Effects of flexibility and scheduling decisions on the performance of an FMS: simulation modelling and analysis

This paper focuses on a simulation-based experimental study of the effects of routing flexibility, sequencing flexibility, and part sequencing rules on the performance of a typical FMS. Three routing flexibility levels, five sequencing flexibility levels, and four scheduling rules for part sequencing decision are considered for detailed investigation. The system work load characterised by the mean interarrival time of parts has been set at different levels. The performance of the FMS is evaluated using various measures related to flow time and tardiness of parts. The simulation results are subjected to statistical analysis. Multiple regression-based metamodels have been developed using the simulation results. The analyses of results reveal that deterioration in system performance can be minimised substantially by incorporating either routing flexibility or sequencing flexibility or both. However, the benefits of either of these flexibilities diminish at higher flexibility levels. When flexibility exists, part sequencing rules such as the earliest due date and earliest operation due date provide a better performance for all the measures.     

Quantifying machine flexibility

There are several studies aiming to quantify several aspects of flexibility in manufacturing systems like routing flexibility, product mix flexibility, volume flexibility, etc. However, there is still a need to develop more generic measures that can be used to quantify flexibility of systems in order to enable decision-makers to reach better decisions in selecting between different system configurations. In this study, a new approach which is based on digraph theory and matrix algebra is proposed to quantify flexibility. Several examples are also provided to illustrate the proposed approach and its practicality and usefulness. The proposed approach is a novel one and can be used to model and quantify several types of flexibilities. In this research, the proposed modelling approach is explained through machine flexibility mainly due to the fact that most of the other flexibility types in manufacturing systems rely on this flexibility type.     

Conceptualising complementarities in manufacturing flexibility: a comprehensive view

In this paper, we develop an extended view of the manufacturing flexibility construct. Instead of a sequential view as emphasised with the competence-capability perspective, the model presented in this study suggests a holistic view of manufacturing flexibility comprised of six complementary dimensions. We contend that the complementary view of manufacturing flexibility (MFLX) consisting of six dimensions – product-mix flexibility (PMX), routing flexibility (RTG), equipment flexibility (EQP), volume flexibility (VOL), labour flexibility (LBR), and supply management flexibility (SPM) – provides a path to implementation of manufacturing flexibility. Manufacturing flexibility is hypothesised as a second order construct comprised of the six complementary dimensions. We test our proposed model using data collected from US manufacturing organisations. Structural equation modelling was used to analyse the data. The results of confirmatory factor analysis support our hypothesis of a second order multi-dimensional construct structure. This comprehensive conceptualisation of manufacturing flexibility should help researchers evaluate the effect of manufacturing flexibility on operational and financial performance.     

Cellular manufacturing systems design with routing flexibility,machine procurement,production planning and dynamic system reconfiguration

This paper investigates the problem of designing cellular manufacturing systems with multi-period production planning, dynamic system reconfiguration, operation sequence, duplicate machines, machine capacity and machine procurement. An important aspect of this problem is the introduction of routing flexibility in the system by the formation of alternate contingency process routings in addition to alternate main process routings for all part types. Contingency routings serve as backups so as to effectively address the reality of part process routing disruptions (in the main routings) owing to machine breakdowns and allow the cellular manufacturing system to operate in a continuous manner even in the event of such breakdowns. The paper also provides in-depth discussions on the trade-off between the increased flexibility obtained versus the additional cost to be incurred through the formation of contingency routings for all parts. Some sensitivity analysis is also performed on some of the model parameters. The problem is modelled and solved through a comprehensive mixed integer programming formulation. Computational results presented by solving some numerical examples show that the routing and process flexibilities can be incorporated within the cellular manufacturing system design without significant increase in the system cost.     

Anarchic manufacturing

This paper introduces anarchic manufacturing, an extremely distributed planning and control philosophy, as the methodology for planning and controlling future smart factories. Anarchic manufacturing delegates decision-making authority and autonomy to the lowest level of entities in system elements with no centralised control or oversight. It is often postulated that traditional hierarchical structures may not be well suited to manage the state-of-the-art hyper-connected smart factories due to their reliance on communication between management layers. Distributed systems, on the other hand, are commonly perceived to be inherently more flexible, robust and adaptable than hierarchical systems due to their structure. This paper characterises distributed systems by evaluating the relative flexibility of a representative hierarchical system against an anarchic system in a job shop scenario. Multi-agent-based simulation is used to model both hierarchical and anarchic systems, which are tested for flexibility following the Taguchi method and compared against Taillard's benchmark job shop problems for overall performance. The results show that the anarchic system performs as well as the hierarchical system when subjected to unforeseen disruption, refuting the argument that hierarchical systems are too rigid and distributed systems are inherently more flexible. However, anarchic manufacturing systems, which show adaptability and self-optimising traits, provide a platform to potentially enable the emerging digital manufacturing paradigm through the free market structure especially when bandwidth for communications is limited.     

Measuring machine and product mix flexibilities of a manufacturing system

This paper provides a brief discussion on flexibility concepts in manufacturing. It emphasizes that flexibility measurements in a manufacturing system should be studied under dynamically changing environments rather than static ones. The approaches for assessing two major flexibility types, machine flexibility and product mix flexibility response, are presented. The machine flexibility model is based on machine-operation efficiency. The product mix flexibility response model is based on the difference between products in terms of tooling requirements, the number of operations that a machine can perform and the efficiency of different machines.     

A Citation Analysis of the Research on Manufacturing and Supply Chain Flexibility

Manufacturing and supply chain flexibility has been discussed extensively in the scientific literature. Given the different definitions, types and dimensions of flexibility, the purpose of this paper is to explore the origins and structure of the scientific research on manufacturing and supply chain flexibility. We identified a sample of 153 internationally published papers and conducted a citation analysis to examine the connections between the many scientific papers and to explore the most influential works and their impact on flexibility research. Our analysis revealed that research on flexibility received the most scientific attention between 1996 and 2005. However, the majority of the currently valid types and dimensions of flexibility have been defined in the late 1980s and the early 1990s. The current paper provides the leading academic journals and papers in the field of flexibility research. Additionally, the results of our citation analysis indicate that flexibility research is significantly influenced by papers that have been presented at subject-specific conferences. Finally, our paper presents a clearly arranged structure of the most frequently cited papers dealing with manufacturing and supply chain flexibility. Furthermore, its contents and findings are briefly discussed to provide an overview of the previous academic research.     

Flexibility-enabled lead-time reduction in flexible systems

This paper presents the results of a conceptual study and simulation experimentation aimed at understanding the underlying mechanisms of sequencing flexibility-enabled manufacturing lead-time reduction. In spite of a large body of literature on flexibility, the exact mechanism that enables flexibility to reduce the lead time is not fully understood. As a part of our research efforts on the proactive application of flexibility for the performance enhancement of manufacturing systems, we are motivated to study how flexibility can be employed in a proactive manner to reduce the manufacturing lead time and to develop an understanding of the underlying mechanisms. Towards this end, we have developed simulation models of simple flexible manufacturing systems and studied the effect of sequencing flexibility on the lead-time performance under different conditions of part load and machine load balancing. The studies indicated that sequencing flexibility has a significant effect on the lead-time performance of the manufacturing system, and the effect of flexibility varies under different conditions of load balancing. Further studies indicate the existence of complex interactions between the sequencing flexibility, process concurrency, processor load balancing, and manufacturing lead time. This paper intends to discuss some of the interesting results of these studies with a focus on the inherent mechanisms of sequencing flexibility-enabled lead-time reduction.     

A method for comparing flexibility performance for the lifecycle of manufacturing systems under capacity planning constraints

The objective of this work is to describe a method for comparing the flexibility performance of manufacturing systems, in an uncertain environment, under lifecycle considerations and capacity planning constraints. The manufacturing systems costs are estimated over a time horizon and for a large variety of possible market scenarios. In order for the lifecycle cost values to be comparable among different systems, their values are calculated with the use of a special purpose algorithm. Statistical analysis of the estimated cost values is then employed for assessing the flexibility of each manufacturing system. The method is applied in an industrial case for checking, also from a flexibility point of view, the investment on a production system, using real life industrial data.     

Development of manufacturing control strategies using unsupervised machine learning

This research considers the control of manufacturing systems that support job routing and process sequence flexibility. A machine learning system is presented that uses a simulation model of the target manufacturing system to discover opportunistic control rules. Learning is unsupervised and is driven by a genetic algorithm. The learning method requires very little a priori control knowledge. For this presentation, the decision-making agents are the part types being processed. Part types evolve cooperative strategies for selecting the best route through the manufacturing system based on simulated real-time information that describes the state of the system. Results are presented that demonstrate the effectiveness of the approach.     

Flexibility in manufacturing enterprises

A manufacturing enterprise is a collection of interrelated, flexible, optimized business processes delivering value to the customers through high quality products and services, faster than competition. This view of an enterprise enables one to consider the entire business system including the suppliers, product development, manufacturing, logistics, distribution, and retailing and to smoothen out the interfaces between them. Performance measures and performance measurement are important for monitoring, control and management. We identify and discuss eight performance measures for generic business processes. These include lead time, customer service, dependability, quality, flexibility, cost, capacity, and asset utilization. In this paper, we concentrate on flexibility of business processes with special emphasis on the supply chain and order-to-delivery processes. We attempt to provide clear definitions and measures of various types of flexibilities as well as discuss the relationship between product structure and supply chain flexibility. The relationship between uncertainties, flexibility, technology, and product structure is clearly brought out in this paper.     

Launching and dispatching strategies for multi-criteria control of closed manufacturing systems with flexible routeing capability

In this paper, we study the problems of launching and dispatching of parts in closed manufacturing systems with flexible routeing. For the manufacturing systems being operated against multiple performance criteria, we postulate that controlling different aspects of the operational control strategy to meet one single performance criterion would improve overall system performance. It is suggested that to achieve production rates, launching rules be utilized and to affect flowtime, dispatching rules be manipulated. Also, for measurement of routeing flexibility, an entropic measure of flexibility is refined. The entropy-based rule is then compared with the dispatching rules commonly used in the industry. Control strategies are developed for a test system and it is shown that a hierarchical control strategy works best when multiple performance criteria are of interest.     

Managing demand variability using requisite variety for improved workflow and operational performance: the role of manufacturing flexibility

In this study, we use the law of requisite variety to investigate the effect of manufacturing flexibility on workflow and operational performance. After developing a requisite variety construct using a parsimonious conceptualisation of manufacturing flexibility, we test our research model with data collected from US manufacturers. The results of the analysis using structural equation modelling support our hypotheses. Our study makes three important contributions. First, we provide a more complete understanding of manufacturing flexibility and its relationship to operational performance. By empirically testing our model, we develop support for manufacturing flexibility as a workflow regulator which provides an organisation with the necessary variety in its response-repertoire to effectively manage demand variability. Second, our analysis of our requisite variety construct allows us to better understand the complex relationship between manufacturing flexibility and efficiency. Our findings suggest that the trade-off between manufacturing flexibility and efficiency can be attenuated. Third, we provide evidence to show that manufacturing flexibility positively influences performance by increasing the speed of material flow and improving organisational efficiency.     

Impact of some flexibility factors in FMSs–a performance evaluation approach

This paper investigates the relative impact of versatility as a selected physical characteristic of the resources (a design adequacy factor), versus some operating strategies (control factors), on flexible manufacturing systems performance. Systems are operated under changes in mix, in the availability of the resources (due to failures), as well as at various levels of planned machines' utilization. Versatility measures at machine and system level are discussed. The extent of the machines' workload imbalance is presented as an off-line system performance indicator, expected to provide rough cut, useful information at the design stage. Simulated experiments are designed and analysed, to offer a factorial on-line view of the problem and thus, to enable estimation of interaction effects between flexibility, environmental factors and changes. A standardized, service-orientated, on-line performance measure of the system is introduced.     

IT能力对竞争优势的影响机制:以制造柔性为中介

为研究IT能力对企业竞争优势的影响机制,以制造柔性为中介变量,运用结构方程模型对来源于珠三角地区制造企业的381份调查数据进行了实证分析。研究结果表明,IT能力对制造柔性具有显著的正向影响,制造柔性对企业竞争优势也有显著的正向影响。此外,制造柔性在IT能力与竞争优势的关系起着显著的部分中介作用,并且IT能力通过制造柔性对竞争优势的影响效应占总效应的44.3%。     

Product flexibility in selecting manufacturing planning and control strategy

The manufacturing systems capable of producing several products simultaneously are frequently subject to changes in product types due to demand fluctuations. In such systems a product flexible manufacturing planning and control (MPC) strategy is needed to change from one product type to another with minimum deterioration to system performance levels. The objective of this research is to develop a systematic analysis and evaluation approach in order to compare the MRP-push and JIT-pull strategies quantitatively based on a product flexibility measure. A new product flexibility measure is developed based on the sensitivity to change concept and presented together with the implementation in a real manufacturing system. Simulation is used to compare the performance of a JIT-pull with an MRP-push strategy based on performance measures, e.g. manufacturing lead time, work-in-process inventory, backorders, machine utilization and throughput. The performances of the two strategies are evaluated in two scenarios: (i) a single product; (ii) a second product is added (the first product being simple and the second being complex in terms of processing). The impacts of adding the second product on the performance measures for the push and pull strategies are then assessed. A multi-attribute evaluation scheme is used to compare the two strategies where the attribute values are the change in performance measures as the second product is added. The proposed product flexibility measure is utilized in the interpretation of the results.     

Managing production level in new product diffusion: an agent-based simulation approach

Managing production level after the launch of a new product is a challenging problem and is critical to the overall profit of manufacturing firms. The problem involves concepts from different fields including production planning, manufacturing flexibility, forecasting, and marketing. In this paper, the gaps in the existing literature are first illustrated. With the goal of addressing the identified research gaps, agent-based modeling and simulation is employed to analyze the performance of different production planning strategies under various levels of volume flexibility and consumer social network structures. The key distinguishing feature of the developed model is the capability of the manufacturing firm to adjust its production level by forecasting the future demand. The analysis of the simulation outputs yields substantial results by challenging some intuitive and traditional understandings of manufacturing systems. The paper also provides a discussion on managerial implications of the results in order to provide the managers with guidelines on the implementation of the model in real-world diffusion environments.     

Dealing with routing in an automated manufacturing cell: a supervisory control theory application

Automated manufacturing cells are capable of producing different products using the same set of resources. This characteristic allows a more efficient use of resources, but makes the control of these systems more difficult and susceptible to errors. This article addresses the problem of routing control in an automated manufacturing cell that consists of three workstations, a transportation system, a feeding conveyor and a retrieving conveyor. In the modelling step, this article employs a systematisation of system and behavioural specifications modelling. This systematisation aims to minimise the impact caused by changes on system constitution and production specifications. In the synthesis step, the local modular control approach of the supervisory control theory is used to obtain the supervisors that match the specifications. In the implementation step, the systematic translation of the results obtained with the application of this theory into the application program to be implemented in the programmable logic controller that controls the cell occurs. The proposed approach in this study was tested and validated in a real manufacturing cell and the experimental results show the viability and success of this project.     

Process flexibility under bill of material constraints: part II–structural properties and improving principles

For the manufacturing system that consists of multiple assembly lines, this paper investigates the method of improving its process flexibility under bill of material (BOM) constraints. Based on the flexibility measurement developed in part I, generic characteristics of the capability configuration of assembly lines that ensure it more flexible are first investigated on two hierarchical and highly interrelated levels: the assembly line level and the system level. These characteristics are termed ‘structural properties’ of process flexibility under BOM constraints, which are valuable because they provide an effective way of improving flexibility of the manufacturing system through investing in limited operational flexibility of machines in the assembly lines. According to the obtained structural properties, guidelines for improving process flexibility on the two levels are then developed. The proposed guidelines can help the manager of a manufacturing system to make effective decisions on flexibility improvement without much computation effort. Results of simulation experiments illustrate that the proposed structural properties and guidelines are effective and widely applicable to real manufacturing systems.     

A methodology for designing flexible cellular manufacturing systems

Cell formation in cellular manufacturing deals with the identification of machines that can be grouped to create manufacturing cells and the identification of part families to be processed within each cell. Dynamic and random variations in part demands can negatively impact cell performance by creating unstable machine utilizations. The purpose of this paper is to introduce and illustrate an interactive cell formation method that can be used to design 'flexible' cells. Flexibility in this context refers to routing flexibility (i.e., the ability for the cellular system to process parts within multiple cells) and demand flexibility (i.e., the ability of the cell system to respond quickly to changes in part demand and part mix). Through an experimental analysis using multiple data sets, we also validate the procedure and provide guidelines for parameter settings depending upon the type of flexibility of interest to the user. Finally, trade-offs and interdependences between alternative types of flexibility in the context of cellular systems are illustrated.