An evaluation of the tandem configuration automated guided vehicle system

The performance of a tandem automated guided vehicle (AGV) system is compared with that of conventional AGV track systems. In the tandem system the track is divided into non-overlapping, single-vehicle closed loops. Using simulation it is shown that, because of trips requiring delivery across loops, the tandem system has a higher expected travel time per load and thus a greater average time in system. When the loads are delivered within the original loop only, the tandem system has a lower average time in system than in conventional systems, although the difference is modest. Track layout appears to influence the average time in system much more than docs the dispatching rule invoked. The success of the tandem system is highly dependent on approximately even AGV utilization, which may be difficult to obtain in practice.     

Flexibility configurations: Empirical analysis of volume and product mix flexibility

In this paper we address flexibility and investigate the relationship between volume and product mix flexibility. One view of flexibility is that of being a capability in itself; another view is that of flexibility as an enabler, providing the manufacturing system with properties on which other competitive capabilities are built. In this research, the latter view of flexibility is used, where flexibility acts as a second order competitive criterion. The aim is to differentiate between two dimensions of flexibility important to the manufacturing value chain, i.e., volume and product mix flexibility, and to investigate how different flexibility configurations are related to various manufacturing practices. A clustering research approach is used to identify groups of companies based on flexibility configurations. The groups are then analyzed with respect to characteristics and impact on operational performance. For the empirical investigation, we use empirical data from the high performance manufacturing (HPM) study, including three industries and seven countries—a total of 211 plants. We find that flexibility configurations based on high or low levels of volume and mix flexibility combinations show significant differences both in terms of operational performance, and in terms of emphasis put into different flexibility source factors.     

“货到人”拣选系统中AGV路径规划与调度研究

本文在电商行业快速发展及人工智能不断成熟的背景下,对智能仓库中采用“货到人”拣选系统的AGV(Automated Guided Vehicle)调度优化问题进行研究,此问题需同时考虑单AGV的路径规划及多AGV间的碰撞避免。基于此,本文提出了两阶段优化算法,首先分别采用数学模型求解和基于货架优先级的任务分配算法得到AGV的货架搬运任务序列,并根据AGV行驶规则生成初始路径。然后设计碰撞检测及避免算法对可能发生冲突的路径交叉点进行主动避撞调度。当发生突发情况时(如设备故障),采取实时重调度措施进行被动路径调整以获得全局AGV无碰路径。最后基于两阶段算法分析了两种AGV搬运任务分配策略的适用情况,并给出了最优的AGV数量配置建议。实验结果表明,本文算法能够调度100台AGV在配备有2015个货架、10个作业平台的仓库中完成100个订单的拣选作业。本研究可为企业采用“货到人”拣选系统实现多AGV的协同调度提供理论依据和实践指导。     

Comparing reactive Kanban and reactive CONWIP

This paper compares a reactive Kanban system to a reactive CONWIP system under conditions of unstable changes in demand using simulation experiments. After an introduction, a model of the JIT ordering systems, the Kanban and the CONWIP systems, is constructed. In order to obtain the fundamental information for developing a control rule of buffer size, the performance of the two types of the JIT ordering systems is analysed under various stable-demand conditions by simulation experiments. Based on the results, the reactive JIT ordering systems are proposed, and the performance of the proposed systems is investigated. The results showed that both of the proposed systems can react to unstable changes in demand and maintain the mean waiting time of demand at less than the required level. In the reactive Kanban system, the total of the mean work-in-process inventories becomes much less than that in the traditional Kanban system without controlling buffer size. However, in the reactive CONWIP system, the total of the mean work-in-process inventories becomes much more than or nearly equal to that in the traditional CONWIP system without controlling buffer size under the strongly correlated or the weakly correlated processing times, respectively. Based on the results, it can be claimed that, in the proposed systems, the reactive Kanban system is more effective to react to unstable changes in demand than the reactive CONWIP system.     

Flexible manufacturing systems performance in U.S. automotive manufacturing plants: a case study

Flexible manufacturing systems (FMS) philosophy is a key weapon in achieving global manufacturing competitiveness. It encompasses a wide range of dimensions to improve all aspects of operational performance metrics. The aim of the study is to examine the current state of flexibility adoption in U.S. automotive manufacturing facilities and its impact on operational performance metrics. It utilizes survey questionnaire developed based on previous work in U.S. manufacturing industry. The survey was originally distributed to 420 facility managers in the U.S. domestic automotive industry. It was revealed that 70% of the respondents had implemented all 15 flexibility dimensions listed in the questionnaire. The data analysis conducted shows that implementation of certain flexibility dimensions will lead to significant improvement in specific operational performance metrics. This considerable finding can be used as a guide for manufacturing managers to achieve certain objectives in operational performance improvement in a rapidly changing environment.     

Sequential and Simultaneous Decision Making for Optimizing Health Care Resource Flexibilities

Health care administrators commonly employ two types of resource flexibilities (demand upgrades and staffing flexibility) to efficiently coordinate two critical internal resources, nursing staff and beds, and an external resource (contract nurses) to satisfy stochastic patient demand. Under demand upgrades, when beds are unavailable for patients in a less acute unit, patients are upgraded to a more acute unit if space is available in that unit. Under staffing flexibility, nurses cross‐trained to work in more than one unit are used in addition to dedicated and contract nurses. Resource decisions (beds and staffing) can be made at a single point in time (simultaneous decision making) or at different points in time (sequential decision making). In this article, we address the following questions: for each flexibility configuration, under sequential and simultaneous decision making, what is the optimal resource level required to meet stochastic demand at minimum cost? Is one type of flexibility (e.g., demand upgrades) better than the other type of flexibility (e.g., staffing flexibility)? We use two‐stage stochastic programming to find optimal resource levels for two nonhomogeneous hospital units that face stochastic demand following a continuous, general distribution. We conduct a full‐factorial numerical experiment and find that the benefit of using staffing flexibility on average is greater than the benefit of using demand upgrades. However, the two types of flexibilities have a positive interaction effect and they complement each other. The type of flexibility and decision timing has an independent effect on system performance (capacity and staffing costs). The benefits of cross‐training can be largely realized even if beds and staffing levels have been determined prior to the establishment of a cross‐training initiative.     

MODELS OF PARTS-ORIENTING SYSTEMS*

An analytical model to estimate, evaluate, and compare the behavior and performance of parts-orienting systems for automated manufacturing systems is developed. The model can be applied to parts-orienting systems for both traditional series-structured manufacturing systems and nontraditionally structured flexible manufacturing systems. Numerical examples are given for typical system configurations such as the pure series system, the pure parallel system, and the pure feedback system.     

针对趋势失控的基于TAR模型的SPC-EPC集成研究

目前在SPC-EPC集成研究中使用线性时间序列模型,该模型对复杂的非线性自相关关系的描述存在偏差并影响最终的控制效果。针对这一问题提出使用一类非线性时间序列模型(即门限自回归模型)描述系统的动态噪声,据此建立基于门限自回归模型的最小均方误差控制器,并进一步建立SPC-EPC集成控制体系。针对在连续生产过程中常见的以趋势形式存在的过程失控,通过实例研究控制器在单独使用和集成控制方法下的控制效果,并与线性控制器相应的结果进行对比,通过模拟研究进一步验证和分析这一集成控制方法的控制效果。研究结果表明,基于非线性时间序列的集成SPC-EPC控制方法,对含趋势形式失控的、复杂的非线性自相关过程,在单独使用控制器调整的基础上可以进一步减小过程变异,模拟研究进一步验证此集成控制方法对不同幅度趋势变异的有效性,并给出不同情况下的集成控制方案。     

Multi-objective simulation optimization using data envelopment analysis and genetic algorithm: Specific application to determining optimal resource levels in surgical services

Simulation is a powerful tool for modeling complex systems with intricate relationships between various entities and resources. Simulation optimization refers to methods that search the design space (i.e., the set of all feasible system configurations) to find a system configuration (also called a design point) that gives the best performance. Since simulation is often time consuming, sampling as few design points from the design space as possible is desired. However, in the case of multiple objectives, traditional simulation optimization methods are ineffective to uncover the efficient frontier. We propose a framework for multi-objective simulation optimization that combines the power of genetic algorithm (GA), which can effectively search very large design spaces, with data envelopment analysis (DEA) used to evaluate the simulation results and guide the search process. In our framework, we use a design point's relative efficiency score from DEA as its fitness value in the selection operation of GA. We apply our algorithm to determine optimal resource levels in surgical services. Our numerical experiments show that our algorithm effectively furthers the frontier and identifies efficient design points.     

Task-Structure Relationship of Information Systems Development Subunit: A Congruence Perspective*

Several studies have examined the influence of organizational context (e.g., organization size, environment, strategy) on information systems (IS) performance/success. But very few studies have examined IS structure, and those studies have focused on a particular element of IS structure (e.g., decentralization). Thus, the multidimensional structural configuration of IS as an organizational subunit has escaped research attention. Further, the nature of the task to be performed by the IS subunit as a contextual factor has been virtually ignored in the IS research literature. In this paper, we first develop a congruence model for the task-structure relationship in an information systems development (ISD) subunit setting. Then we test this model as a multivariate relationship followed by subordinate bivariate analyses. Analysis of data from 41 hospitals’ ISD subunits reveals that a congruence relationship between task context and the structure of an ISD subunit is present. Our findings also show that an exclusive association exists between uncertainty in the task environment and the decision-making structure. Likewise, the association between equivocality of task content and the control structure of the ISD subunit is also exclusive. Implications of these findings for decision makers and researchers are discussed.     

Partially Flexible Operating Rooms for Elective and Emergency Surgeries

In hospitals, the management of operating rooms faces a trade‐off between the need to be responsive to emergency surgeries and to conduct scheduled elective surgeries efficiently. Operating rooms can be configured as flexible and handle both electives and emergencies, or as dedicated to focus on either electives or emergencies. With flexible rooms, the prioritization of emergencies over scheduled electives can lead to schedule disruptions. Focused rooms can lead to imbalances between capacity and surgery workload. Whereas hospital administrators typically handle this trade‐off by employing either flexible rooms (complete flexibility) or dedicated rooms (complete focus), we investigate whether a combination of flexible and dedicated rooms (partial flexibility) could be a preferable alternative. The ensuing question is what is the right combination of flexible and dedicated rooms? A versatile simulation model is developed to evaluate different resource allocation policies under various environmental parameters and performance metrics, including patient wait time, staff overtime, and operating room utilization. The main result is that partial flexibility configurations outperform both complete flexibility and complete focus policies by providing solutions with improved values of expected wait time for both emergency and elective patients.     

Scheduling with Sequencing Flexibility*

This study examines the effects of sequencing flexibility on the performance of rules used to schedule operations in manufacturing systems. The findings show that taking advantage of even low levels of sequencing flexibility in the set of operations required to do a job results in substantial improvement in the performance of scheduling rules with respect to mean flowtime. Differences in the mean flowtime measure for various rules also diminish significantly with increasing sequencing flexibility. Performance improvements additionally result for such due-date related performance measures as mean tardiness and the proportion of jobs tardy. At high levels of sequencing flexibility, some nonparametric scheduling rules outperform the shortest processing time rule in terms of the mean flowtime criterion. Rules based on job due dates also outperform rules based on operation milestones in terms of tardiness related criteria at high levels of sequencing flexibility. The implications of these findings for the design of manufacturing systems and product design are noted.     

Supporting Quick Response Through Scheduling of Make-to-Stock Production/Inventory Systems*

Scheduling of traditional job shops in make-to-order systems has seen extensive research over the past three decades. In such systems, performance is often related to various job completion metrics such as average flow time, average lateness, etc. This paper examines a scheduling problem in a make-to-stock environment where individual job completion measures are irrelevant. In this case, customer orders are satisfied through on-hand inventory where customer service is more closely related to the manufacturer's ability to quickly satisfy demand. We consider the role of scheduling in reducing inventories and improving customer service in the context of a manufacturer who assembles several different products on a single assembly line. We develop scheduling rules for such a system and experimentally compare their performance to those typically used in such environments. Our results indicate that rules which consider the inventory position and demand forecast outperform traditional fixed cycle rules.     

Fuzzy multi-attribute decision-making for evaluating manufacturing flexibility

Evaluation of manufacturing flexibility in operations management is important to determine the competitiveness of manufacturing system, and is being increasing discussed in the literature on manufacturing system. The objective of this research is to develop a multi-attribute decision-making model based on fuzzy set theory to improving the manufacturing flexibility evaluation process. Since most information available in this stage is not numerical, fuzzy linguistic assessment is used to represent the performance rating of all flexibility metrics under flexibility dimensions and importance grade of all flexibility dimensions. This paper proposes a modified LOWA operator, which uses the maximum entropy weights, and uses it to direct computation on linguistic terms. The developed model evaluates the degree of manufacturing flexibility that can assist decision-makers, when making capital investment decisions and measuring performance, in finding the need for improving manufacturing flexibility, and in determining the dimensions of flexibility as the best directions to improvement. Examples using a case study of leading Taiwan firm in the bicycle industry are used to illustrate the concept developed. In addition, sensitivity analysis can be performed to examine the solution reasonability.     

Appointment scheduling in tandem-type service systems

Appointment-based service systems arise in a broad variety of healthcare settings (for example an outpatient clinic or a dentist). Where most existing algorithms specifically consider the situation of the patient undergoing a single service, in many practical situations multiple services have to be sequentially performed. Modeling the service system as a tandem queue, the main objective of this paper is to generate schedules that soundly balance the interests of patients (i.e., low waiting times) and staff (i.e., low idle times). Importantly, following up on prior work for the single-node queue, we advocate a phase-type based technique that can deal with any service-time distribution (which may, in addition, vary across patients). Relying on a novel recursive scheme to evaluate the sojourn-time distribution of clients in such tandem systems, we show how optimal schedules can be computed. Our technique is illustrated by extensive numerical experimentation, also leading to practical guidelines that apply to a broad range of parameter settings.     

Pull system performance measures: a review of approaches for system design and control

Abstract

Research into the effect of three factors on four major system performance measures for pull system design and control are reviewed. The three factors are: (1) qualitative or quantitative nature of the approach, (2) traditional production control or pull system approach, and (3) systems/integrative characteristics of the approach for the performance measures. The four performance measures are: (I) throughput, (2) quality, (3) material Dow, and (4) flexibility. Articles were evaluated in terms of the performance measures and factors with several articles discussed in depth, Journals and other sources were chosen to be searched based on the source having published refereed articles related to pull system performance evaluation. Articles on quantitative pull systems are not well represented in the literature. Traditional approach methods     

The company-wide effects of just-in-time manufacturing within an integrated materials handling system: a case study

Abstract

The paper describes the formulation of a cellular manufacturing methodology to allow the use of an automated guided vehicle system (AGVS) in a difficult environment. The resulting methodology allowed just-in-time operation on the factory floor. The methodology was also shown to provide benefits additional to those associated with AGV usage, namely increased integration of the manufacturing control departments of the organization, and increased efficiency of their MRP system.     

供应中断风险下OEM供应链弹性运作与提升策略

供应中断是OEM供应链中企业面临的主要风险。本文基于供应链弹性分析的角度，将OEM供应链弹性运作问题描述为多变量耦合控制模型，构建了可变结构的弹性控制系统，研究了在供应中断风险冲击下OEM供应链弹性交互影响机制。在此基础上，提出了一种有针对性的提升供应链弹性的深度学习机制，此算法比传统的BP神经网络更加能够提高供应链绩效，并结合案例进行验证。研究结果表明：当供应中断发生时，深度学习算法可有效提升OEM供应链弹性，最大程度减轻企业损失。     

Setting tolerance limits for statistical project control using earned value management

Project control has been a research topic since decades that attracts both academics and practitioners. Project control systems indicate the direction of change in preliminary planning variables compared with actual performance. In case their current project performance deviates from the planned performance, a warning is indicated by the system in order to take corrective actions.Earned value management/earned schedule (EVM/ES) systems have played a central role in project control, and provide straightforward key performance metrics that measure the deviations between planned and actual performance in terms of time and cost. In this paper, a new statistical project control procedure sets tolerance limits to improve the discriminative power between progress situations that are either statistically likely or less likely to occur under the project baseline schedule. In this research, the tolerance limits are derived from subjective estimates for the activity durations of the project. Using the existing and commonly known EVM/ES metrics, the resulting project control charts will have an improved ability to trigger actions when variation in a project׳s progress exceeds certain predefined thresholdsA computational experiment has been set up to test the ability of these statistical project control charts to discriminate between variations that are either acceptable or unacceptable in the duration of the individual activities. The computational experiments compare the use of statistical tolerance limits with traditional earned value management thresholds and validate their power to report warning signals when projects tend to deviate significantly from the baseline schedule.     

An Efficient and Robust Design for Transshipment Networks

Transshipment, the sharing of inventory among parties at the same echelon level of a supply chain, can be used to reduce costs. The effectiveness of transshipment is in part determined by the configuration of the transshipment network. We introduce chain configurations in transshipment settings, where every party is linked in one connected loop. Under simplifying assumptions we show analytically that the chain configuration is superior to configurations suggested in the literature. In addition, we demonstrate the efficiency and robustness of chain configurations for more general scenarios and provide managerial insights regarding preferred configurations for different problem parameters.