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.     

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.     

离散制造装配系统的活性控制1)

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.     

自动化仓库输送调度问题的建模与控制研究

基于面向对象着色Petri网模型和时态逻辑方法,对自动化仓库输送系统运行过程的调度问题进行研究。建立了系统的面向对象着色Petri网模型,讨论了该过程的死锁分析问题,给出了系统行为的时态逻辑规范和死锁避免的最大允许反馈控制策略。     

柔性制造系统的改进粒子群无死锁调度算法

基于柔性制造系统的Petri网模型,以制造期最小为优化目标,将死锁避免策略嵌入粒子群算法中,提出一种无死锁改进粒子群调度算法.该算法将粒子与工件的工序序列相对应,以位置数值的大小表示对应工件工序在执行顺序中的优先级.采用一步向前看的死锁避免策略方法对序列的可行性进行验证,提出一种跳出局部极值的策略.实例仿真结果表明了粒子群调度算法的可行性和有效性,以及改进粒子群调度算法的优越性.     

自动制造系统设计和仿真中的死锁控制

通过对自动制造系统的Pctri网模型的分析．可以揭示出被模拟系统的死锁跟其初始标识和其结构有关。要想消除自动制造系统的死锁异常,可以通过修改Petri网的初始标识或Pctri网的结构来解决。该文结合具体示例,给出了具体的可操作算法并设计出了控制自动制造系统死锁的Petri网模型,从而为自动制造系统的异常处理提供了一条有效途径。     

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.     

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.     

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Petri网的柔性制造系统一种预防死锁方法

基于Petri网的结构特性分析,研究了FMS(柔性制造系统)一种预防死锁方法.提出了 Petri网的一种特殊拓扑结构--基本信标的概念.在Petri网中基本信标的集合是SMS(严格极 小信标)集合的一个真子集.尤其在大型Petri网系统中,基本信标的集合比SMS的集合要小得 多.对于Petri网的一个子类S3PR,只对每一个基本信标添加一个库所使其不被清空,就可实现 预防死锁,也就是说无须控制S3PR的所有SMS而达到无信标被清空的目的.此外,对于S3PR, 还提出了一种求取SMS和基本信标的方法.相对于现在普遍采用的控制所有SMS来预防死锁 的策略,其具三方面优势.1)只需控制少量的SMS即所谓的基本信标.相应地,添加少量的控制 库所和连接弧,就可得到无死锁或活的Petri网.2)不需要先行计算出极小信标的集合.3)明显 地,这种方法更适合大型Petri网系统.我们通过穿插在文中的一个例子来说明这些方法.     

Optimal deadlock avoidance Petri net supervisors for automated manufacturing systems

Deadlock avoidance problems are investigated for automated manufacturing systems with flexible routings. Based on the Petri net models of the systems, this paper proposes, for the first time, the concept of perfect maximal resourcetransition circuits and their saturated states. The concept facilities the development of system liveness characterization and deadlock avoidance Petri net supervisors. Deadlock is characterized as some perfect maximal resource-transition circuits reach their saturated states. For a large class of manufacturing systems, which do not contain center resources, the optimal deadlock avoidance Petri net supervisors are presented. For an general manufacturing system, a method is proposed for reducing the system Petri net model so that the reduced model does not contain center resources and, hence, has optimal deadlock avoidance Petri net supervisor. The controlled reduced Petri net model can then be used as the liveness supervisor of the system.     

Optimal deadlock avoidance Petri net supervisors for automated manufacturing systems

Deadlock avoidance problems are investigated for automated manufacturing systems with flexible routings. Based on the Petri net models of the systems, this paper proposes, for the first time, the concept of perfect maximal resource-transition circuits and their saturated states. The concept facilitates the development of system liveness characterization and deadlock avoidance Petri net supervisors. Deadlock is characterized as some perfect maximal resource-transition circuits reaching their saturated states. For a large class of manufacturing systems, which do not contain center resources, the optimal deadlock avoidance Petri net supervisors are presented. For a general manufacturing system, a method is proposed for reducing the system Petri net model so that the reduced model does not contain center resources and, hence, has optimal deadlock avoidance Petri net supervisor. The controlled reduced Petri net model can then be used as the liveness supervisor of the system.     

Deadlock-free genetic scheduling algorithm for automated manufacturing systems based on deadlock control policy

Deadlock-free control and scheduling are vital for optimizing the performance of automated manufacturing systems (AMSs) with shared resources and route flexibility. Based on the Petri net models of AMSs, this paper embeds the optimal deadlock avoidance policy into the genetic algorithm and develops a novel deadlock-free genetic scheduling algorithm for AMSs. A possible solution of the scheduling problem is coded as a chromosome representation that is a permutation with repetition of parts. By using the one-step look-ahead method in the optimal deadlock control policy, the feasibility of a chromosome is checked, and infeasible chromosomes are amended into feasible ones, which can be easily decoded into a feasible deadlock-free schedule. The chromosome representation and polynomial complexity of checking and amending procedures together support the cooperative aspect of genetic search for scheduling problems strongly.     

Matrix approach to deadlock-free dispatching in multi-class finite buffer flowlines

For finite-buffer manufacturing systems, the major stability issue is "deadlock," rather than "bounded-buffer-length stability." The paper introduces the concept of "system deadlock," defined rigorously in Petri net terms, and system operation with uninterrupted part-flow is characterized in terms of the absence of this condition. For a large class of finite-buffer multiclass re-entrant flowline systems, an analysis of "circular waits" yields necessary and sufficient conditions for the occurrence of "system deadlock." This allows the formulation of a maximally permissive one-step-look-ahead deadlock-avoidance control policy for dispatching jobs, while maximizing the percent utilization of resources. The result is a generalized kanban dispatching strategy, which is more general than the standard multiclass last buffer first serve (LBFS) dispatching strategies for finite buffer flowlines that typically under-utilize the resources. The problem of computational complexity associated with Petri net (PN) applications is overcome by using certain sub-matrices of the PN incidence matrix. Computationally efficient matrix techniques are given for implementing the deadlock-free dispatching policy.     

Deadlock-free scheduling for flexible manufacturing systems using Petri nets and heuristic search

Deadlock-free control and scheduling are two different problems for flexible manufacturing systems (FMSs). They are significant for improving the behaviors of the systems. Based on the Petri net models of FMSs, this paper embeds deadlock control policies into heuristic search algorithm, and proposes a deadlock-free scheduling algorithm to minimize makespan for FMSs. Scheduling is performed as heuristic search in the reachability graph of the Petri net. The searching process is guided by a heuristic function based on firing count vectors of state equation for the Petri net. By using the one-step look-ahead method in the optimal deadlock control policy, the safety of a state is checked. Experimental results are provided to show effectiveness of the proposed heuristic search approach in deadlock-free scheduling for FMSs.     

A deadlock prevention approach for flexible manufacturing systems with uncontrollable transitions in their Petri net models

Deadlocks are a highly undesired situation in a fully automated flexible manufacturing system, whose occurrences are tied to the existence of shared resources that are competed by different production processes. In the last two decades, a fair amount of research has been done on deadlock analysis and control for flexible manufacturing systems, leading to a variety of strategies in the literature. Petri nets are a promising mathematical tool to handle deadlock problems in flexible manufacturing systems. However, most deadlock control policies based on a Petri net formalism assume that all the transitions in a plant model are controllable. However, uncontrollability of events are a natural feature in a real‐world production system. This paper proposes a deadlock prevention policy for a class of Petri nets by considering the existence of uncontrollable transitions. Deadlocks are prevented by adding monitors to a plant Petri net model, whose addition does not inhibit the firings of uncontrollable transitions. Linear programming techniques are employed to find transitions to which a monitor points in order that a more permissive liveness‐enforcing Petri net supervisor can be found. A number of manufacturing examples are used to demonstrate the proposed methods. Copyright © 2011 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society