Cristovao Silva

Three decades of workload control research: a systematic review of the literature

The workload control (WLC) concept has received much attention in the past three decades; however, a comprehensive literature review has not been presented. In response, this article provides a systematic review of the conceptual, analytical, empirical and simulation-based WLC literature. It explores the evolution of WLC research, determines the current state-of-the-art and identifies key areas for further study. The research finds that the field has evolved substantially. Early research focused on theoretical development and experimental testing of order release strategies; order release was then integrated with other planning stages, e.g., the customer enquiry stage, making the concept more suitable for customised manufacturing and leading to a comprehensive concept which combines input and output control effectively; recent attention has focused on implementing the resulting concept in practice and refining theory. While WLC is well placed to meet the needs of producers of customised products, future research should include: conducting further action research into how WLC can be effectively implemented in practice; studying human factors that affect WLC; and feeding back empirical findings to simulation-based WLC research to improve the applicability of WLC theory to real-life job shops.

Heuristic lot size scheduling on unrelated parallel machines with applications in the textile industry

In this paper, we present an industrial problem found in a company that produces acrylic fibres to be used by the textile industry. The problem is a particular case of the discrete lot sizing and scheduling problem (DLSP). In this problem, lots of similar products must be generated and sequenced in ten unrelated parallel machines, in order to minimize tool changeovers and the quantity of fibre delivered after the required due date. The company problem is original because a changeover can occur between two lots of the same product due to tool wear. We analyse the problem in detail and present an adaptation of a heuristic found in the literature to solve it. Results obtained with the proposed heuristic are compared with results that used to be obtained by the production planner, using historical data.

Theoretical development of a workload control methodology: evidence from two case studies

Workload control (WLC) is a leading production planning and control (PPC) concept for manufacturing environments subjected to high levels of uncertainty, such as in the make-to-order (MTO) industry. Despite the importance of this concept, few case study applications of WLC have been presented in the literature. This paper takes advantage of a rare opportunity to explore two independent longitudinal empirical WLC projects recently undertaken in Portugal and the United Kingdom. Uniquely, the projects were conducted in parallel and both chose to incorporate the exact same influential WLC methodology in the development of a decision support system (DSS), thus providing an ideal platform for cross-case comparison. The paper focuses primarily on theoretical refinements which ultimately had to be made to the WLC methodology applied to the two cases. Reasons for the refinements can be broadly split into two groups: (1) refinements due to the time that has elapsed since the development of the original methodology; and (2) refinements due to company specific characteristics. The paper also reflects upon a number of implementation difficulties common to both case studies, providing insight into how these could be avoided in the future. Finally, eight future research challenges are presented.

Workload control release mechanisms: from practice back to theory building

Much Workload Control research has focussed on the order release stage but failed to address practical considerations that impact practical application. Order release mechanisms have been developed through simulations that neglect job size variation effects while empirical evidence suggests groups of small/large jobs are often found in practice. When job sizes vary, it is difficult to release all jobs effectively—small jobs favour a short period between releases and a tight workload bounding while large jobs require a longer period between releases and a slacker workload bounding. This paper represents a return from a case study setting to theory building. Through simulation, the impact of job sizes on overall performance is explored using all three aggregate load approaches. Options tested include: using distinct load capacities for small/large jobs and prioritising based on job size or routing length. Results suggest the best solution is assigning priority based on routing length; this improved performance, especially for large jobs, and allowed a short release period to be applied, as favoured by small jobs. These ideas have also been applied to a second practical problem: how to handle rush orders. Again, prioritisation, given to rush orders, leads to the best overall shop performance.

Optimising workload norms: the influence of shop floor characteristics on setting workload norms for the workload control concept

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.

Improving the applicability of workload control (WLC) : the influence of sequence-dependent set-up times on workload controlled job shops

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.

A case study of the successful implementation of workload control : A practitioner-led approach

Purpose – Workload control (WLC) is a Production Planning and Control concept of particular relevance to small and medium sized make-to-order companies. Despite the simplicity of its core principles, few successful implementations have been reported, and both understanding and awareness of the concept amongst practitioners is limited. The authors describe a rare successful implementation of WLC in which elements of the concept were embedded in a company to support both customer enquiry management and order release. The purpose of this paper is to focus on the implementation process itself rather than the impact on performance. Design/methodology/approach – A particularly novel aspect of the case is that the implementation was practitioner (rather than researcher) led. A manager chose to read up on and implement the concept, creating a strong in-house commitment to the initiative. The researchers played a facilitating role, e.g. intervening where necessary to answer questions and advise. A factory visit/to...

The application of workload control in assembly job shops: an assessment by simulation

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.

Towards an Integrated Workload Control (WLC) Concept: The Performance of Due Date Setting Rules in Job Shops with Contingent Orders

Workload control (WLC) is a production planning and control concept developed for make-to-order companies. Its customer enquiry management methodology supports due date setting, while its order release mechanism determines when to start production. For make-to-order companies, due date setting is a strategically important, complex task where unconfirmed jobs place demands on capacity which are contingent on a quotation being accepted by the customer. Yet most prior WLC research has begun at the order release stage with a set of confirmed orders with predetermined due dates. In contrast, this paper focuses specifically on customer enquiry management and uses simulation to compare and contrast the performance of 11 due date setting rules in a job shop where part of the workload consists of unconfirmed or contingent orders. The best results are achieved by a finite loading rule which explicitly considers the workload of contingent orders when estimating lead times. This enables demand to be levelled over time, allowing due dates to be short and reliable – thereby improving both the competitiveness of a make-to-order company and the customer service level it is able to offer. Future research should focus on integrating customer enquiry management, and its due date setting rule, with order release control.

Controlled order release: A performance assessment in job shops with sequence dependent set-up times

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 whether the results hold if set-up times are not distributed equally across job types and work centres.