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This paper shows the effectiveness of labour transfers in addressing premature idleness caused by controlled order release. Controlled order release restricts order entry to the shop floor and is commonly employed in high-variety manufacturing where it results in benefits such as stable work-in-progress. However, it can increase waiting times when orders are blocked from release, while capacities are idling. This issue, known as premature idleness, negatively impacts delivery performance. Previous studies have primarily focused on addressing premature idleness through input control by releasing new orders to idling workstations. This approach overlooks the potential of output control during premature idleness, transferring labour to assist at other workstations in a dual resource constrained setting. Using simulation, this study demonstrates that output control significantly improves delivery performance—in terms of mean tardiness and percentage tardy—and reduces total and shop floor throughput times. Importantly, this result proves robust, even when the efficiency of the assisting worker is severely limited. Shop-level performance improves despite the efficiency loss of the worker. The impact of the where-rule is minimal, while the efficacy of the priority dispatching rule depends on the joint efficiency of collaborating workers. Finally, we show that combining input control and output control enhances performance, providing opportunities for further research on the role of both control approaches in high-variety manufacturing. [ABSTRACT FROM AUTHOR]

Many companies in practice want to dynamically adjust planned lead times in their production planning and control systems in response to demand fluctuations. But for decades it has been recognised that this can lead to escalating planned lead times and realised throughput times. Authors have highlighted the negative impact of this 'lead time syndrome', especially in the context of Material Requirements Planning systems, prompting the development of alternative concepts intended to overcome its vicious cycle, such as Workload Control. Yet some authors have shown that increasing planned lead times has advantages – it can improve end-item service levels. To resolve this paradox, we conjecture that the effects of the lead time syndrome are limited when demand is independent of internally planned lead times, such as in make-to-order companies, and subsequently use simulation to prove this conjecture. We show that although dynamic planned lead times have a detrimental effect on performance in make-to-order systems, it is not an increase in planned lead times that leads to a performance loss. Rather, it is the decrease of lead times in low load periods that increases workloads in upcoming periods of high load. This questions the use of upper bounds (WIP-cap) in these contexts. [ABSTRACT FROM AUTHOR]

In manufacturing shops in practice, machine capacity is often constrained by more than one type of resource. Yet research mainly focusses on the effects of only one type of resource that constrains machine capacity, e.g. labour, tooling or auxiliary constraints. In response, we use simulation to assess the impact of order release, dispatching and resource assignment rules in make-to-order job shops with multiple resource constraints. The capacity wasted while a machine stands idle waiting for other resources increases with the number of constraints, and all three production planning and control functions have little impact on this waiting time. Effective production planning and control can however improve operational performance in terms of time and tardiness-related measures. In general, combining order release control with a dispatching rule that prioritises jobs for which all resources are available at dispatching and a longest queue resource assignment rule leads to the best performance. Most importantly, and rather counterintuitively, prioritising orders with the fewest missing resources worsens the performance of both the dispatching and resource assignment rule since it reduces resource utilisation during periods of high load. Results from dual resource-constrained shops are consequently not directly transferable to more complex resource-constrained shops. [ABSTRACT FROM AUTHOR]

Workload Control withholds orders from the shop floor in a backlog from which they are released to meet certain performance metrics. This release decision precedes the execution of orders at shop floor stations. For each station there are consequently three types of workload: indirect, released work that is still upstream of the station; direct, work that is currently at the station; and, completed, work that is still on the shop floor but is downstream of the station. Most release methods control an aggregate workload made up of some representation of at least two of these three workload types. Yet the core objective of Workload Control release methods relates to only one of the three types – that is, to create a small, stable direct load in front of each station. Clearly, order release would be greatly simplified if only the direct load had to be considered. Using discrete event simulation, we show that Direct Workload Control leads to performance levels that match those of more complex and sophisticated approaches to Workload Control. Further, it greatly simplifies continuous order release, decentralising the release decision by allowing it to be executed at each gateway station. This has important implications for research and practice. Highlights Presents a new Workload Control release method that controls the direct load only. The new method significantly simplifies workload calculations. The new method can be decentralised with control exercised locally at gateway stations. Simulation results demonstrate comparable performance to more sophisticated methods. The new method improves the performance of large jobs. [ABSTRACT FROM AUTHOR]

Manufacturing industries struggle to devise precise planning and scheduling solutions due to unpredictable business situations. Additionally, uncertainties in production such as machine breakdowns, labour absenteeism, cycle time deviations, etc., would further deteriorate production plans and lead to uncertainty in decision-making processes. Flow shops with bottlenecks are particularly susceptible to these disturbances. Moreover, the random variations in cycle time variations can cause the bottleneck to shift between different stages. Literature indicates that conventional job release methods are ineffective in addressing these difficulties. In contrast, workload control methods would provide better solutions. Hence, a flow shop model has been developed and simulated using the variables like process time variations and bottleneck shifting on the discrete-event simulation software. The flow shop model incorporates realistic shop characteristics which are subjected to random process time variations, so as to assess the performance. The outcomes of the experimentation demonstrate that order release methods play a pivotal role in improving the performance of flow shops in more volatile situations. [ABSTRACT FROM AUTHOR]

An essential task in manufacturing planning and control is to determine when to release orders to the shop floor. A prominent approach is the workload control (WLC) concept which originated from the idea of controlling flow times by controlling order releases. Despite recent advances in rule based WLC models, the recent semiconductor literature has neglected them, although it has been shown that they outperform most other periodic and continuous order release models. Therefore, we adapt the most successful rule based WLC model, the LUMS-COR approach and compare it with two approaches from the semiconductor manufacturing literature: Starvation Avoidance (SA) and ConLOAD approach. We include three pool sequencing rules, namely First-Come First-Served (FCFS), Earliest Due Date (EDD) and Critical Ratio (CR). We analyse their performance using a simulation model of a scaled-down wafer fabrication facility. The results show that, in comparison to the other two order release approaches, the LUMS-COR model yields lower total costs due to a more balanced shop and better timing performance which is robust across different settings. This suggests that the adapted LUMS-COR model has high potential to become a viable alternative to the rule based order release mechanisms used in semiconductor industry to date. [ABSTRACT FROM AUTHOR]

Additive Manufacturing (AM) shops typically produce high variety, low volume products on a to-order basis. Products are first created in parallel batches at a single AM station before being subjected to several post-processing operations. While there exists an emerging literature on AM station scheduling and order book smoothing, this literature has largely neglected downstream post-processing operations, which also affect overall performance. Workload Control provides a unique production control solution for these post-processing operations, but the specific AM shop structure has been neglected in the literature. Using simulation, this study shows that load balancing via the use of workload norms, as is typical for Workload Control, becomes ineffective since the norm must allow for the operation throughput time at the AM station and for its variability. A sequencing rule for the jobs waiting to be released that inherently creates a mix of jobs that balances the workload is therefore identified as the best-performing rule. These findings reinforce the principle that load limiting should be used at upstream stations whereas sequencing should be applied at downstream stations. Finally, although the focus is on AM shops, the findings have implications for other shops with similar structures, e.g. in the steel and semi-conductor industries. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 maintaining high output and due-date performance. One approach to this problem is the workload control (WLC) concept. Within WLC research two directions have been developed, largely separately, over time: Rule based and optimisation-based models. If a company intends to introduce an order release concept based on WLC it first has to decide which of these two approaches should be applied. Therefore, this paper compares two of the most widely used and considered best performing periodic order release models out of both streams: the LUMS (rule based) and the clearing function model (optimisation based). The parameters of both approaches are set using simulation optimisation. The performance is compared using a simulation study of a hypothetical job shop in a rolling horizon setting. The results show that the optimisation model outperforms the rule-based mechanism in all instances with stochastic demand (exponential inter-arrival times), but is outperformed in aggregate cost of backorders and inventory holding and balancing measures by the LUMS approach for scenarios with high utilisation and seasonal demand. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

An important scheduling function of manufacturing systems is controlled order release. While there exists a broad literature on order release, reported release procedures typically use simple sequencing rules and greedy heuristics to determine which jobs to select for release. While this is appealing due to its simplicity, its adequateness has recently been questioned. In response, this study uses an integer linear programming model to select orders for release to the shop floor. Using simulation, we show that optimisation has the potential to improve performance compared to 'classical' release based on pool sequencing rules. However, in order to also outperform more powerful pool sequencing rules, load balancing and timing must be considered at release. Existing optimisation-based release methods emphasise load balancing in periods when jobs are on time. In line with recent advances in Workload Control theory, we show that a better percentage tardy performance can be achieved by only emphasising load balancing when many jobs are urgent. However, counterintuitively, emphasising urgency in underload periods leads to higher mean tardiness. Compared to previous literature we further highlight that continuous optimisation-based release outperforms periodic optimisation-based release. This has important implications on how optimised-based release should be designed. [ABSTRACT FROM AUTHOR]

Order release is a key production planning and control function, specifically in high variety contexts. A large literature on release methods that balance the workload consequently emerged. These Workload Control methods can be rule based, using a simple greedy heuristic, optimization based or optimization based with lead times that are exogenous. Although all three types of methods have the same objective, their performance has never been compared. Using simulation, this study shows that a better on time delivery performance of jobs can be achieved by the two optimization based release methods. Most importantly, optimization based methods that assume lead times to be exogenous significantly outperform alternative methods in terms of tardiness performance. Rule based and optimization based Workload Control without exogenous lead times overemphasize average lateness reduction, which leads to sequence deviations that offset performance improvements through balancing. In contrast, Workload Control methods that assume lead times to be exogenous limit sequence deviations, which leads to a significant reduction in dispersion of lateness. This has important implication for the future design of order release methods, and managerial practice. [ABSTRACT FROM AUTHOR]

Most Workload Control literature assumes that delivery performance is determined by tardiness related performance measures only. While this may be true for companies that directly deliver to end-customers, for make-to-stock companies or firms that are part of supply chains, producing early often means large inventories in the finished goods warehouse or penalties incurred by companies downstream in the supply chain. Some earlier Workload Control studies used a so-called time limit, which constrains the set of jobs that can be considered for order release, to reduce earliness. However, recent literature largely abandoned the time limit since it negatively impacts tardiness performance. This study revisits the time limit, assessing the use of different adaptive policies that restrict its use to periods of either low or high load. By using a simulation model of a pure job shop, the study shows that an adaptive policy allows to balance the contradictory objectives of delaying the release of orders to reduce earliness and to release orders early to respond to periods of high load as quick as possible. Meanwhile, only using a time limit in periods of high load was found to be the best policy. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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: labour and machines. This study, therefore, uses simulation to investigate the performance of WLC in Dual Resource Constrained (DRC) high-variety shops with fully interchangeable labour. By considering several environmental factors and different labour 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 labour availability at release, as proposed in previous research, could not, however, be confirmed. Thus, the original release method can be applied if labour is fully interchangeable. In terms of labour 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 labour absenteeism than lowering the input frequency of work. [ABSTRACT FROM AUTHOR]

The reduction in emissions and the increase in energy costs push companies to identify solutions to reduce energy consumption in production systems. One of the approaches proposed in the literature is the shutdown of machines to reduce energy consumption in the idle state. This solution does not affect production processes and can be applied in various manufacturing fields. This paper proposes switch-off policies in manufacturing systems under a workload control system. The shutdown policies developed consider the number of items in the queue and the calculation derived from the workload control mechanism. Simulation models have been developed to test the proposed policies using the case always on as a benchmark, considering different levels of absorbed power in the inactivity and warm-up states and different warm-up times. The results highlight how the switch policies that include the workload evaluation drastically reduce the number of on/off activities, assuring lower energy consumption. [ABSTRACT FROM AUTHOR]

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Either make-to-order (MTO) or make-to-stock (MTS) alone cannot satisfy the customer demand. Therefore, companies in different sectors are trying to adopt a hybrid MTO-MTS production strategy. Unlike MTO or MTS, there are few research on hybrid MTO-MTS environment and very few considering the operational issues. The workload control seems to handle better the operational issues in the hybrid environment considering job shop configuration whereas drum buffer rope has been designed to handle the bottleneck issue in a pure flow shop. Therefore, the objective of this paper has been defined to compare bottleneck-based release models and load-based ones in a hybrid MTO-MTS flow shop. Simulation results are not clearly in favour of one over the other, since besides severity, there is another important factor to consider: the choice of control at the dispatching level, and its connected trade-offs. In particular, implementing this control with the bottleneck-based rule is effective for MTO performances, especially in cases of high severity. Instead, when control at dispatching is absent, the workload control as a release rule is preferred, leading also to the best lead times for low severity. The results demonstrate important managerial and research implications for bottleneck-based and load-based release models in MTO-MTS flow shops. [ABSTRACT FROM AUTHOR]

Kanban systems are simple yet effective means of controlling production. Production control is decentralised or exercised locally on the shop floor, i.e. a downstream station signals to an upstream station that an item is needed. If items are always the same and known, then demands can be satisfied instantaneously from stock; but if items differ and are unknown, demands must first be propagated backwards from station to station before being satisfied. The former is defined as an inventory control problem and the latter as an order control problem. Handling the order control problem via kanban involves a decentralised card acquisition process (during which information is propagated from station to station) that is separated from the actual production process. COBACABANA (control of balance by card-based navigation), an alternative card-based solution, shares kanban's control structure but centralises the card acquisition process. Evaluating the two systems therefore provides a unique opportunity to compare decentralised and centralised control. Using simulation, we demonstrate that it is specifically the centralised card acquisition process that allows COBACABANA to balance the workload across resources and thus to outperform kanban in an order control problem. This has major implications for research and practice. [ABSTRACT FROM AUTHOR]

This paper deals with improving the lead-time performance of a large crystal manufacturer that uses a state-of-the-art commercial Enterprise Resource Planning system. Since the company encountered some limitations of the standard production planning and control (PPC) system it sought for improvements by implementing an order release mechanism based on workload control (WLC). WLC employs certain rules for releasing orders in order to maintain a certain level of work in process to achieve a certain utilisation of the production system and thus control the flow times in order to meet the required due dates of the orders. We describe the successful implementation of an order release mechanism based on the WLC concept in this make-to-stock company. The paper describes the implemented order release mechanism, the implementation process and its impact on the company’s performance. We show that the core function of WLC - the order release mechanism - can be integrated successfully into an existing PPC system. Furthermore, this study highlights the applicability of WLC to a wider range of companies, especially to make-to-stock manufacturers. [ABSTRACT FROM AUTHOR]

This study uses simulation to assess the performance of alternative methods for detecting momentary bottlenecks in high-variety contexts that produce on a to-order basis. The results suggest that using the utilisation level of a station to detect bottlenecks leads to the best performance, but that this method suffers from high nervousness. Using the active period of a station appears to be a better overall choice for practice given its good performance and low nervousness. Meanwhile, methods that focus on the workload at a station are a viable alternative, but they may become dysfunctional in shops with directed routings and a limit on the queue. This negative effect is even stronger if the corrected workload measure is used, as recently suggested in the literature on short term capacity adjustments. Finally, using the inter-departure time detection method leads to the worst performance since: (i) it counterintuitively detects non-bottlenecks instead of bottlenecks; and, (ii) it is based on historical data, leading to a response delay. [ABSTRACT FROM AUTHOR]

Workload Control is used in manufacturing systems to obtain more predictable throughput times and accurate delivery dates. The models proposed in the literature are typically focused on machines with fixed processing time. This study, therefore, uses simulation to investigate the performance of the Workload Control method with a controllable processing time of the machines. This research proposes four models to support the decisions on the time to reconfigure the machines and the number of machines reconfigured. The time decision follows two strategies as periodic and continuous, while the machines reconfigured can be all or two considering the workload. The combinations of these strategies lead to four models that are tested in different conditions of reconfiguration times and the number of bottlenecks. The results suggest as the proposed models allow to improve delivery time performance and a more uniform distribution of workload among the machines of the manufacturing systems. [ABSTRACT FROM AUTHOR]

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 control problems in an Italian manufacturer of valves and flanges, configured as a job shop system. To the best of our knowledge, this is the first industrial implementation of COBACABANA. Although COBACABANA can be extended to estimating delivery time allowances, this research focuses only on its shop floor control mechanism. In the case study, the COBACABANA boards and cards are carefully reviewed. The criticalities and limitations of traditional COBACABANA are fully explored and, subsequently, two new boards and a new form are introduced to facilitate its practicality and ease of implementation. The new tools enable planners to better monitor the pre-shop pool status and the progress of orders. [ABSTRACT FROM AUTHOR]

In consulting services, call centres serve several types of customers. The demand of different customers who ask for specific skills hinders the ability to balance the workload among servers; thus, task reallocation may be needed. In this paper, we model a call centre with multi-type customers as a multi-class tandem queue and examine how to reallocate the workload among servers. To evaluate the performance of the system, we propose an approximate analytical method based on the aggregation of Markov chain models. To optimally reallocate the workload, we propose a search algorithm based on the optimal computing budget allocation method. A simulation is employed to validate the analytical method and identify regions in which it can be successfully applied. A case study shows the applicability of the approach and quantifies its benefits in a realistic situation. [ABSTRACT FROM PUBLISHER]

Constant Work-in-Process (ConWIP) is a card-based control system that was developed for simple flow shops – a lack of load-balancing capabilities hinders its application to more complex shops. In contrast, load balancing is an integral part of Workload Control, a production planning and control concept developed for high-variety environments. One means of load balancing evident in the Workload Control literature is through the use of a capacity slack-based backlog-sequencing rule. This study therefore investigates the potential of the backlog-sequencing decision to improve load balancing in the context of ConWIP, thereby making it suitable for more complex, high-variety environments. Using simulation, we demonstrate that: (i) the choice of backlog-sequencing rule significantly impacts throughput times and tardiness-related performance measures; and (ii) capacity slack-based sequencing rules achieve significant performance improvements over ‘classical’ ConWIP backlog-sequencing rules. These results significantly extend the applicability of ConWIP. Results from the Workload Control literature however do not directly translate across to ConWIP. The simplified release procedure of ConWIP makes backlog-sequencing based on planned release dates dysfunctional. This negatively impacts the performance of modified capacity slack-based sequencing rules that were recently shown to be the best choice for Workload Control. [ABSTRACT FROM PUBLISHER]

To effectively react and meet the current ever growing demand for individualised motor vehicles, built to customer specific requirements, automotive industry has accelerated its transition towards mass-customisation. As a result, the number of new model introductions has drastically increased over the past three decades. To cope with this intensified customisation, the current automotive assembly platforms are designed to assemble a wide range of relatively different models, and are turned into mixed-model assembly lines (MMALs). This implies that the set of tasks to be performed on each workstation is no longer stable but varies highly with the model-mix. As a consequence, the manufacturing complexity increases at the workstations and throughout the whole assembly system. This paper proposes a method to monitor manufacturing complexity at each workstation while the MMAL is being balanced. An entropy-based quantitative measure of complexity, which incorporates the variability of each task duration, is developed. This measure is used to monitor the manufacturing complexity level at each workstation. An integrated mixed-line balancing and complexity monitoring heuristic is proposed, to determine workload balance solutions, in which manufacturing complexity is levelled throughout the workstations composing the line. This procedure is tested on a real data-set provided by an automotive manufacturer. The results are reported and thoroughly discussed. [ABSTRACT FROM AUTHOR]

Purpose: Due to high customization, companies, which used to follow the make-to-stock (MTS) strategy, are now adopting the make-to-order (MTO) strategy along with MTS in order to cope with high variability in customer demand. Although lean is quite popular in general, its advantage in MTO-MTS environment is quite rare. Therefore, the purpose of this paper is to show the benefits of lean techniques in this environment. Design/methodology/approach: A popular furniture company in Italy is selected for the longitudinal action research and as a part of the lean project, workload control (WLC) is deployed in the selected company. Data were collected through on-site interviews, potential areas of improvement were identified and then suitable countermeasures were proposed and adopted. Findings: This study reveals that the implementation of lean techniques including WLC in a furniture company reduces the throughput time and improves the inventory turnover. Research limitations/implications: This research is based on a single company. Additional empirical studies can help to refine our proposition. Practical implications: The paper shows the practical implementation of WLC as a part of the lean project in MTO-MTS environment. It provides the practitioners and consultants with the way of implementing lean techniques in companies that are planning to adopt MTO-MTS hybrid strategy. Originality/value: This is the first paper that aims to assess the efficiency of the adaption of WLC based on lean principles in a company using MTO-MTS hybrid strategy. Hence, the results are highly valuable for similar companies, especially when are facing transition situations (e.g. from MTS to MTO-MTS). [ABSTRACT FROM AUTHOR]

Card-based systems are simple, effective means of controlling production. Yet most systems concentrate on controlling the shop floor. They neglect other planning tasks, like estimating short, feasible due dates during customer enquiry management. A card-based version of the workload control concept for job shops – COBACABANA (COntrol of BAlance by CArd-BAsed Navigation) – was proposed in the literature to overcome this shortcoming. COBACABANA uses cards for due date setting and order release, making it a potentially important solution for small shops with limited resources. But many such firms operate as flow shops rather than job shops. Research demonstrated that COBACABANA’s release mechanism must be adapted if applied to a pure flow shop, but its approach to due date setting has not been evaluated in such an environment. We show COBACABANA has the potential to improve pure flow shop performance, but its due date setting procedure should be adapted compared to job shops. In a flow shop, due date estimation can also be further simplified by considering the load awaiting release to the first (gateway) station only while maintaining most performance benefits. The results are important for all card-based systems that aim to stabilise work-in-process, includingkanbanand ConWIP (Constant Work-in-Process). [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

This paper addresses the problem of lot splitting in the context of workload control (WLC). Past studies on WLC assumed that jobs released to the shop floor proceed through the different stages of processing without being split. However, in practice, large jobs are often split into smaller transfer sublots so that they can move more quickly and independently through the production process and allow operations overlapping relating to the same job. This paper assesses the performance of different lot splitting policies for job release and dispatching strategies under lot splitting. A new dispatching rule was designed to specifically take advantage of lot splitting and operations overlapping in the context of WLC. Discrete-event simulation is used to assess system performance in relation to the ability to provide shorter delivery times and on time deliveries. Results highlight the importance of releasing the sublots of the same job together and demonstrate that combining an effective lot splitting policy with an appropriate dispatching rule can enhance the performance of production systems. [ABSTRACT FROM AUTHOR]

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 from the pool. Our simulation results show that although this may improve load balancing at release, it does not in fact improve overall shop performance. Therefore, the greedy heuristic of first sequencing and then selecting jobs can be maintained, which allows the release decision-making process to retain its simplicity. The work has important implications for the use of sequencing rules in practice. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

The purpose of this study is to choose an order dispatching rule and measure the work-in-process and lead-time in the production process of a conveyor chain manufacturer. The main strategic issue for the manufacturer is dependability, which requires meeting deadlines and managing internal lead-times. The study integrates two techniques, workload control (WLC) and an analytical hierarchy process (AHP), respectively systems for production planning and control, and multi-criteria decision support, both widely used in handling manufacturing strategic issues. The research method is a field experiment. Supported by the AHP and according to strategic criteria, practitioners selected the early due date rule (the order with the closest due date comes first) to release 231 orders. Then, employing a methodology designed to support WLC applications, the study measured key parameters that provide information regarding the overall performance of the manufacturer, the input rate, work-in-process, lead-time, throughput performance, and the level of safety stock. Using the model and a graphical tool derived from queuing theory, the throughput diagram, the study provides evidence that, although the manufacturing process is satisfactorily balanced and achieves acceptable performance, the level of safety stock is small and should be increased to prevent starvation on the shop floor. [ABSTRACT FROM AUTHOR]

In many real-life high-variety make-to-order shops, jobs are physically transported from one station to another, and transportation capacity may constrain order progress on the shop floor. Yet, the material handling literature on vehicle assignment rules remains largely inconclusive on which rule to apply, and it neglects order release control. Similarly, and despite the importance of material handling, its impact is widely neglected in the order release literature. In response, this study assesses the combined performance effect of vehicle assignment, order release and dispatching rules. It uses discrete event simulation. Results show that assigning a vehicle to the station with the largest outgoing queue leads to the best performance. This simple vehicle assignment rule has been largely neglected in the literature since the 1980s. In contrast, the two rules that have received the most attention in the literature – the shortest distance and first-come-first-served rules – lead to the worst performance. Meanwhile, order release has a direct detrimental performance effect in pure job shops with material handling constraints. This identifies an important contingency factor so far neglected in the literature that assesses the applicability of order release. More specifically, the use of order release should be restricted to general flow shops with more directed routings since, in these contexts, it can reduce throughput times without jeopardizing tardiness performance. [ABSTRACT FROM AUTHOR]

The paradigm shift toward Industry 4.0 is facilitating human capability, and at the center of the research are the workers—Operator 4.0—and their knowledge. For example, new advances in augmented reality and human–machine interfaces have facilitated the transfer of knowledge, creating an increasing need for labor flexibility. Such flexibility represents a managerial tool for achieving volume and mix flexibility and a strategic means of facing the uncertainty of markets and growing global competition. To cope with these phenomena, which are even more challenging in high-variety, low-volume contexts, production planning and control help companies set reliable due dates and shorten lead times. However, integrating labor flexibility into the most consolidated production planning and control mechanism for a high-variety, low-volume context—workload control—has been quite overlooked, even though the benefits have been largely demonstrated. This paper presents a mathematical model of workload control that integrates labor flexibility into the order review and release phase and simulates the impact on performance. The main results show that worker transfers occur when they are most needed and are minimized compared to when labor flexibility is at a lower level of control—shop-floor level—thus reducing lead time. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The workload control (WLC) is a popular concept in manufacturing planning and control, which plays a significant role in enhancing the efficiency of manufacturing firms that have uncertainty in meeting customer orders. Owing to changes in several set of factors, such as processing time variations, fluctuation in orders, and rise in quality issues etc., would disrupt production schedules and adversely affect the shop performance. Improvements are certainly possible by integrating WLC policies in distinct stages of production that in turn help to keep a steady workflow and balanced shop floor activities. In this study, we have considered the production of a part of a windmill that poses difficulties in production due to changes in processing times. A production shop simulation model was developed by considering real-time data. The model is simulated to analyze the performance under different order release methods at process time changes. In addition, we consider the influence of downtime and capacity cushion at bottleneck station. The objective of this research is to investigate the influence of the processing time variation, downtime, and capacity cushion on the performance of the shop floor and to evaluate the best release method suitable in different situations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

Workload control (WLC) is a well-established production control concept for job shops that put primary emphasis on load-based order release. Recent advances in load-based order release research have led to an improved delivery performance at reduced shop floor workloads. But although order release is the primary focus of WLC research, it must be coupled with priority dispatching on the shop floor if order progress is to be regulated. Prior simulation research suggests that load-based order release methods should only be coupled with simple dispatching rules because other, more powerful rules can conflict with the functioning of the release method. Yet, recent empirical research suggests that powerful priority dispatching rules – such as due date-oriented dispatching rules – are in fact needed for a high level of delivery performance to be obtained in practice. This paper focuses on overcoming the conflict between order release and dispatching, so load-based order release can be combined with due date-oriented dispatching. Preliminary analysis reveals that part of the conflict is because existing due date-oriented dispatching rules overcompensate for schedule deviations that occur when orders are either released earlier or later than planned. Two alternative new dispatching rules based on an improved method of determining operation due dates are then developed to better account for schedule deviations and overcome the conflict with load-based order release. Further improvements in delivery performance are obtained, while the large workload reductions achieved by recently developed load-based order release methods are retained. [ABSTRACT FROM PUBLISHER]

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, norm setting requires careful attention. Disregarding the norms of non-bottleneck work centres, a common sense approach, might strongly deteriorate performance when the level of protective capacity is not sufficiently high. Contrarily, tighter norms for non-bottleneck work centres are shown to perform better in this situation. [ABSTRACT FROM PUBLISHER]

Material flow control mechanisms determine: (i) whether an order should be released onto the shop floor; and (ii) whether a station should be authorized to produce. Well‐known approaches include Kanban, Drum‐Buffer‐Rope (DBR), Constant Work‐in‐Process (ConWIP), Paired‐cell Overlapping Loops of Cards with Authorization (POLCA), Workload Control (WLC), and Control of Balance by Card Based Navigation (COBACABANA). The literature typically treats these approaches as competing, meaning studies argue for the superiority of one over another. However, a closer look reveals that existing mechanisms either focus on order release (ConWIP, DBR, WLC, and COBACABANA) or on production authorization (Kanban and POLCA). This study therefore calls for a paradigm shift and argues that the different mechanisms may play complementary rather than competing roles. Using simulation, we assess the performance of COBACABANA and POLCA in a high‐variety make‐to‐order shop, a type of shop arguably in most need of material flow control given the importance of throughput times and delivery time adherence. Results demonstrate that COBACABANA outperforms POLCA, but the simultaneous adoption of both control mechanisms outperforms the use of either one in isolation. More specifically, adding POLCA production authorization to COBACABANA order release enables the superfluous direct load to be further reduced, resulting in shop floor throughput time reductions of between 15% and 26% while further reducing the percentage tardy and mean tardiness by up to 14%. Compared to no material flow control, the new combined mechanism realizes a reduction of almost 50% in the percentage tardy and more than 30% in mean tardiness. [ABSTRACT FROM AUTHOR]

Most literature on make-to-order shops assumes that service rates are independent of the system state. In practice however, the service rate is often dependent on the workload level experienced by the worker. While a body of knowledge on state-dependent service rates exists, the available literature has not given sufficient attention to make-to-order shops, which are often characterized by complex routings and defined due dates, which means delivery performance becomes a major concern. This study uses simulation to assess the performance impact of state-dependent service rates under different degrees of routing directedness. We show that including information on the load upstream of a station when making service rate adjustments has the potential to improve performance compared to considering the load directly queuing at a station only, as has been the case in previous research on state-dependent service rates. Moreover, using the same threshold to trigger service rate adjustments at each station in shops with directed routings leads to higher service rates at upstream stations. This service rate imbalance can be avoided by using different triggering thresholds for upstream and downstream stations. Further, and most importantly, we show that although speeding up behavior during high load periods significantly improves performance, if worker fatigue leads to a decrease in the service rate in response to the initial increase then performance may in fact deteriorate. [ABSTRACT FROM AUTHOR]

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 "enters" 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

Workload control (WLC) is a production planning and control concept developed to meet the needs of small- and medium-sized make-to-order companies, where a job shop configuration is common. Although simulation has shown WLC can improve job shop performance, field researchers have encountered significant implementation challenges. One of the most notable challenges is the presence of ‘assembly job shops’ where product structures are more complex than typically modelled in simulation and where the final product consists of several sub-assemblies (or work orders) which have to be co-ordinated. WLC theory has not been developed sufficiently to handle such contexts, and the available literature on assembly job shops is limited. In response, this paper extends the applicability of WLC to assembly job shops by determining the best combination of: (i) WLC due date (DD) setting policy, (ii) release method and (iii) policy for coordinating the progress of work orders. When DDs are predominantly set by the company, the DD setting policy should play the leading role while the role of order release should be limited and the progress of work orders should not be co-ordinated in accordance with the DD of the final product. But when DDs are predominantly specified by customers, the importance of order release as a second workload balancing mechanism increases and work orders should be coordinated by backward scheduling from the DD of the final product. Results indicate that WLC can improve performance in assembly job shops and outperform alternative control policies. Future research should implement these findings in practice. [ABSTRACT FROM PUBLISHER]

Workload control (WLC) is a well established production planning and control concept for make-to-order companies. The main insights into WLC are being gained from stationary simulation studies. Knowledge on its functioning in practice is limited, especially in a dynamic setting. The aim of this paper is to identify and classify the key issues when WLC is being used to control logistical performance in a dynamic setting. For this purpose an empirical research project in a company is carried out, in which both quantitative scanning data and qualitative workshop data are obtained and analysed over one year. Throughput diagrams and order progress diagrams support the analysis of the quantitative data obtained. Various types of dynamics could be discerned, but all implying similar requirements in terms of adaptations to the control concept. Whereas research in stationary settings addresses parameter choices, this study shows that it is issues on timing and magnitude of control adjustments that need to be addressed if WLC is to be used effectively in a dynamic setting. The empirical study shows that if the dynamics are not adequately taken into account, buffers may starve while throughput times become uncontrolled over time, despite the use of WLC. [ABSTRACT FROM AUTHOR]

This paper presents a decentralised multi-objective scheduling methodology for semiconductor manufacturing. In this methodology, a new classification method is designed based on utilisations and entropies in order to decentralise global objectives into local ones of work stations. Then, a decentralised multi-objective scheduling policy is proposed to control virtual production lines (VPLs) and machine workload. Results of numerical experiments show that the proposed methodology outperforms common rule-based scheduling policies and a compound scheduling strategy. [ABSTRACT FROM AUTHOR]

Workload control (WLC) is a leading production planning and control (PPC) solution for small to medium sized enterprises (SMEs) and make-to-order (MTO) companies, but when WLC is implemented, practitioners find it difficult to determine suitable workload norms to obtain optimum performance. Theory has provided some solutions (e.g., based on linear programming) but, to remain optimal, these require the regular feedback of detailed information from the shop floor about the status of work-in-process (WIP), and are therefore often impractical. This paper seeks to predict workload norms without such feedback requirements, analysing the influence of shop floor characteristics on the workload norm. The shop parameters considered are flow characteristics (from an undirected pure job shop to a directed general flow shop), and the number of possible work centres in the routing of a job (i.e., the routing length). Using simulation and optimisation software, the workload norm resulting in optimum performance is determined for each work centre for two aggregate load-oriented WLC approaches: the classical and corrected load methods. Results suggest that the performance of the classical approach is heavily affected by shop floor characteristics but no direct relationship between the characteristics and norm to apply could be established. In contrast, results suggest that the performance of the corrected load approach is not influenced by shop floor characteristics and the workload norm which results in optimum performance is the same for all experiments. Given the changing nature of MTO production and the difficulties encountered with the classical approach, the corrected load approach is considered a better and more robust option for implementation in practice. Future simulations should investigate the influence of differing capacities across work centres on the workload norm while action research should be conducted to apply the findings in practice. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]