Methods of measuring the size and complexity of PLC programs in different logic control design methodologies

Currently there is a wide variety of logic control design methodologies used in industrial logic design. These methodologies include ladder diagrams, function block diagrams, sequential function charts, and flow charts, but driven by a desire for verifiability, academics are developing additional logic control design methodologies, such as modular finite state machines and Petri nets. Using these, important properties of programs can be verified and some logic can be generated automatically from a part plan. The main contribution of this paper is to define methods for measuring programs written in different methodologies, so that the performance of the methodologies can be compared.We demonstrate these methods of measurement using four program samples that perform similar functions on the same machine, written in four logic control design methodologies: ladder diagrams, Petri nets, signal interpreted Petri nets and modular finite state machines.     

A new approach to supervisor design with sequential control Petri-net using minimization technique for discrete event system

This paper proposes a systematic approach for the design of a supervisory controller for discrete event systems (DES) and their ladder logic diagrams (LLD). The method is based on Petri nets, which is used for modeling the systems. It involves, defining the control policy and simplifying it by using Espresso software to form the compiled controller by adding inhibiting and enabling arcs to the original model and then the method is used to drive structured ladder logic code. A design example is presented to clarify the full procedure.     

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.     

Modular synthesis of Petri nets for modeling flexible manufacturing systems

This paper proposes a modular Petri net synthesis method for modeling flexible manufacturing systems based on synchronization among control processes of the manufacturing resources (such as robots and machines). In the method, the target system is modeled in a bottom-up and uniform manner by first describing the system's control processes using strongly connected state machines (SCSMs) as the basic modules. Each SCSM may contain multiple tokens to represent resources from the same type such as spaces in a buffer. Next, the common transitions and common transition subnets of the modules are merged to represent their synchronization. The system model constructed is proven to be conservative and thus bounded. Moreover, a restricted class of merged nets is proven to be live and reversible. For general classes of merged nets, this paper shows theorems that easily calculateP-invariants of the final net without solving the linear system equations. TheseP-invariants can be used to help in verifying the model's qualitative properties such as liveness.     

Building AGV traffic-control models with place-transition nets

Owing to the lack of robust design and control algorithms, most current applications of automated guided vehicle systems (AGVSs) employ simple control methods despite the fact that the system is far from efficient. Recently, Petri nets have evolved into a powerful tool for modelling complex manufacturing systems. One of the advantages of the use of Petri nets is that analysis, simulation and on-line control can all be done on the same model once the model is built. The purpose of this paper is to establish the research fundamentals in the field of the Petri-net modelling of an AGVS. The main contribution of the paper is to define basic traffic-control nets which can be used directly to model an AGVS without too much thinking. Some basic AGV Petri-net control elements are described and illustrated in detail. Also, difficulties in the use of bidirectional flows are discussed, and it has been determined how Petri nets can be used to solve these problems.     

Generation of optimal control policy for flexible manufacturing cells: A Petri net approach

For flexible manufacturing cells (FMCs), the major control problem is to determine the optimal part routeing and machines dispatching policy. In this research, we present a methodology for generating optimal operating strategies for FMCs. The methodology is built on two techniques: stochastic timed Petri net (STPN) and Markov decision process (MDP). The operating strategy devised with the proposed methodology is deadlock-free and provides the best performance given a set of system parameters (machines, parts, machining times, etc.). Generation of programmable logic controller (PLC) codes is greatly facilitated as the generated strategy can be mapped directly to a ladder diagram representing the PLC code. The objective of real-time optimal control could then be achieved with this type of controller. An example is provided to illustrate the methodology.     

基于柔性制造系统的Petri网模型简化规则

为解决Petri网模型中的状态爆炸问题，提出了一种将简化规则应用到Petri网模型中的方法。该方法通过库所和变迁的合并与消除，来达到简化模型的目的。以柔性制造系统的加工站为例，建立加工站的Petri网模型，将简化规则应用到改进的加工站模型中。为了说明简化网保持了原网的外部功能，利用基于时序Petri网的时态逻辑规则进行了证明。结果显示，简化网在保持功能性的基础上，提高了验证和分析的效率。     

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.     

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。     

Constraint graph-based approach for the analysis and the control of time critical systems

The increasing complexity of manufacturing systems gives rise to the development of new formal methods allowing to handle their particularities. Petri nets are a powerful formalism for the specification and verification of concurrent systems including sequential systems and manufacturing systems. To deal with systems whose time issues become fundamental, different time Petri nets extensions have been developed in the literature, each one being dependent on the application considered. For the time critical systems, their correctness depends not only on the logic correctness but also on the time constraints. Timing issues are essential. In this paper, a new method of analysis and control for P-time Petri nets is proposed. It is based on the firing instant notion and exploits a constraint graph approach.     

基于PLC多层架构构建数控语言的方法与实现

针对"使用梯形图等编程方式编写PLC运动控制程序,在调试和优化过程中,需要不断修改PLC控制程序和调整参数而导致开发效率低"的问题,提出了一种在PLC上构建数控语言的方法。详细介绍了构建数控语言的实现过程,包括数控指令的设计与实现、数控程序的编译和执行;重点说明了如何采用PLC模块化编程方式实现数控指令的功能,同时给出了数控程序解析执行控制算法;最后在可定制嵌入式PLC上构建了数控语言层,并通过具体实例进行了验证分析。研究结果表明,利用该方法能够方便地进行运动控制序列的调整和参数的设置,并且可以有效减少PLC控制程序的代码量,从而提高开发效率;此外,还可针对不同的实际应用,快速定制出相应的数控语言,满足灵活多变的控制要求,适合在工业生产中使用。     

钻孔组合机床液压传动和电气控制系统的设计

以单面钻孔组合机床为对象,依据液压系统设计的基本原理,拟出合理的液压系统图,通过系统主要参数的计算确定了液压元件的规格。以FX1N-24MRPLC为主控制器,基于FXGP/WIN-C软件,采用梯形图编程方式,用顺序控制设计开发了控制程序,实现机床的自动循环加工。     

基于PLC的双面钻孔组合机床控制系统的改造

采用西门子s7-200系列可编程控制器（PLC）对双面钻孔组合机床的控制系统进行技术改造,并介绍了双面钻孔组合机床的工作过程,给出了PLC控制的I／O端子接线图和梯形图程序。实践证明,改造后的双面钻孔组合机床在实际生产中运行稳定,生产效率大大提高。     

基于扩展随机Petri网的可重组制造系统建模与分析方法

可重组制造系统(Reconfigurable manufacturing system,RMS)可根据市场变化进行组态调整和组元升级,系统的建模与分析方法必须能适应上述特点。提出基于扩展随机Petri网(Extended stochastic Petri nets,ESPN)的模块化建模方法,将RMS不同的加工资源对应于相应的ESPN基本模块,并通过过渡变迁合成ESPN模型,该模型能适应任意分布的制造系统,可更加精确地反映生产过程。在此基础上采用基于行为表达式的分析方法,得到系统性能指标,该分析方法可不必画出可达图而直接得到系统性能关系函数,使分析过程更加直观、简洁。可重组电动机生产线的实例证明了该建模与分析方法的有效性。     

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.     

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。     

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.     

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。     

Process Variety Modeling for Process Configuration in Mass Customization: An Approach Based on Object-Oriented Petri Nets with Changeable Structures

Mass customization strategies could be usefully deployed by companies whose products are characterized by a modular design. Typically, each module serves a specific aspect of the overall product function at varying performance levels. Each product variant (constructed through a unique combination of modular performance levels), therefore, serves to customize the overall performance of the product, thus serving the unique needs of each customer. The high demand for each module guarantees economies of scale and, eventually, low cost to customer. The rationale of configuring production processes for producing individual product variants originates from the fact that massive process data is commonly available in a firm and there exists a generic process structure underlying the production of similar products in a family. To design a decision support mechanism that constructs process configuration corresponding to a given product configuration, this paper develops a formal modeling of process variety using Petri nets. Object-oriented Petri nets (PNs) with changeable structures (OPNs-CS) are applied to deal with the issues of generic representation, constraints compliance, and operational sequence requirements. Object-oriented PN (OPN) models facilitate generic representation of product and process variety as well as their instantiations. The OPNs-CS model is tested with simulation. Based on simulation results, the optimal configuration of production processes can be determined for each individual product as well as the cohort of a product family. To illustrate the feasibility and potential of OPNs-CS based process variety modeling, a case study of process configuration for mass customized textile spindles is reported.     

支持混流生产的可重组逻辑控制器设计方法

针对可重组制造系统混流生产、快速重组的特点,提出了一种基于Petri网的可重组模块化逻辑控制器设计方法。该控制器包括产品决策逻辑控制器和加工设备逻辑控制器,通过施加不同的条件变量,以明确相互之间的时序关系,并给出了相应的模块连接算法,产品变化通过变量调整可快速重组逻辑控制器。由Petri网自身特性证明,支持混流生产的可重组模块化逻辑控制器是活性、安全和可逆的,可直接转换为用于工业现场控制的顺序功能流程图。实际应用证明,该控制器具有高度模块化、易于重构、支持混流的特点。