Nuno O. Fernandes

Order release in a workload controlled flow-shop with sequence-dependent set-up times

In this paper, we report a simulation study on the role of sequence-dependent set-up times in decision making at the order release level of a workload controlled make-to-order flow-shop. The study evaluates the potential for set-ups savings, dependent on the level of workload in the shop, for two alternative strategies, namely considering set-up times centrally, within the release decision or locally, within the dispatching decision. These strategies are compared and assessed on the basis of two main performance measures namely time in system and standard deviation of the job lateness. Results indicate that the local strategy, which has been traditionally adopted in practice and in most of the studies dealing with sequence-dependent set-up times, does not always give the best results. The release frequency and the shop workload appear critical to the selection of the strategy to adopt, strongly influencing system performance.

Workload control in unbalanced job shops

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.

On the backlog-sequencing decision for extending the applicability of ConWIP to high-variety contexts : an assessment by simulation

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.

Aligning workload control theory and practice: lot splitting and operation overlapping issues

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.

Introduction to Multiple Helix Ecosystems for Sustainable Competitiveness

This chapter summarizes the evolution of the metaphorical concept of the triple helix, through the quadruple helix and quintuple helix; the second Leydesdorff (J Knowl Econ 3(1):25–35, 2012), a founder of Triple Helix, invites the submission of other model proposals with more than three helices. Based on the literature review on these currents of collaborative interaction for innovation, knowledge and technology transfer, we set out to build a conceptual model that can help explain the improvement of sustainable competitiveness of economies and companies. The model has been designed from the concept of “Multiple Helix Ecosystems for Sustainable Competitiveness”, opening doors to its empirical verification.

Centralised vs. decentralised control decision in card-based control systems: comparing kanban systems and COBACABANA

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.

Load-Based POLCA: An Assessment of the Load Accounting Approach

POLCA (i.e. Paired-cell Overlapping Loops of Cards with Authorization) is a card-based decision support system for production control developed to support the adoption of Quick Response Manufacturing. In POLCA, the flow of jobs through the production system is controlled through a combination of release authorisations and production control cards - POLCA cards. In this paper, we discuss a load-based version of the POLCA system (LB-POLCA), which draws on recent insights from the Workload Control literature. In this context, a question arises: what is the load (e.g. in hours) that a POLCA card should represent? Using simulation, we demonstrate that insights from the Workload Control literature cannot be straight transferred to the POLCA system. Results further demonstrate that significant performance improvements for all card acquisition rules considered in this study can be realized when the POLCA card represents the operation time the job imposes to the second station of the pair.

Generic POLCA: An Assessment of the Pool Sequencing Decision for Job Release

We present a simulation study for assessing the impact of pool sequencing rules for job release in a make-to-order general flow shop under the Generic POLCA order release and materials flow control system. Four pool sequencing rules are tested when the workload released to the shop floor is measured: (1) in jobs; and (2) in processing time units. Performance results based on both, the ability to deliver jobs on time and to provide short delivery times, show that a capacity slack rule based on the corrected aggregate workload perform best.

Iterative Optimization-Based Simulation: A Decision Support Tool for Job Release

Job release is an essential scheduling function and a core part of every production planning and control system. Essentially, job release has to do with the timing and the jobs to release on to the shop floor, in such way that, a balanced and restricted workload is achieved. In this paper, an Iterative Optimization-based Simulation (IOS) decision support tool is proposed for job release. This is in line with Industry 4.0 paradigm, allowing the autonomous selection of jobs based on the current shop floor situation. This decision support tool is implemented using SIMIO as a simulation manager, MATLAB as an optimization manager and MySQL as a database manager.

Improving materials flow through autonomous production control

Abstract Autonomous Production Control (APC) aims at improving production systems performance through fast and flexible reaction to changes in dynamic environments. In this paper, a new APC method for job routing decision-making is proposed and its performance compared with that of two other APC methods, namely the Queue Length Estimator and the Pheromones, and with two conventional control strategies – a centralized and a random decision-making strategy. A discrete-event simulation model of a flexible flow shop operating under make-to-order was used to evaluate performance. Results show that the new method outperforms those with which it was compared, under high system workload and high variability of orders’ arrival and operation times. The study gives a contribution for better understanding of the performance behavior of APC methods, having important implications for industrial practice and for future research on autonomous production control.