基于变迁覆盖的制造系统死锁控制策略

基于系统Petri网模型,研究柔性制造系统的死锁控制问题.论文利用变迁覆盖为系统设计活性控制器.变迁覆盖是由一组极大完备资源变迁回路组成的集合,其变迁集覆盖了Petri网中所有极大完备资源变迁回路的变迁集.验证变迁覆盖的有效性,然后仅对有效变迁覆盖中的极大完备资源变迁回路添加控制位置,就得到系统的活性受控Petri网.这种受控Petri网包含的控制位置个数少,从而结构相对简单.最后通过一个例子说明了所提出的死锁控制策略的构成与特点.     

一类FMS的最佳活Petri网模型的综合

利用Petri网为一类柔性制造系统建模,并讨论避免系统死锁问题.通过Petri网模 型的结构分析,证明了系统产生死锁的一个充分必要条件.给出了避免死锁的最佳控制器,它 可以通过给系统的Petri网模型增加一些新的位置与相应的弧来实现.从而导出了这类制造 系统的最佳活Petri网模型.     

基于Petri网的无死锁控制器设计

对柔性制造系统提出了一种新的死锁预防控制算法。运用区域理论对Petri网模型设计一个控制器,对控制器中所有严格极小信标求取控制库所与资源库所的代数式,保证了所有的严格极小信标受控。与现有方法相比,该策略不必考虑控制器结构,只需根据这些代数式分布控制器中的资源,就可以得到相应无死锁监督控制器。     

基于Petri网模型的FMS死锁结构

死锁是柔性制造系统中容易发生的异常现象,如不加以消除,将会导致整个系统的瘫痪.文中通过Petri网模型对FMS的结构分析,揭示出导致FMS死锁的结构元素和活性特征,在此基础上,提出了FMS的无死锁Petri网控制器算法,并举例说明这种控制器算法在FMS中的应用.     

基于Petri网的柔性制造系统无死锁遗传调度算法

对以最小化加工时间为目标的柔性制造系统无死锁调度问题, 提出了一种遗传调度算法. 算法考虑到同类工件具有预先确定的相同加工路径, 而各工序的处理时间与工件有关. 用Petri网对工序和资源分配进行逻辑建模,利用遗传算法, 采用工序自然编码方式, 基于系统的最佳避免死锁Petri网控制器, 检测染色体的可行性, 修复不可行染色体使其对应的调度满足资源约束和无死锁控制约束, 从而保证算法所利用的所有染色体都对应系统的可行调度. 仿真结果表明了算法的可行性和有效性.     

基于Petri网模型的FMS死锁结构

死锁是柔性制造系统中容易发生的异常现象，如不加以消除，将会导致整个系统的瘫痪。文中通过Petri网模型对FMS的结构分析，揭示出导致FMS死锁的结构元素和活性特征，在此基础上，提出了FMS的无死锁Petri网控制器算法，并举例说明这种控制器算法在FMS中的应用。     

部分可控Petri网分布式死锁监控器设计

研究了部分可控Petri网柔性制造系统中的死锁避免的问题。为了保证死锁避免和资源最大允许利用,提出了基于分支定界法的Petri网死锁监控器的优化设计方法,采用多个子控制节点对全局状态建立分布式监控器,通过行为可行和分布可行对分布式监控器下合法状态空间进行检测,对最大行为可行子集建立线性规划模型求解最大分布可行合法状态集,得到分布式监控器下的最大合法状态子空间。最后,建立了柔性制造系统的部分可控Petri网模型,针对系统的死锁避免等多个行为特性要求,分别设计了集中式监控器和分布式监控器,分布式监控器能有效地避免死锁。     

基于混合整数规划的一般Petri网死锁检测方法

信标的受控性是检测柔性制造系统(flexible manufacturing system,FMS)Petri网模型是否存在死锁的关键因素.对于普通Petri网,在任何可达标识下所有信标不被清空是检测网系统非死锁的充分条件.然而,该条件对于建模能力更强的一般Petri网并不适用,max可控性条件由此产生.研究证明,该条件对于一般Petri网的死锁检测过于严格了.虽然其后有很多研究者通过改进max可控性条件以求给出条件更宽松的一般Petri网非死锁的充分条件,但大部分的研究成果都仅仅局限于一种顺序资源共享分配系统Petri网模型S4PR(systems of sequential systems with shared resources)网.因此,本文在max可控性条件的基础上提出了新的名为max#可控的信标可控性条件,并在此条件的基础上实现了基于混合整数规划(mixed integer programming,MIP)的死锁检测方法.与现有研究成果相比,max#可控性条件更宽松,可适用于更多类型的一般网,为解决大规模柔性制造系统中死锁监督控制器的结构复杂性问题提供了有力的理论支撑.     

信标基底的可选择性在制造系统死锁控制中的应用

本文基于Petri网模型,讨论柔性制造系统的死锁控制问题.为了建立结构简单的Petri网控制器,本文在以前的工作中提出了信标基底的概念.信标基底是一组满足特定条件的严格极小信标集合.本文证明基于不同的信标基底,建立的受控系统其容许性能也不同.而容许性是评价死锁控制策略优劣的重要标准之一.故如何选择信标基底,提高受控系统的容许性能是值得研究的问题.本文讨论了使受控系统容许性能大大提高的信标基底的选择条件.基于该条件,为柔性制造系统建立有效的死锁控制策略.最后,通过两个例子解释该条件和策略.     

离散制造装配系统的活性控制

首次研究离散制造装配系统的活性控制问题.建立了系统的工件加工过程Petri网 模型.通过对系统Petri网模型的结构分析,提出了导致系统死锁的两类元素结构及活性特 征.对一类离散制造装配系统提出了避免死锁的Petri网控制器,这类控制器容易实现,对系 统的限制小,而且使得受控系统仍具Petri网模型.对一般离散制造装配系统提出了保证系统 活性的控制策略.     

Fault-tolerant deadlock avoidance algorithm for assembly processes

Unreliable resources pose challenges in design of deadlock avoidance algorithms as resources failures have negative impacts on scheduled production activities and may bring the system to dead states or deadlocks. This paper focuses on the development of a suboptimal polynomial complexity deadlock avoidance algorithm that can operate in the presence of unreliable resources for assembly processes. We formulate a fault-tolerant deadlock avoidance controller synthesis problem for assembly processes based on controlled assembly Petri net (CAPN), a class of Petri nets (PNs) that can model such characteristics as multiple resources and subassembly parts requirement in assembly production processes. The proposed fault-tolerant deadlock avoidance algorithm consists of a nominal algorithm to avoid deadlocks for nominal system state and an exception handling algorithm to deal with resources failures. We analyze the fault-tolerant property of the nominal deadlock avoidance algorithm based on resource unavailability models. Resource unavailability is modeled as loss of tokens in nominal Petri Net models to model unavailability of resources in the course of time-consuming recovery procedures. We define three types of token loss to model 1) resource failures in a single operation, 2) resource failures in multiple operations of a production process and 3) resource failures in multiple operations of multiple production processes. For each type of token loss, we establish sufficient conditions that guarantee the liveness of a CAPN after some tokens are removed. An algorithm is proposed to conduct feasibility analysis by searching for recovery control sequences and to keep as many types of production processes as possible continue production so that the impacts on existing production activities can be reduced.     

Verification of hypercube communication structures via parametric Petri nets<Superscript>1</Superscript>

A model of a hypercube communication structure of arbitrary size with an arbitrary number of dimensions is constructed in the form of a parametric Petri net. A technique for calculating linear invariants of parametric Petri nets is developed that allows one to analyze information transmission processes in such a hypercube. The structure of complicated deadlocks caused by a path (loop) of blockings and the isolation of devices is investigated. In real-life networks, the described deadlocks lead to a considerable decrease in performance and can be induced by some ill-intentioned traffic.     

On Siphon Computation for Deadlock Control in a Class of Petri Nets

As a structural object, siphons are well recognized in the analysis and control of deadlocks in resource allocation systems modeled with Petri nets. Many deadlock prevention policies characterize the deadlock behavior of the systems in terms of siphons and utilize this characterization to avoid deadlocks. This paper develops a novel methodology to find interesting siphons for deadlock control purposes in a class of Petri nets, i.e., a system of simple sequential processes with resources . Resource circuits in an are first detected, from which, in general, a small portion of emptiable minimal siphons can be derived. The remaining emptiable ones can be found by their composition. A polynomial-time algorithm for finding the set of elementary siphons is proposed, which avoids complete siphon enumeration. It is shown that a dependent siphon can always be controlled by properly supervising its elementary siphons. A computationally efficient deadlock control policy is accordingly developed. Experimental study shows the efficiency of the proposed siphon computation approach.     

Controller design and performance evaluation for deadlock avoidance in automated flexible manufacturing cells

In this paper, the design of a deadlock avoidance controller is described. The uncontrolled system is modeled using colored Petri nets. The system controller is based on a restrictive (not maximally permissive) deadlock avoidance policy to resolve deadlocks and control the real-time resource allocation decisions in the system. Performance evaluation of systems controlled by not maximally permissive algorithms is essential in determining the applicability and effectiveness of the control algorithms. The performance of the controlled system is compared with performance of optimal control policies to quantify the effects of the restrictiveness of the deadlock avoidance policy on system performance.     

Robustness analysis of Petri nets for assembly/disassembly processes with unreliable resources

In most application of Petri nets, resources are modeled as tokens. Unreliable resources pose challenges as existing Petri net theory is deficient in analyzing the impacts of the perturbation due to resource failures. One strategy to analyze a perturbed system is to study its robustness with respect to uncertainties. In this paper, we study the robustness of a class of controlled Petri nets called controlled assembly/disassembly Petri net (CADPN) for assembly/disassembly processes with unreliable resources. The number of tokens in CADPN is not conservative to capture the effects of resource failures. Based on the CADPN model, we characterize different types of tolerable resource failures allowed for a nominal marking of a live CADPN. We show that liveness of a CADPN can still be preserved under tolerable resource failures.     

Detection of Ada static deadlocks using Petri net invariants

A method is presented for detecting deadlocks in Ada tasking programs using structural; and dynamic analysis of Petri nets. Algorithmic translation of the Ada programs into Petri nets which preserve control-flow and message-flow properties is described. Properties of these Petri nets are discussed, and algorithms are given to analyze the nets to obtain information about static deadlocks that can occur in the original programs. Petri net invariants are used by the algorithms to reduce the time and space complexities associated with dynamic Petri net analysis (i.e. reachability graph generation)     

信标可控性定义及问题

死锁是资源分配系统中极不希望出现的现象,目前死锁控制的一个重要的方法是信标控制法,信标控制法的基础是信标可控性的定义.对于普通Petri网,已有一个完善的信标可控性定义,而对于一般Petri网,这方面的工作还需改进和完善.近年来,学者们针对一般Petri 网及其子类提出了不少信标可控性定义,但这些定义并不完善,仍有大量的问题亟待解决.首先回顾了文献中的各个信标可控性定义,提出了两个新的信标可控性定义,然后从可控性定义的宽松程度、应用范围以及等价性等方面分析比较了现有的信标可控性定义优缺点.最后给出了今后的研究方向.     

A Simulation Engine for Stochastic Timed Petri Nets and Application to Emergency Healthcare Systems

In many service delivery systems, the quantity of available resources is often a decisive factor of service quality. Resources can be personnel, offices, devices, supplies, and so on, depending on the nature of the services a system provides. Although service computing has been an active research topic for decades, general approaches that assess the impact of resource provisioning on service quality matrices in a rigorous way remain to be seen. Petri nets have been a popular formalism for modeling systems exhibiting behaviors of competition and concurrency for almost a half century. Stochastic timed Petri nets (STPN), an extension to regular Petri nets, are a powerful tool for system performance evaluation. However, we did not find any single existing STPN software tool that supports all timed transition firing policies and server types, not to mention resource provisioning and requirement analysis. This paper presents a generic and resource oriented STPN simulation engine that provides all critical features necessary for the analysis of service delivery system quality vs. resource provisioning. The power of the simulation system is illustrated by an application to emergency health care systems.      

Analysis of modularly composed nets by siphons

This paper uses siphons to analyze the class of Petri nets constructed by a modular approach by Jeng and DiCesare (1995) for modeling manufacturing systems with shared resources. A resource point of view is taken. First the behavior of each resource is modeled using resource control nets, strongly connected state machines with one place being marked initially. Interactions among the resources are modeled through merging of common transition subnets. This paper provides conditions, expressed in terms of siphons, under which reversibility and liveness of the integrated model are obtained. Relations between siphons and circular-wait are formally established. Superiority of the siphon-based analysis over a previous analysis using circular wait is shown