and analytical capabilities of Petri nets, on the other hand, are the major motivations for this research.     

Decomposition in automatic generation of Petri nets for manufacturing system control and scheduling

Despite the efforts in developing Petri net models for manufacturing control and scheduling, the generation of Petri net models cannot be automated for agile manufacturing control and scheduling without difficulties. The problems lie in the complexity of Petri net models. First of all, it is difficult to visualize the basic manufacturing process flow in a complex Petri net model even for a Petri net modelling expert. The second problem is related to the complexity of using Petri net models for manufacturing system scheduling. In this paper, a decomposition methodology in automatic generation of Petri nets for manufacturing system control and scheduling is developed. The decomposition methodology includes representing a manufacturing process with the Integrated Definition 3 (IDEF3) methodology, decomposing the manufacturing process based on the similarity of resources, transforming the IDEF3 model into a Petri net control model, and aggregating sub Petri net models. Specifically, a sequential cluster identification algorithm is developed to decompose a manufacturing system represented as an IDEF3 model. The methodology is illustrated with a flexible disassembly cell example. The computational experience shows that the methodology developed in this paper reduces the computational time complexity of the scheduling problem without significantly affecting the solution quality obtained by a simulated annealing scheduling algorithm. The advantages of the methodology developed in this paper include the combined benefits of simplicity of the IDEF3 representation of manufacturing processes and analytical and control properties of Petri net models. The IDEF3 representation of a manufacturing process enhances the manmachine interface.     

Deadlock prevention policy based on Petri nets and siphons

This paper presents a deadlock prevention method for a class of flexible manufacturing systems where deadlocks are caused by unmarked siphons in their Petri net models. This method is an iterative approach consisting of two main stages. At each iteration, a fast deadlock detection technique developed by mixed integer programming is used to find an unmarked maximal siphon. An algorithm is formalized that can efficiently obtain an unmarked minimal siphon from the maximal siphon. The first stage, called siphons control, of the proposed method is to add, for each unmarked minimal siphons, a control place to the original net with its output arcs to the sink transitions of the minimal siphon. The objective is to prevent a minimal siphon from being unmarked. The second stage, called augmented siphons control, is to add a control place to the modified net with its output arcs to the source transitions of the resultant net if the resource places are removed. The second stage is required since adding control places in the first stage may create new unmarked siphons. In addition, the second stage assures that there are no new unmarked siphons generated. The relation of the proposed method and the liveness and reversibility of the controlled net have been obtained. Finally, manufacturing examples are presented to illustrate the method and to allow comparison with earlier methods.     

Modelling and simulation of automated manufacturing systems for evaluation of complex schedules

Automated manufacturing systems have been studied widely in terms of scheduling. As technology evolves, the behaviour of tools in automated manufacturing systems has become complicated. Therefore, mathematical approaches to the analysis of complex schedules no longer reflect reality. In this paper, we propose a systematic way of conducting simulation experiments to evaluate the complex operating schedules of automated manufacturing systems. A simulation model is based on a timed Petri net to take advantage of its mathematical strength. Since a Petri net cannot itself have token firing rules, we introduce additional states called operational states. Operational states are not directly related to a Petri net, and are only used for decision making. In addition, a decision function that is responsible for the conflict resolution of a Petri net model and an operational state transition function are introduced. The parallel simulation concept is also suggested by dividing a Petri net into several independent decision sub-nets. A multi-cluster tool system for semiconductor manufacturing is analysed as an application.     

应用Petri网改进基于故障树的诊断方法

故障树是表示故障发生和故障传播关系的一种逻辑模型,基于故障树诊断方法的广泛应用由于实际故障树分析过程的ＮＰ困难问题而受到妨碍。而Ｐｅｔｉｒ网是一种特殊的有向网,适合于描述故障的传播关系。本文提出了两种基于Ｐｅｔｒｉ网的改进方法,与下行法相比,用该方法寻找最小割集和最小路集能有效地节省上计算时间,提高推理速度和效率;最后还应用Ｐｉｔｒｉ网的状态方程分析方法提出了基于Ｐｅｔｒｉ网的故障监测和诊断方法。     

基于有限状态机模型的全自动烫印机控制系统设计

针对现有烫印设备存在自动化水平低和人机界面交互性差的问题,采用三维参数化设计方法对全自动烫印机的结构进行设计,以三菱新一代FX3GE系列PLC （programmable logic controller,可编程逻辑控制器）和T5L智能触摸屏为核心搭建了全自动烫印机的网络化控制系统,实现了伺服电机与气缸的高速、高精度驱动,PID （proportion integration differentiation,比例积分微分）温度控制以及多参数人机数据交互。针对传统固定路径顺序指令编程模式无法满足小批量定制化生产模式的要求的问题,提出了基于有限状态机（finite state machine, FEM）模型的全自动烫印机控制程序设计方法。首先,对全自动烫印工艺和运动工位的分布、数量和逻辑顺序关系进行功能规划与模块化设计,建立了层次化有限状态机模型;然后,根据状态迁移事件与状态转换条件构造了烫印动作的状态转换图;最后,结合图论思想,将状态转换图映射到邻接链表变量空间,并采用变量配置方式驱动邻接链表,从而完成对状态转换图的遍历和拓扑扩展。仿真结果表明,与传统的指令编程模式相比,基于有限状态机模型的全自动烫印机控制程序设计方法不仅提升了开发效率和组态化编程水平,有效降低了控制程序的维护成本,还保证了伺服电机与气缸的时序动作不会干涉和交叉,提高了控制系统的可靠性,具有广阔的工程应用前景。     

基于PLC的橡胶包装码垛生产线控制系统设计

针对橡胶后处理装置中半自动包装、人工码垛的现状，研制了橡胶称重、包装、码垛全自动生产线。该生产线采用基于PLC的主从控制系统来控制，可靠性高、操作方便。在控制系统软件的设计中采用的是单元化的状态流程图法，该方法的控制逻辑清晰，特别适合于复杂系统的软件开发。另外还对橡胶包装码垛控制系统中的关键技术环节进行了剖析。     

The application of Petri nets to failure analysis

Unlike the technique of fault tree analysis that has been widely applied to system failure analysis in reliability engineering, this study presents a Petri net approach to failure analysis. It is essentially a graphical method for describing relations between conditions and events. The use of Petri nets in failure analysis enables to replace logic gate functions in fault trees, efficiently obtain minimal cut sets, and absorb models. It is demonstrated that for failure analysis Petri nets are more efficient than fault trees. In addition, this study devises an alternative; namely, a trapezoidal graph method in order to account for failure scenarios. Examples validate this novel method in dealing with failure analysis.     

Reliability analysis of discrete event dynamic systems with Petri nets

This paper deals with dynamic reliability of embedded systems. It presents a method for deriving feared scenarios (which might lead the system to a critical situation) in Petri nets. A classical way to obtain scenarios in Petri nets is to generate the reachability graph. However, for complex systems, it leads to the state space explosion. To avoid this problem, in our approach, Petri net reachability is translated into provability of linear logic sequents. Linear logic bases are introduced and used to formally define scenarios and minimality of scenarios. These definitions allow the method to produce only pertinent scenarios. The steps of the method are described and illustrated through a landing-gear system example.     

A Petri-net-based state-transition model for an optimal operator cyclic walking pattern development in GT cells

An approach for developing the optimal operator scheduling solution for a group technology (GT) production problem is studied. A state-transition model is developed to analyse and gain insight into the operator-machine interaction of the problem. Operator cyclic walking patterns are then denned. A Petri net model has succeeded in determining the optimal cyclic walking pattern. The computational efforts needed for the Petri net model are compared with those for an integer programming model. The results show large savings in computational effort by using the Petri net model. In addition, the extendability of the Petri net model for various system aspects is addressed     

Simulating free-space optical computing architectures

Major issues in optoelectronic system design include timing, synchronization, and control. Designing free-space optical computing architectures is difficult because of the high degree of system complexity, parallelism, and concurrency in conjunction with the high cost and lack of availability of devices. Current simulation tools lack the expressiveness to model the system structure and behavior of parallel and concurrent architectures, thus making them inefficient and ineffective. We show that Petri nets, compared with other system-modeling methodologies, are more efficient and effective at expressing the functional, behavioral, and structural properties of parallel and concurrent architectures. We show how an extended version of the standard Petri net, a timed-colored Petri net, is used to model and simulate free-space optoelectronic computing architectures. We also present methods for analysis of system timing, synchronization, and control behavior.     

Petri-net based applications for supply chain management: an overview

Supply networks are discrete event dynamic systems consisting of suppliers, manufacturers, distributors, and customers. It is a difficult and challenging task to model such a complex system. Recently, characterised as being capable of model parallelism and synchronisation, Petri nets (PNs) have attracted great attention for modelling and studying a supply network. Since each type of Petri net has its unique properties to model specific applications, this paper reviews and classifies Petri net applications for supply chain management (SCM). According to the decision problems facing SCM, we have the Petri net applications classified into three areas: competitive strategy, firm focused tactics, and operational efficiency. Categorising the Petri net applications will provide a guideline to industry practitioners and research academics to choose the appropriate Petri net for the problem. In addition, the paper attempts to offer future directions in applying the Petri net to SCM including: (1) modelling with uncertainty elements; (2) addressing the scalability issue of Petri nets; (3) integrating Petri nets with other tools; and (4) extending Petri nets to emerging applications.     

Coloured timed Petri net model for real-time control of automated guided vehicle systems

Automated guided vehicle systems (AGVS) are material-handling devices representing an efficient and flexible option for products management in automated manufacturing systems. In AGVS, vehicles follow a guide-path while controlled by a computer that assigns the route, tasks, velocity, etc. Moreover, the design of AGVS has to take into account some management problems such as collisions and deadlocks. The paper presents a novel control strategy to avoid deadlock and collisions in zone-controlled AGVS. In particular, the control scheme manages the assignments of new paths to vehicles and their acquisition of the next zone. Moreover, the use of coloured Petri nets is proposed to model the dynamics of AGVS and implement the control strategy stemming from the knowledge of the system state. Additionally, extending the coloured Petri net model with a time concept allows investigation of system performance. Several simulations of an AGVS with varying fleet size while measuring appropriate performance indices show the effectiveness of the proposed control strategy compared with an alternative policy previously presented.