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COntrol of BAlance by CArd-BAsed NAvigation (COBACABANA) is a card-based system for job shop control. This paper presents a real industrial application of COBACABANA aimed at solving production con...

Workload control (WLC) is a lean oriented system that reduces queues and waiting times, by imposing a cap to the workload released to the shop floor. Unfortunately, WLC performance does not systematically outperform that of push operating systems, with undersaturated utilizations levels and optimized dispatching rules. To address this issue, many scientific works made use of complex job-release mechanisms and sophisticated dispatching rules, but this makes WLC too complicated for industrial applications. So, in this study, we propose a complementary approach. At first, to reduce queuing time variability, we introduce a simple WLC system; next we integrate it with a predictive tool that, based on the system state, can accurately forecast the total time needed to manufacture and deliver a job. Due to the non-linearity among dependent and independent variables, forecasts are made using a multi-layer-perceptron; yet, to have a comparison, the effectiveness of both linear and non-linear multi regression model has been tested too. Anyhow, if due dates are endogenous (i.e. set by the manufacturer), they can be directly bound to this internal estimate. Conversely, if they are exogenous (i.e. set by the customer), this approach may not be enough to minimize the percentage of tardy jobs. So, we also propose a negotiation scheme, which can be used to extend exogenous due dates considered too tight, with respect to the internal estimate. This is the main contribution of the paper, as it makes the forecasting approach truly useful in many industrial applications. To test our approach, we simulated a 6-machines job-shop controlled with WLC and equipped with the proposed forecasting system. Obtained performances, namely WIP levels, percentage of tardy jobs and negotiated due dates, were compared with those of a set classical benchmark, and demonstrated the robustness and the quality of our approach, which ensures minimal delays.

Most manufacturing shops in practice have limited physical space in front of each workstation, due, for example, to physical, economical or operational constraints. As a result, a job may cause blocking because it has to remain at a given station after an operation has been completed until space in front of the next station in its routing becomes available. Despite this practical reality, the Workload Control literature typically assumes infinite buffer limits and therefore neglects the impact of blocking. Using simulation, we highlight the direct, detrimental impact of blocking in both the pure and general flow shop. Workload Control order release dampens the effect of blocking and improves overall performance. This makes Workload Control order release even more important in the context of shops with blocking or physical space constraints. Further analysis reveals that the impact of blocking is less pronounced in the pure flow shop given its directed routing. Finally, most of the blocking that occurs is because jobs cannot enter the shop, i.e. there is no space in front of the gateway station. This re-emphasises the close relationship between blocking and release methods that limit the workload, and it highlights the importance of workload balancing.

An important goal of Production Planning and Control systems is to achieve short and predictable flow times, especially where high flexibility in meeting customer demand is required, while maintain...

Workload Control is a production control concept for high-variety shops built on the principle of input/output control. The literature, however, has argued that input/output control overemphasises throughput improvements to the detriment of the timing of individual orders and, consequently, that it needs to be supplemented by a preceding customer enquiry stage where due dates are set. Yet, although there are broad separate literature streams on due date setting, order release, and output control, there is a lack of research on the three functions together. In response, this study uses simulation to assess the combined performance effect of all three functions. Results show that each control function can be related to a specific performance objective. The degree of emphasis that should be placed on each function, therefore, depends on a company's specific performance needs. Due date setting and capacity adjustments (output control) are shown to support each other as they address different performance objectives. Meanwhile, order release (input control) is effective in reducing work-in-process and can play a role in making throughput improvements when capacity adjustments are not possible. Findings enhance existing literature on the diagnosis of delivery reliability performance in high-variety shops, with important implications for research and practice.

Although there is a broad literature on capacity management, there has been only limited attention on how to support short-term capacity control decisions, especially in high-variety make-to-order shops. While finite loading has been identified as a potential means of guiding capacity adjustments, the actual performance impact of this solution has not been adequately assessed. Using a simulation model of a make-to-order job shop, we compare the performance impact of four different forward and backward finite loading methods and a load trigger method recently presented in the literature. Results confirm the potential of finite loading to improve performance when compared to a general capacity increase. Yet all four methods are outperformed by the load trigger method. The capacity adjustments made under finite loading methods are determined by individual jobs and their properties. This may lead to no adjustments despite an overload period (e.g. if a job has a long due date but only one overload station in its routing) or to unnecessary adjustments when there is no overload (e.g. if a large job has a tight due date). This finding draws into question the use of finite loading altogether and reinforces the importance of the load trigger method.

Workload control mechanisms are widely studied in the literature for the control of job-shop systems. The control of these systems involves acceptance, order release and priority dispatching. At the release level, the workload norm controls the &ldquo, enters&rdquo, of the jobs, it is relevant how the aggregate workload is computed. Few works have studied new computation methods of the aggregate workload but use the adjusted aggregate workload proposed in the literature. This paper proposes a dynamically adjusted aggregate workload to improve the performance of the workload control mechanism in job-shop systems. The adjusted aggregate workload is updated when each part exits from a workstation, this means that the workload used to release the orders is related to the state of the job shop in real-time. Simulation is used to evaluate and compare the proposed model to the classical models proposed in the literature. The simulation experiments demonstrate improvement of performance and how the model proposed is robust under different manufacturing system conditions.

The purpose of this article is to present a method for calculating the lead-time, the inventory, and the safety stock or buffer in job shop manufacturing, which are essentially stochastic variables. The research method is quantitative modelling. The theoretical foundation of the method relies on the techniques belonging to the WLC (workload control) body of knowledge on the manufacturing management. The study includes an application of the method in a manufacturing plant of the furniture industry, whose operation strategy requires high dependability. The company operates in a supply chain and must have high a reliability in deliveries. Adequate safety stocks, lead times, and inventory levels provide the protection against the lack of reliability in the deliveries. The inventory should remain within a certain range, being as small as possible to maintain low lead times, but not too small that it could provoke a starvation, configuring an optimization problem. The study offers guidelines for a complete application in industries. Further research shall include the influence of the variability of the lot size in the stochastic variables.

Workload Control (WLC) seeks to align capacity with demand, where capacity is typically assumed to be restricted by a single constraint – machine capacity. In practice however, shops are often restricted by dual resource constraints: labor and machines. This study therefore uses simulation to investigate the performance of WLC in Dual Resource Constrained (DRC) highvariety shops with fully interchangeable labor. By considering several environmental factors and different labor assignment and dispatching rules, it is demonstrated that the order release function of WLC maintains its positive impact on performance in a DRC shop under different staffing levels. The positive effect of considering labor availability at release, as proposed in previous research, could not however be confirmed. Thus, the original release method can be applied if labor is fully interchangeable. In terms of labor assignment, we show that a distinct assignment pattern that differs between upstream and downstream stations improves performance if the routing is directed. Meanwhile, dispatching plays a less important role but creates important interaction effects with the assignment rule. Finally, the results suggest that increasing the service rate is a better response to the reduction in capacity that results from labor absenteeism than lowering the input frequency of work

The research discusses in this paper concerns the improvement allocation policies to reduce the process time in job-shop manufacturing systems. The policies proposed are based on the evaluation of the workload control of the entire manufacturing system. Three policies are proposed: centralized, distributed and proportional. A simulation model is used to test the proposed policies under different conditions as: static and dynamic demand; introduction of machine breakdowns; different level of average manufacturing system utilization. The performance measures are compared to a manufacturing system without policies. The simulation results show that the improvement allocation allows to improve the performance with limited investment (average reduction of process time needed) and how the machine breakdowns and demand changes lead to different better policy. The decision maker can use these results to decide the better policy to use.

In the workload control literature, the Load-Oriented Order Release (LOOR) approach has been neglected since its robustness was questioned at the end of the 1990s. This paper revisits LOOR and evaluates whether its performance can be improved in two ways. First, an intermediate pull release mechanism is added to avoid starvation between periodic release events. This mechanism was recently shown to be effective at improving the performance of a state-of-the-art release method known as LUMS COR. Second, an integer linear programming model is used to manage the trade-off between the timing and load balancing functions of order release. The two refinements are assessed using simulations of different shop configurations, which allow us to evaluate robustness. Results demonstrate that the refinements contribute to improving the performance of LOOR such that it can even outperform LUMS COR. Perhaps counter-intuitively, putting more emphasis on load balancing than on the urgency of individual orders is shown to lead to a lower percentage of tardy orders. Overall, the improvements mean that concerns about LOOR’s robustness are no longer valid – it now appears suitable for a wide range of shops found in practice.

One of the key functions of Workload Control is order release. Jobs are not released immediately onto the shop floor – they are withheld and selectively released to create a mix of jobs that keeps work-in-process within limits and meet due dates. A recent implementation of Workload Control’s release method highlighted an important issue thus far overlooked by research: How to accommodate re-entrant flows, whereby a station is visited multiple times by the same job? We present the first study to compare the performance of Workload Control both with and without re-entrant flows. Simulation results from a job shop model highlight two important aspects: (i) re-entrant flows increase variability in the work arriving at a station, leading to a direct detrimental effect on performance; (ii) re-entrant flows affect the release decision-making process since the load contribution of all visits by a job to a station has to fit within the norm. Both aspects have implications for practice and our interpretation of previous research since: (i) parameters given for work arriving may significantly differ from those realised; (ii) increased workload contributions at release mean that prior simulations may have been unstable, leading to some jobs never being released.

The workload control literature highlights the importance of balancing the shop floor workload, but also acknowledges that this can conflict with processing the most urgent orders – hence, there is a trade-off. In practice, shops contain many complexities, e.g. simultaneous batching and sequence-dependent set-up times that may conflict with processing the most urgent orders and require other solutions than workload balancing to avoid capacity losses. This adds to the trade-off dilemma, which traditionally only considers timing and balancing. This paper develops a framework that determines whether to address a complexity through order release or dispatching. It comprises two dimensions: (i) the typical position of a complexity in the routing of an order and (ii) the criticality of the complexity. A case study is presented, which demonstrates the framework’s utility and illustrates the development of specific solutions designed to handle the complexities. Most complexities present in the case require handling at the order release stage. The challenges of handling multiple complexities at this decision level are evaluated. Finally, the implications for managers and future research are outlined.

Recent research has highlighted the potential impact of pool sequencing on order release performance but it suffered from two shortcomings. First, arguably the best release solution for workload control in practice combines periodic with continuous release. Although the two types of releases serve different functions, recent work assumed the same sequencing rule should be used for both. Here, the use of different sequencing rules for periodic and continuous releases is evaluated. Using a job-shop simulation, we demonstrate that the rule applied during continuous releases has only a negligible impact on performance. Therefore, jobs can be pulled intermediately from the pool by workers using a more straightforward sequencing rule than the one applied for periodic release. Second, it was assumed that all jobs in the pool are sequenced and then a subset is selected for release. But for some load-oriented sequencing rules, the priority value used for sequencing jobs should be updated after each job selection f...

Order release is a key component of the Workload Control concept. Jobs do not enter the shop floor directly – they are retained in a pre-shop pool and released in time to meet due dates while keeping work-in-process within limits or norms. There are two important groups of release methods: continuous methods, for which the workload falling to a specified level can trigger a release at any moment in time; and, periodic release methods, for which releases take place at fixed intervals. Continuous release methods in general have been shown to outperform periodic release methods. Yet, there is incongruence in the results presented in the literature on the relative performance of the various continuous release methods. We use a job shop simulation model to examine the performance of continuous release methods from the literature and find that the contradictory results are explained by the different rules applied to sequence jobs in the pool – a factor neglected in previous work. Finally, a new breed of continuous release methods has recently emerged, but these have not been compared with prior approaches. Therefore, we also examine these methods and show that they significantly improve overall performance, although this is to the detriment of jobs with large processing times.

Most literature on job shops assumes that an infinite storage space is available on the shop floor. But in practice, storage space is limited or finite. The job shop control problem with finite storage space has received little attention; it is a complex problem for which optimised scheduling may lead to unfeasible and poor quality solutions. The alternative to a scheduling solution is the use of controlled order release. This study uses simulation to assess the performance of four order release control rules. Results indicate that the best results are achieved by the workload control order release WLCOR rule. This rule produces results comparable to the optimum solution identified by analytical modelling in terms of the storage space required to achieve a certain production rate. The advantage of WLCOR is that it does this while simultaneously controlling lead times and the percentage of tardy jobs. Results have significant implications for practice, particularly where space is at a premium.

Workload control (WLC) has been developed as a production planning and control approach for job shop manufacturing. By balancing loads across work centres, WLC anticipates the fact that multiple work centres may become potential bottlenecks in the short term. This approach is generally tested in job shop models that assume equal utilisation levels for all work centres, which will create maximum bottleneck shiftiness. However, job shop practice clearly shows differences in utilisation levels, which means that some work centres can be seen as non-bottlenecks, having protective capacity. This study investigates the effect of different levels of protective capacity on the performance of state-of-the-art WLC release methods. More in detail, it shows how the level of protective capacity interacts with the influence of workload norms at work centres. Despite the fact that WLC has been developed for highly balanced utilisations, results indicate that WLC could also be effective in unbalanced situations. However, ...

Most studies on the performance of workload control (WLC) order release methods assume products have simple structures. But, in practice, products are often complex and consist of a number of sub-assemblies that flow through a level 1 job shop before converging on several final assembly operations in a level 2 assembly shop. Evaluating the performance of release methods in this context referred to as the two-level multi-stage job shop is an important step towards improving the alignment between WLC theory and practice. We use simulation to assess the performance of four of the best-performing WLC order release methods. Results suggest that WLC order release has the potential to limit work-in-process (WIP) while reducing the percentage of tardy jobs. It is also important to consider when and where release should be controlled. Results suggest that: (1) orders should be considered for release to level 2 when the first sub-assembly is complete, rather than only when all of the sub-assemblies that make up an assembly order are complete at level 1; and, (2) exercising control at level 2 (with or without control at level 1) leads to a greater reduction in the percentage of tardy jobs than control at level 1 only.

A relevant segment of job shop environments has to cope with specific routing complexity. When order routings are long or assembly structures exist, performance problems arise in the application of traditional job shop control concepts based on workload control (WLC). This paper investigates how to adapt the WLC concept to the needs of these job shops. A first review of literature provides only few studies considering long or assembly type routings. Consequently field research is executed, analyzing and capitalizing the influences of a control approach applied in a successful company. The derived guidelines for adaptation of the WLC concept aim at improving the synergy between controlled release, to avoid irresolvable delays at early stages, and priority dispatching at later stages of the long order routings. (C) 2012 Elsevier B.V. All rights reserved.

Simulation has demonstrated that the workload control (WLC) concept can improve performance in job shops, but positive empirical results are scarce. A key reason for this is that the concept has not been developed to handle a number of practical considerations, including sequence-dependent set-up times. This paper investigates the influence of sequence-dependent set-up times on the performance of a workload-controlled job shop. It introduces new set-up-oriented dispatching rules and assesses the performance impact of controlled order release. Simulation results demonstrate that combining an effective WLC order release rule with an appropriate dispatching rule improves performance over use of a dispatching rule in isolation when set-up times are sequence dependent. The findings improve our understanding of how this key implementation challenge can be overcome. Future research should investigate whether the results hold if set-up time parameters are dynamic and set-up times are not evenly distributed across resources.

Findings from recent implementations of Workload Control (WLC) have called for research to investigate how sequence-dependent set-up times can best be handled within the design of the concept. In response, this study assesses the performance of four of the best-performing release methods from the literature in a job shop with sequence-dependent set-up times by simulation. First, the four methods are compared without considering set-up requirements at release. Second, the methods are refined to consider set-up requirements before being compared against the original methods. A release method that combines continuous and periodic release emerges as the best-performing method. Findings further suggest that considering set-up requirements at release do not have a significant positive effect on performance and may even be counterproductive: conflicting goals between the selection rules employed at release and dispatching may lead to an increase in the percentage of tardy jobs. Future research should consider wh...

An existing taxonomy of workload control rules is adapted to classify 25 workload control rules and their components examined in prior research to create a basis to model how three workload control rule components – order selection, work aggregation and buffer limits – affect shop performance in environments that differ in their bottleneck utilization and protective capacity levels. Two of the most frequently studied rules in the literature – CONWIP and DBR – were included in the simulation. Data analysis found that the workload control rules had a significant positive effect on the shop performance measures only in environments with the highest bottleneck utilization and the lowest protective capacity. However, the work aggregation method was found to be an important component of the workload control rule. Suggestions were made for future research that could increase our understanding of workload control rules and integrate this research with our understanding of lean production.

Existing card-based production control systems such as Kanban are mostly dedicated to repetitive production environments. Cards-based systems for job shop control are lacking, while particularly this industry segment shows a need for simple control systems. This paper aims at filling the gap by presenting a simple card-based system for job shop control, the Cobacabana (control of balance by card-based navigation) system. It is based on the concept of workload control (WLC), which has already proven its value in job shops. Developments towards more robust norms now allow for transforming the WLC concept into a simple card-based system, with loops of cards accompanying orders from release until completion at critical work centers. Cards returning from work centers authorize the planner to release new orders. A card-based display supports the planner with an overview of the shop floor situation, based on non-released cards. An additional loop of cards between sales and planning enables support of the order acceptance and due date promising function. A card-based acceptance display indicates what delivery dates are realistic, considering the current workload situation. Both the scientific roots of the system and practical implementation issues are discussed in this paper. (C) 2008 Elsevier B.V. All rights reserved.

The paper focuses on refining a Workload Control (WLC) concept to improve the applicability of the approach to the shop characteristics found in practice. This is a two-stage process leading to significant conceptual refinements to a key WLC methodology. The first stage focuses on the development of a Decision Support System (DSS) based on a WLC concept designed for Make-To-Order (MTO) companies. Refinements made include changes to the backwards scheduling procedure and the way in which jobs are released onto the shop floor. The second stage focuses on the process of implementation. Using a case study of a MTO company, the paper describes the strategy taken to overcome a number of prerequisites to the successful implementation of a Production Planning and Control (PPC) concept. Issues addressed include grouping machines and determining capacities. This case study adds to the available literature by looking specifically at implementing WLC from the customer enquiry stage, while the case study experience also provides further refinements to the WLC concept.

To be successful in companies, a production planning and control (PPC) concept should fit to the production environment. Essential elements of the concept should correspond with the characteristics of the production system. For classical concepts such as MRP these elements have become common sense. For example, BOM-explosion and constant lead times make MRP known to perform best in environments with high material and low capacity complexity. For many other concepts the situation is less clear. In this paper the workload control (WLC) concept is considered for which the requirements for a successful application have never been investigated. A framework is proposed to explore the applicability of WLC in small- to medium-sized make-to-order (MTO) companies. It supports an initial consideration of WLC in the first phase of a PPC selection and implementation process.As a first step in developing the framework, the inherent characteristics of the WLC concept and the relevant MTO production characteristics are identified. Confronting the indicators of the company characteristics with the WLC elements results in best-fit indications for the WLC concept. Contrarily to other PPC evaluation schemes the framework considers variability indicators besides averages.Use of this framework for a medium-sized MTO company demonstrates its suitability in getting a systematic and quick impression of the applicability of WLC. Essential elements are treated and assessed. (C) 2003 Elsevier B.V. All rights reserved.

In this paper we propose an extension of the job shop models that are used in the theoretical studies, that can explain the differences between the results of theoretical studies and empirical studies into the effects of workload control on performance. The extension assumes that the workload level affects the effective processing times in a shop and is based on the results of empirical research on the relationship between workload and shop performance. Using computer simulation we investigate the effects of workload based work order release on the shop performance under the new job shop model. The simulation results confirm the effect of workload control on shop performance found in empirical studies, and show that, under certain conditions, without workload control the shop performance can become unstable

Several order release methods have been developed for workload control in job shop production. The release methods of the traditional workload control concerts differ in how they deal with the flow of work to each station. Previous research has pointed at strengths and weaknesses of each method. Till now the choice of the appropriate method for a particular situation has hardly received attention. This research shows that shop characteristics are an important factor to this choice, A simulation study indicates that the relative performance of the release methods changes completely with for instance the presence or absence of a dominant flow direction in the shop. Adjustments to the traditional release methods are suggested which prove to make these methods more robust. (C) 2000 Elsevier Science B.V. All rights reserved.

Several order release methods have been developed for workload control in job shop production. The release methods of the traditional workload control concepts differ in how they deal with the flow of work to each station. Previous research has pointed at strengths and weaknesses of each method. Till now the choice of the appropriate method for a particular situation has hardly received attention. This research shows that shop characteristics are an important factor to this choice. A simulation study indicates that the relative performance of the release methods changes completely with for instance the presence or absence of a dominant flow direction in the shop. Adjustments to the traditional release methods are suggested which prove to make these methods more robust.

A specific class of production control concepts for jobs shops is based on the principles of workload control. Practitioners emphasise the importance of workload control. However, order release methods that reduce the workload on the shop floor show poor due date performance in job shop simulations. This paper traces the reasons behind the poor results of release methods that subject the workload to norms. An alternative release approach is proposed which improves the due date performance significantly. Simulation results show that the method is able to maintain the control of workload without the use of rigid norms. (C) 1998 Elsevier Science B.V. All rights reserved.

In many job shop environments production orders arrive continuously at the production system over time. If an 'order review and release'(ORR) technique is applied, the arrival itself does not necessarily involve the release of a job to the shop floor; in fact, the ORR activities determine which jobs have to be selectively dispatched to theshop floor and when job releaseis to takeplace, so as to improve job shop management and performances. This paper introduces a review and a classification framework of the main research work carried out to date. We have considered eight main dimensions that describe the fundamental principles, characteristics and logic of existing order review and release techniques. The classification of selected models from the specialised literature by means of the proposed framework provides a better understanding for the practitioners and, on the other hand, aims to indicate to researchers the most promising research paths.

Protecting throughput from variance is the key to achieving lean. Workload control (WLC) accomplishes this in complex make-to-order job shops by controlling lead times, capacity, and work-in-process (WIP). However, the concept has been dismissed by many authors who believe its order release mechanism reduces the effectiveness of shop floor dispatching and increases work center idleness, thereby also increasing job tardiness results. We show that these problems have been overcome. A WLC order release method known as LUMS OR (Lancaster University Management School order release) combines continuous with periodic release, allowing the release of work to be triggered between periodic releases if a work center is starving. This paper refines the method based on the literature (creating LUMS COR [Lancaster University Management School corrected order release]) before comparing its performance against the best-performing purely periodic and continuous release rules across a range of flow directions, from the pure job shop to the general flow shop. Results demonstrate that LUMS COR and the continuous WLC release methods consistently outperform purely periodic release and Constant WIP. LUMS COR is considered the best solution in practice due to its excellent performance and ease of implementation. Findings have significant implications for research and practice: throughput times and job tardiness results can be improved simultaneously and order release and dispatching rules can complement each other. Thus, WLC represents an effective means of implementing lean principles in a make-to-order context.

Workload control concepts are advocated as one of the new production planning and control methods. In its elaborated form, workload control includes three major decision levels: job entry, job release and priority dispatching. In each decision level, several decision points which have significant impact on the effectiveness of the production planning and control are defined (i.e., acceptance/rejection, due date assignment, etc.). Workload control systems should consider all of these decision points simultaneously in order to improve the effectiveness of production planning and control. In addition to these decision levels, flexibility of the shop can also be included as the fourth decision level which allows the shop capacity to be adjusted as new orders enter the system and as they are released to the shop floor. In this study, simulation models which enable the effect of each decision level within a workload control concept to be explored are developed and tested. The results reveal that simultaneous consideration of decision levels is critical and can improve the effectiveness of production planning and control.