Inventory issues in incremental FMS implementation

In recent years, numerous studies have demonstrated convincingly that impressive benefits can be obtained by the adoption of flexible manufacturing systems (FMSs). To obtain the benefits of an FMS requires the development of a completely integrated system. However, FMS implementations are frequently done incrementally through the introduction of subsystems such as flexible machining centers into an existing conventional system. The purpose of this research is to investigate some of the operational issues associated with the introduction of a CNC (computer numerically controlled) machine tool into a conventional system. The primary objective of the present study is to explore the relative effects on inventory holding cost of installing a single CNC at different locations within three different system configurations. Additionally, the study examines the sensitivity of these impacts to changes in (1) System utilization; (2) the ratios of setup times to run times in the conventional work centers; and (3) the rates of increase in holding costs for parts as they move through the system. Results indicated that, in general, introduction of a CNC into an otherwise conventional system reduces inventory holding cost for the system as a whole. However, the degree of this reduction varies depending on the position of the CNC in the system. In some cases the reduction in inventory holding cost is substantial, while in other cases it is relatively small.     

Experimental investigation of an FMS due-date scheduling problem: Evaluation of machine and AGV scheduling rules

Although extensive research has been conducted to solve design and operational problems of automated manufacturing systems, many of the problems still remain unsolved. This article investigates the scheduling problems of flexible manufacturing systems (FMSs). Specifically, the relative performances of machine and automated guided vehicle (AGV) scheduling rules are analyzed against various due-date criteria. First, the relevant literature is briefly reviewed, and then the rules are tested under different experimental conditions by using a simulation model of an FMS. The sensitivity to AGV workload, buffer capacity, and processing-time distribution is also investigated to assess the robustness of the scheduling rules.     

A case study in FMS production planning and dispatching

The speedy development and extensive application of computers have helped play a significant role in a new technological revolution. The importance of FMS flexibility in producing a variety of products and adapting rapidly to customer requirements makes FMSs attractive. Further, FMSs are most appropriate for largevariety and medium- to high-volume production environments. However, the module of the FMS production planning system is not perfect. This paper focuses on a new scheme for FMS production planning and dispatching under the realistic assumptions promoted by a particular flexible manufacturing factory. Some practical constraints such as fixture uniqueness, limited tool magazine capacity, and a given number of pallets are considered. The simulation results indicate that the scheme provides a good production plan, according to the short-term plans from the MIS Department. Some conclusions are drawn and a discussion is presented.     

An efficient heuristic for scheduling batches of parts in a flexible flow system

Flexible manufacturing systems (FMSs) are a class of automated systems that can be used to improve productivity in batch manufacturing. Four stages of decision making have been defined for an FMS—the design, planning, scheduling, and control stages. This research focuses on the planning stage, and specifically in the area of scheduling batches of parts through the system.The literature to date on the FMS planning stage has mostly focused on the machine grouping, tool loading, and parttype selection problems. Our research carries the literature a step further by addressing the problem of scheduling batches of parts. Due to the use of serial-access material-handling systems in many FMSs, the batch-scheduling problem is modeled for a flexible flow system (FFS). This model explicitly accounts for setup times between batches that are dependent on their processing sequence.A heuristic procedure is developed for this batch-scheduling problem—the Maximum Savings (MS) heuristic. The MS heuristic is based upon the savings in time associated with a particular sequence and selecting the one with the maximum savings. It uses a two-phase method, with the savings being calculated in phase I, while a branch-and-bound procedure is employed to seek the best heuristic solution in phase II. Extensive computational results are provided for a wide variety of problems. The results show that the MS heuristic provides good-quality solutions.     

性能指标驱动的半导体生产线动态派工方法

为了求解不确定生产环境下的半导体生产线调度问题,提出一种基于极限学习机的性能指标驱动的动态派工方法。首先通过半导体生产线仿真系统仿真,得到所需样本,进而通过极限学习机建立半导体生产线性能指标预测模型;然后通过预测出的性能指标,结合生产线的实时状态信息,学习产生调度过程中的动态派工规则算法所需的最佳系数,驱动生产线派工决策,使其性能指标趋向预测值,最终达到提高生产线整体性能的目的。通过仿真平台验证了该方法能够有效提高生产线的性能指标。     

A conflict-based priority dispatching rule and operation-based approaches to job shops

Job shop scheduling (JSS) problems consist of a set of machines and a collection of jobs to be scheduled. Each job consists of several operations with a specified processing order. In this paper, a job shop model problem is scheduled with the help of the Giffler and Thompson algorithm using a priority dispatching rule (PDR). A conflict based PDR is used to schedule the job shop model by using Genetic Algorithms (GAs). An iterative method is applied to the job model to find the optimal conflict-based PDR order and the operation sequence. The same job shop model is also scheduled based on an operation using simulated annealing (SA) and hybrid simulated annealing (HSA). A makespan of the job model is used as an objective. These four methods are considered as different solutions for each problem. A two-way analysis of variance (ANOVA) is applied to test its significance.     

柔性制造系统中生产调度问题求解的一种符号算法

讨论了基于有序二叉决策图的有界Petri网符号分析,给出了其中镜像计算的一种新算法,进而建立了柔性制造系统生产调度问题的符号有序二叉决策图求解算法.该算法在求解过程中通过对状态空间及其搜索过程中相关数据的有序二叉决策图表示,避免了状态和搜索的显示枚举,实现了隐式高效操作,进而改善了算法的计算性能.仿真实验表明了算法的有效性.     

A study on the weighted composite dispatching rule in modular production systems

Scheduling semi-conductor manufacturing process systems is a complicated and difficult job, due to such characteristics as re-entry into manufacturing processes, high uncertainty of processes, and products and technologies changing rapidly. The researchers have carried out many studies to find efficient techniques for semi-conductor manufacturing systems with a view to accomplishing the goals of systems, such as saving cycling time and increasing production quantity per unit time. The production flow in the semi-conductor industry has the most unique characteristics, which makes it difficult to plan production and to schedule semi-conductor manufacturing. Currently, the scheduling methods in semi-conductor assembly processes follow the dispatching rule on a simple first come first serve (FCFS) basis, and a backlog is operated as a buffer based on the daily production quantity. In this study, therefore, we will apply various dispatching rules on a real-time basis and verify the effect and result of exact scheduling through simulation, based on the assumption that competitive advantages in production come from efficient inventory control and exact scheduling.     

A decision-making approach to the operation of flexible manufacturing systems

This paper introduces a generic decision-making framework for assigning resources of a manufacturing system to production tasks. Resources are broadly defined production units, such as machines, human operators, or material handling vehicles; and tasks are activities performed by resources. In the specific context of FMS, resources correspond to individual machines; tasks correspond to operations to be performed on parts. The framework assumes a hierarchical structure of the system and calls for the execution of four consecutive steps to make a decision for the assignment of a resource to a task. These steps are 1) establishment of decision-making criteria, 2) formation of alternative assignments, 3) estimation of the consequences of the assignments, and 4) selection of the best alternative assignment. This framework has been applied to an existing FMS as an operational policy that decides what task will be executed on which resource of this FMS. Simulation runs provide some initial results of the application of this policy. It is shown that the policy provides flexibility in terms of system performance and computational effort.     

Deadlock Avoidance for Sequential Resource Allocation Systems: Hard and Easy Cases

Deadlock is a major problem for systems that allocate resources in real time. The key issue in deadlock avoidance is whether or not a given resource allocation state is safe: that is, whether or not there exists a sequence of resource allocations that completes all processes. Although safety is established as NP-complete for certain broad resource allocation classes, newly emerging resource allocation scenarios often exhibit unique features not considered in previous work. In these cases, establishing the underlying complexity of the safety problem is essential for developing the best deadlock avoidance approach. This work investigates the complexity of safe resource allocation for a class of systems relevant in automated manufacturing. For this class, the resource needs of each process are expressed as a well-defined sequence. Each request is for a single unit of a single resource and is accompanied by a promise to release the previously allocated resource. Manufacturing researchers have generally accepted that safety is computationally hard, and numerous suboptimal deadlock avoidance solutions have been proposed for this class. Recent results, however, indicate that safety is often computationally easy. The objective of this article is to settle this question by formally establishing the NP-completeness of safety for this class and investigating the boundary between the hard and easy cases. We discuss several special structures that lead to computationally tractable safety characteristics.     

生产时间可变间歇过程的Petri网模型及其调度

讨论了生产时间可变的多产品间歇过程的最优调度问题,给出了间歇过程在复杂中间的无限存储策略、有限存储策略、无中间存储策略和混杂存储策略下p-时间Petri网模型的描述方法,进而给出了基于可行调度集和修正分枝界定的间歇过程最短生产时间的最优调度算法.该算法利用一间歇过程最短生产时间不大于另一间歇过程最短生产时间的条件,有效地限制了对解空间的搜索,进而改善了算法的计算性能.仿真算例表明了所述方法的有效性.     

Deadlock prevention and avoidance in FMS: A Petri net based approach

The use of structure theory of Petri nets to develop efficient deadlock prevention and deadlock avoidance methods for flexible manufacturing systems (FMSs) modelled by S⁴R nets is demonstrated. Major synchronisation patterns, such as generalised parallel and sequential mutual exclusion, frequently observed in FMS contexts can be represented by this class. The liveness property of a given S⁴R net (deadlock-freeness in the context of FMSs) is characterised in terms of structural Petri net elements called siphons. An efficient method for controlling minimal siphons of a given S⁴R net is developed where local control places are added to the net. A sufficient condition for liveness of the augmented net is provided. This constitutes a deadlock prevention approach. When the net liveness condition is not satisfied, an on-line controller, using a dynamic resource allocation policy, is developed for the augmented net. The performance of the proposed approaches is illustrated using several examples.     

柔性装配制造系统中生产优化的Petri网方法

本文在基本Ｐｅｔｒｉ网的基础上,给出了一种进行周期性生产的柔性装配制造系统的模型;同时,针对该系统,给出了一个求稳定需求下最佳生产指量的启发式调度方法。     

The Effects of Scheduling Rules and Routing Flexibility on the Performance of a Random Flexible Manufacturing System

The increased use of flexible manufacturing systems to efficiently provide customers with diversified products has created a significant set of operational challenges for managers. Many issues concerning procedures and policies for the day-to-day operation of these systems still are unresolved. Previous studies in this area have concentrated on various problems by isolating or simplifying the systems under study. The primary objective of this study is to extend previous research by examining the effects of scheduling rules and routing flexibility on the performance of a constrained, random flexible manufacturing system (FMS). Other experimental factors considered are shop load, shop configuration, and system breakdowns. Within the bounds of this experiment, the results indicate that, in the presence of total routing flexibility, the effects of shop load, system breakdowns, and scheduling rules are significantly dampened. In particular, when total routing flexibility exists, the choice of scheduling rules is not critical. We also show that the behavior of scheduling rules in a more constrained FMS environment (i.e., where system breakdowns occur and material handling capability is limited) is consistent with the findings of previous research conducted under less constrained environments. Finally, results indicate that the shop configuration factor has little or no impact on a system's flow-time performance.     

Logical Cell Formation in FMS, Using Flexibility-Based Criteria

Flexible manufacturing systems often are organized into a cellular architecture for ease of operation. The formation of these cells sometimes has been treated as an extension of the conventional cell-formation problem. This paper argues that, owing to the existence of flexible routing and transfer capabilities, the cell-formation problem in FMSs should be treated as quite distinct from that in conventional manufacturing systems and shows that a flexibility-based procedure is apt for overcoming the deficiencies of earlier forays into this area. Manufacturing cell flexibility is defined as a composite of three flexibility measures: producibility,processivity , and transferability. The problem of cell formation is modeled as flexibility maximization, and a procedure is developed for the simultaneous formation of machine cells and part families, while heuristically maximizing within-cell flexibility.     

A pattern-directed approach to flexible manufacturing: A framework for intelligent scheduling, learning, and control

The planning, scheduling, and control of manufacturing systems can all be viewed as problem-solving activities. In flexible manufacturing systems (FMSs), the computer program carrying out these problem-solving activities must additionally be able to handle the shorter lead time, the flexibility of job routing, the multiprocessing environment, the dynamic changing states, and the versatility of machines. This article presents an artificial intelligence (AI) method to perform manufacturing problem solving. Since the method is driven by manufacturing scenarios represented by symbolic patterns, it is referred to as pattern-directed. The method is based on three AI techniques. The first is the pattern-directed inference technique to capture the dynamic nature of FMSs. The second is the nonlinear planning technique to construct schedules and assign resources. The third is the inductive learning method to generate the pattern-directed heuristics. This article focuses on solving the FMS scheduling problem.In addition, this article reports the computation results to evaluate the utility of various heuristic functions, to identify important design parameters, and to analyze the resulting computational performance in using the pattern-directed approach for manufacturing problem-solving tasks such as scheduling.     

Stochastic coloured Petri nets for modelling and evaluation,and heuristic rule base for scheduling of FMS

This paper addresses scheduling of flexible manufacturing systems considering stochastic behaviour such as failure and repair of machines, and variation in processing times. Stochastic coloured Petri nets have been used for obtaining a compact model of the system. A heuristic rule base has been proposed for resolving conflicts in the allocation of jobs to machines. The superiority of the proposed approach has been exemplified with a case study.     

Analysis of dispatching rules in a stochastic dynamic job shop manufacturing system with sequence-dependent setup times

Stochastic dynamic job shop scheduling problem with consideration of sequence-dependent setup times are among the most difficult classes of scheduling problems. This paper assesses the performance of nine dispatching rules in such shop from makespan, mean flow time, maximum flow time, mean tardiness, maximum tardiness, number of tardy jobs, total setups and mean setup time performance measures viewpoint. A discrete event simulation model of a stochastic dynamic job shop manufacturing system is developed for investigation purpose. Nine dispatching rules identified from literature are incorporated in the simulation model. The simulation experiments are conducted under due date tightness factor of 3, shop utilization percentage of 90 % and setup times less than processing times. Results indicate that shortest setup time (SIMSET) rule provides the best performance for mean flow time and number of tardy jobs measures. The job with similar setup and modified earliest due date (JMEDD) rule provides the best performance for makespan, maximum flow time, mean tardiness, maximum tardiness, total setups and mean setup time measures.     

Scheduling tasks and vehicles in a flexible manufacturing system

Due to their increasing applicability in modern industry, flexible manufacturing systems (FMSs), their design, and their control have been studied extensively in the recent literature. One of the most important issues that has arisen in this context is the FMS scheduling problem. This article is concerned with a new model of an FMS system, motivated by the practical application that takes into account both machine and vehicle scheduling. For the case of a given machine schedule, a simple polynomial-time algorithm is presented that checks the feasibility of a vehicle schedule and constructs it whenever one exists. Then a dynamic programming approach to construct optimal machine and vehicle schedules is proposed. This technique results in a pseudopolynomialtime algorithm for a fixed number of machines.     

基于CORBA技术的远程监控的实现

在论述CORBA技术的基础上,详细说明如何利用该技术实现网络范围内的远程监控,并结合一个实验系统,具体地说明了CORBA远程服务器和CORBA客户程序的建立过程,实验系统对远程监控进行了模拟与实现。