Reliability Optimisation of Flexible Manufacturing Systems with Spare Tooling

Tool reliability plays an important role in the performance and justification of flexible manufacturing systems (FMSs). Failure of a single tool can cause downtimes over the entire system. This would cause due dates to be missed and can result in inferior products. Therefore, in order to justify the large capital investment associated with FMSs, the system must perform in a reliable manner to give an acceptable or required rate of return on the investment. In order to arrive at this objective, FMS reliability must be studied at the planning and design stages, tool failures pose a major obstacle to achieving this objective. In this paper, a mathematical model has been developed to determine the spare tooling requirement for the tooling system in an FMS, so that a desired system reliability is achieved and the cost is minimised. The influence of tool sharing on cost, reliability, spares requirement, and tool magazine capacity of the FMS are analysed. The tools and tool transporter are subject to general failure distributions.     

FMS的可靠性建模研究

ＦＭＳ的可靠性建模是ＦＭＳ可靠性设计与分析的基础,本文对ＦＭＳ可靠性建模的几种方法进行了介绍和分析,提出了ＦＭＳ可靠性建模的要求和建模的途径。     

Production planning of FMS under tool magazine constraints: A dynamic programming approach

Production planning is one of the most important activities for efficient operation of a flexible manufacturing system. This complex acivity is concerned with the decisions related to system set-up, involving solving the problems of selection of a set of part types for simultaneous processing, determining the production ratios, assigning the pallets and fixtures, and assigning operations and tools to machines. In this paper, a dynamic programming algorithm is developed to solve the above problems simultaneously by considering the flexibilities and constraints of the system in order to minimise the unbalanced workload of machines. In this context, various factors such as the tools required for operations, alternative routes available for operations, tool magazine capacity and the limited number of pallets and fixtures are considered. The method is validated with a case study.Notation B a large number - i part type,i=1,...,N - o operation,o=1,...,O _i - k machine,k=1,...,K - j stage,j=1,...,N (N is maximum number of stages required, i.e. equal to the number of part types) - n indicates the number of parts to be introduced into the system (represents the state in DP),n=j,...,nmax. In stagej at least one number of each part type should be introduced into the system. The maximum number of parts can be a user defined maximum,nmax. - P _iok processing time of part typei, operationo on machinek - OA _io set of operation alternatives available for part typei, operation - o an alternative consists of machine number, tool number along with its processing time - pw _ik average workload required by a part typei on machinek - a _i number of parts (ratio) of typei - f _i number of fixtures available to part typei - S(j) {S ₁(j),S ₂(j),...,S _p (j),...}, whereS _p (j) ispth set of selected part types with cardinalityj - |S(j)| ^N C _j - W _k a constant, indicating the average unbalanced workload on a machinek and is a user defined value - L _j,k (S _p (j),n,a _i ) load of machinek at stagej when selection isS _p (j), number of part types isn, and number of parts of type isa _i - L* _j,k (S _p (j),n) load of machinek in stagej at minimum unbalanced workload when selection isS _p (j) and number of parts in system isn - F _j (S _p )(j),n,a _i ) unbalanced workload of all machines at stagej, when selection isS _p (j), number of part types isn, and number of parts of type isa _i - F _j (S _p (j),n) minimum unbalanced workload of all machines, whenS _p (j) is the set of selected part types andn is the number of parts in the system     

The State of the Art in Simulation Study on FMS Scheduling: A Comprehensive Survey

Scheduling of flexible manufacturing systems (FMSs) has been one of the most attractive areas for both researchers and practitioners. A considerable body of literature has accumulated in this area since the late 1970s when the first batch of papers was published. A number of approaches have been adopted to schedule FMSs, including simulation techniques and analytical methods. Numerous articles can be found on each of these approaches. This paper reviews scheduling studies of FMSs which employ simulation techniques as an analysis tool, since simulation is the most widely used tool for modelling FMSs. Scheduling methodologies are categorised into simulation of general scheduling studies, multi-criteria scheduling approaches, and artificial intelligence (AI) approaches in FMSs. Comments on the publications, and suggestions for further research and development are given.     

Development of a Tool Database Management System

This paper describes a systematic approach to the design and development of a tool management system for the valve production unit of Bharat Heavy Electricals Limited (BHEL) located in Trichirapalli, India. The salient features of the design and development of a centralised database for tools that contains information regarding tooling, tool location, allo-cation, tool flow, tool storage and retrieval, parts, machines, etc. are discussed. It is estimated that the system being developed will significantly reduce the downtime of machines due to unavailability of the appropriate tools.     

A review of some issues and identification of some barriers in the implementation of FMS

Global competition, advancements in technology and ever changing customers’ demand have made the manufacturing companies to realize the importance of flexible manufacturing systems (FMS). These organizations are looking at FMS as a viable alternative to enhance their competitive edge. But, implementation of this universally accepted and challenging technology is not an easy task. A large number of articles have been reviewed and it is found that the existing literature lacks in providing a clear picture about the implementation of FMS. In this paper, work of various researchers has been studied and it is found that it is really a very difficult task for any organization to transform into FMS on the basis of existing research results. A wide gap exists between the proposed approaches/algorithms for the design of different components of FMS and the real-life complexities. Besides describing the gap in various issues related to FMS, some barriers, which inhibit the adaptation and implementation of FMS, have also been identified in this paper.     

FMS中AGV的实时控制系统设计与实现

将AGV控制系统作为FMS控制系统中的一个通用组件来研究其建模与设计方法.首先,采用面向对象Petri网建模技术建立了AGV控制组件的动态模型.其次,进行了AGV控制组件与FMS中其它组件的基于CORBA的通信接口的定义.在此基础上,为了使AGV控制组件具有良好的维护性、重用性和柔性,建立了AGV控制组件的面向对象类的统一建模语言(UML)模型.最后进行了AGV控制组件设计和开发.     

Flexible Fixturing with Phase-Change Materials. Part 1. Experimental Study on Magnetorheological Fluids

Flexible fixturing is an important issue in manufacturing, where a single set of fixtures are used for locating and holding a variety of workpieces. Flexible fixturing with phase-change materials involves the use of functional materials which change the status from liquid to solid under certain conditions. The workpiece is located in the liquid fixture material and held firmly when the material is changed into the solid state. Therefore, the material strength in the solid state is crucial for a successful application to production. This paper presents an experimental study of magnetorheological (MR) fluid material. With the application of a compression technique, a thick column structure is formed and enhanced. Hence, a high shear strength of the MR fluid in solid status is achieved. Experimental results are reported in this paper. Further application of this technique is under development for flexible fixturing in industrial applications.     

Reliability sensitivity of multi-degree-of-freedom uncertain nonlinear systems with independent failure modes

The reliability sensitivity analysis of uncertain multi-degree-of-freedom nonlinear vibration systems with independent failure modes subjected to random excitation is examined. An earlier version of the statistical fourthmoment method is extended to deal with vector-valued and matrix-valued functions and is developed to determine the first four moments of the system response and state function. Random variables and system derivatives are conveniently arranged into 2D matrices by means of Kronecker algebra. The distribution function of the system state function is approximately determined by the standard normal distribution functions using Edgeworth series technique, and its reliability and reliability sensitivity are presented and discussed.     

An integrated methodology for operations analysis of computer integrated manufacturing systems: Application to an FMS for manufacture of medical instruments

This paper contributes a novel methodology for operations analysis of computer integrated manufacturing systems. The methodology employs an integrated set of personal computer-based manufacturing systems modelling software, and is conducted in three stages. The methodology is illustrated in detail for analysis of materials flow and staffing for a flexible manufacturing system (FMS) installed at a medical instruments manufacturing plant. First, a rough-cut capacity and materials flow analysis was performed using a queueing model. Second, a detailed analysis of FMS system characteristics was conducted by creating and analysing a discrete-event simulation model. Finally, a graphical animation package was used to verify previous results and provide visual feedback on FMS system performance. Analysis results were obtained rapidly and conveniently, and were used to advise manufacturing managers regarding FMS operating policy. This analysis methodology can be applied to a wide range of computer integrated manufacturing systems and manufacturing cells.     

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.     

Simultaneous optimisation of control factors in automated storage and retrieval systems and FMS using stochastic coloured Petri nets and the Taguchi method

Automated storage and retrieval systems (AS/RS) are playing an important role in modern manufacturing systems such as flexible manufacturing systems (FMS). The integration of AS/RS systems and FMS is very complex owing to the complexity of the individual elements. Stochastic coloured petri nets (SCPN) can be used to model, simulate and analyse such a system efficiently and realistically. In this paper, SCPN based simulation, aided by the Taguchi method of design of experiments, has been used for analysing the influence of important factors on makespan, unproductive travel time, and the mean flow time, and is supported by an example.     

Models of production lines with flexible workstations

This article presents models of production line systems with two flexible workstations. Flexible stations have the capability of switching from one operation to another when they become either blocked or starved. The objective is to determine and compare the production rates of serial and parallel arrangements of the workstations. It is shown that, in general, the serial arrangement achieves higher efficiency. Under some specific assumptions, however, both arrangements have the same production rate. An extension of the analysis of two stations to n stations for the parallel case is also presented.     

Identification and elimination of redundant control places in petri net based liveness enforcing supervisors of FMS

In the past two decades, a number of Petri-net-based approaches were proposed for deadlock prevention in flexible manufacturing systems (FMS). An FMS is modeled as a Petri net, and then the controller or the liveness enforcing supervisor (LES) is computed as a Petri net. A live Petri net (LPN) guarantees deadlock-free operations of the modeled FMS. An LES consists of a number of control places (CPs) and their related arcs. To-date most of the attention has been paid to make the underlying Petri net models live without questioning whether or not all of the computed CPs are necessary. It is often the case that the number of CPs determined by these approaches is not minimal. Reducing it in order to reduce the complexity of the controlled system is an important issue that was not tackled before. To address this problem, this paper proposes a redundancy test for an LES of an FMS. The proposed approach takes an LPN model, controlled by n CPs, as input and in the existence of any redundant CPs it produces redundant and necessary CPs. The proposed approach is applicable to any LPN consisting of a Petri net model (PNM), controlled by means of a set of CPs.     

Performance Modelling and Evaluation of Dynamic Tool Allocation in Flexible Manufacturing Systems using Coloured Petri Nets: An Object-Oriented Approach

Manufacturing systems are faced with ever-increasing customisation and unstable demand. The traditional hierarchical control structures for shop floor (pre-release planning, scheduling, dispatching and activity control) are often inflexible in responding to unexpected scenario changes and are thus not robust to system disturbances. In this paper, an object-oriented approach to modelling of FMS dynamic tool allocation and control under a non-hierarchical shop floor control scheme using coloured Petri nets is presented. A client–server paradigm is used in the proposed modelling method. The complete FMS model is partitioned into individual classes (machines, magazines, tool transport system, SGVs, tool storage, etc.) thereby significantly reducing the complexity of the model to a tractable size. The system performance under different tool request selection rules is also evaluated using coloured Petri net simulation. The proposed method can provide the designer of a tool management system with a high-level and structured representation of the tool-sharing control. It also provides an effective method for prototyping and evaluating performance of object-oriented shop floor control software.     

基于Petri网的非串行制造系统的可靠性分析

对带有中间缓冲区的非串行系统深入研究其可靠性问题,建立基于广义随机Petri网的可靠性模型,着重考虑出现的装配、拆分以及按不同比例进行装配或拆分的加工工序,其中间缓冲区可用度对机器可用度的影响,在此模型基础上,采用缓冲区可用度定理,对中间缓冲区和机器本身的状态转移概率分别分类求解,计算出相应的缓冲区可用度和等效机器可用度;同时通过功能可靠度指标,对整个加工任务车间的可靠性进行总体评价,从时间量和产品量因子出发,来衡量由于生产率的不平衡以及中间缓冲区的库存太小,引起生产流程中传输道的阻塞和缺料,进而造成的相关工件的加工时间延迟等问题。以三台机床和两个待加工工件的加工装配单元为例,验证该模型与分析方法的有效性。     

Real-Time Manipulation of Alternative Routeings in Flexible Manufacturing Systems: A Simulation Study

This paper presents the results of a simulation study of a typical flexible manufacturing system (FMS) that has routeing flexibility. The objective is this study is to test the effectiveness of the dissimilarity maximisation method (DMM) for real-time FMS scheduling. DMM is an alternative process plan selection method developed for routeing selection in off-line FMS sched-uling. An integrated framework that consists of a computer simulation model, which mimics a physical system, a C++ module, and a linear program solver is used to evaluate the effects of various operational control rules on the system performance. The hypothetical FMS employed in this study consists of seven machining centres, a loading and an unloading area, and six different part types. Owing to the existence of identical machining centres in the system, the part types have alternative routeings. For selecting an incoming part and later routeing it to a machining centre for its next operation, three control rules, namely, first-in first-out/first available (FIFO/FA), equal probability loading (EPL), and dissimilarity maximisation method/first-in first-out (DMM/ FIFO) are used. In this study, DMM is 1. Used as a real-time decision-making tool to select routeings for the parts that are in the system. 2. Tested and benchmarked against FIFO/FA and EPL. The results show that DMM/FIFO outperforms FIFO/FA and EPL on system throughput. Other measures such as average waiting time, average transportation time, and percentage utilisation rates are also investigated to provide insights for the effectiveness of the DMM rule for real-time FMS control applications.     

Hierarchical Production Planning in Flexible Automated Workshops with Delay Interaction

This paper addresses the hierarchical production planning (HPP) problem for flexible automated workshops (FAWs) with delay interaction, each with a number of flexible manufacturing systems (FMSs). The delay interaction aspect arises from taking into consideration the transfer of parts between FMSs. Any job which requires processing on more than one FMS cannot be transferred directly from one FMS to the next. Instead a semi-finished-product completed in one period must be put into shop storage until the next period at which it can be transferred to the next FMS for further processing. The objective is to decompose medium-term plans (assigned to an FAW by ERP/MRP II) into short-term plans (to be executed by FMSs in the FAW) so as to obtain the lowest production cost. The HPP problem is formulated in this paper by a nonlinear programming model whose constraints are linear but whose objective function is piecewise linear. For the convenience of solving the nonlinear programming model, it is transformed into a linear programming model. Because the model for a general workshop is too large to be solved by the simplex method on a personal computer within acceptable time, Karmarkar’s algorithm and an interaction/prediction algorithm, respectively, are used to solve the model, the former for medium- or small-scale problems and the latter for large-scale problems. With the implementations of these algorithms and with many HPP examples, Karmarkar’s algorithm, the interaction/prediction algorithm and the linear programming method in Matlab 5.0 are compared, showing that the proposed approaches are very effective.     

Effects of Dispatching and Routeing Decisions on the Performance of a Flexible Manufacturing System

This paper is concerned with the effects of dispatching and routeing decisions on the performance of a flexible manufacturing system (FMS). Three routeing policies, no alternative routeings, alternative routeings dynamic, and alternativerouteings planned are first considered with four dispatching rules with infinite buffer capacity. However, in real-life, the size of the local buffers in FMSs may be limited by the size of the part types. Therefore, further experiments to investigate the above three routeing policies with the four dispatching rules with finite buffer capacity are also conducted in order to study the impact of limited buffer capacity on system performance. In addition, the effect of changing part mix ratios is discussed with both infinite and finite buffer capacity. The performance measures considered are makespan, average machine utilisation, average flow-time, and average delay at local input buffers. The simulation results indicate that for infinite buffer capacity, the alternative routeings planned policy, combined with the shortest total processing time dispatching rule, gives the best results for all the above performance measures. For finite buffer capacity, the alternative routeings dynamic policy gives the best results in three performance measures, except for average delay at local input buffers. Further, the effect of changing part mix ratios is not significant. ID="A1"Correspondance and offprint requests to: Dr F. T. S. Chan, Department of Industrial and Manufacturing Systems Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong. E-mail: ftschan@hkucc.hku.hk     

An Optimal Deadlock Prevention Policy for Flexible Manufacturing Systems Using Petri Net Models with Resources and the Theory of Regions

In this paper, an optimal deadlock prevention policy for flexible manufacturing systems (FMSs) is proposed. In an FMS, dead-locks can arise because of a limited number of shared resources, i.e. machines, robots, buffers, fixtures etc. Deadlock is a highly undesirable situation, where each of a set of two or more jobs keeps waiting indefinitely for the other jobs in the set to release resources. The proposed optimal deadlock prevention policy is based on the use of reachability graph analysis of a Petri net model (PNM) of a given FMS and the synthesis of a set of new net elements, namely places with initial marking and related arcs, to be added to the PNM, using the theory of regions, which is a formal synthesis technique toderive Petri nets from automaton-based models. The policy proposed is optimal in the sense that it allows the maximal use of resources in the system according to the production requirements. Two examples are provided for illustration. RID=" ID=" <E5>Correspondence and offprint requests to</E5>: M. Uzam, Ni&gbreve;de &Uuml;niversitesi, M&uuml;hendislik-Mimarlik Fak&uuml;ltesi, Elektrik-Elektronik M&uuml;hendisli&gbreve;i B&ouml;l&uuml;m&uuml;, Kamp&uuml;s, 51100, Ni&gbreve;de, Turkey. E-mail: murat_uzam&commat;hotmail.com