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.     

Integrating load-based order release and priority dispatching

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.     

Handling the complexities of real-life job shops when implementing workload control: a decision framework and case study

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.     

Book review

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.     

卡片导航平衡控制系统性能仿真研究

为了减少装配作业车间内因物料齐套产生的等待浪费,使具有装配约束的关联零件加工进度得到有效协同,设计一类卡片导航平衡控制系统 (control of balance by card-based navigation,COBACABANA)。其基于两类卡片循环回路实现任务投放与作业分派的可视化进度协同控制逻辑。本文详细介绍系统的运行机制和系统控制参量,通过构建一般化的装配作业车间仿真模型,探讨在不同装配关联度下各控制参量的性能变化。实验结果表明,COBACABANA系统性能良好,并且选择合适的控制参量就能够有效提升关联零件的进度协同性。     

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.     

具有两道工序的柔性同序加工车间任务投放策略

任务投放上限值是负荷控制理论中任务投放策略的关键量之一。为帮助任务投放决策者确定每个周期任务投放量的上限值,针对具有两道工序的柔性同序加工车间任务投放上限值确定问题,提出了一种在等周期条件下通过仿真求解任务投放量上限值的方法,并且通过仿真计算说明了这种方法的合理性与有效性。     

Performance of an ant colony optimisation algorithm in dynamic job shop scheduling problems

The goal of the current study is to identify appropriate application domains of Ant Colony Optimisation (ACO) in the area of dynamic job shop scheduling problem. The algorithm is tested in a shop floor scenario with three levels of machine utilisations, three different processing time distributions, and three different performance measures for intermediate scheduling problems. The steady-state performances of ACO in terms of mean flow time, mean tardiness, total throughput on different experimental environments are compared with those from dispatching rules including first-in-first-out, shortest processing time, and minimum slack time. Two series of experiments are carried out to identify the best ACO strategy and the best performing dispatching rule. Those two approaches are thereafter compared with different variations of processing times. The experimental results show that ACO outperforms other approaches when the machine utilisation or the variation of processing times is not high.