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- Load-based generic polca: performance assessement using simulationPublication . Fernandes, Nuno O.; Silva, Sílvio do CarmoPOLCA (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. Two variants of POLCA have been proposed in the literature to improve POLCA performance: Load Based POLCA and Generic POLCA. In this paper, we combine these two variants into a single production control system and analyse its performance for different backlog-sequencing rules. The results of a simulation study carried out for a make-to-order flow shop, support the strategy of combining these two POLCA variants and show that capacity-slack backlog sequencing based on corrected aggregate load have the potential for improving performance.
- Simulation optimization: a new job release approach for Industry 4.0Publication . Fernandes, Nuno O.; Thurer, Matthias; Pinho, Tatiana; Torres, Pedro; Silva, Sílvio do CarmoThe rise of Industry 4.0 has highlighted simulation optimisation as a decision-making tool for scheduling complex-manufacturing systems, specifically when resources are expensive and multiple jobs compete for the same resources. In this context, simulation optimisation provides an important mean to predict, evaluate and improve the short-term performance of the manufacturing system. An important scheduling function is controlled job release; jobs (or orders) are not released immediately to the shop floor, as they arrive to the production system, but release is controlled to stabilize work-in-process, reduce manufacturing lead times and meet customer delivery requirements. While there exists a broad literature on job release, reported release procedures typically use simple rules and greedy heuristics to determine which job to select for release. While this is justified by its simplicity, the advent of Industry 4.0 and its advanced scheduling techniques question its adequateness. In this study, an integer linear programming model is used to select jobs to be released to the shop floor. While there are some recent studies that use a similar procedure, these studies assume the release decision for a given set of jobs is optimized in discrete time intervals. In contrast, in this study, we analyse the impact of different triggering intervals. Experimental results for a pure flow shop support our contention that simulation optimisation as a decision-making tool for job release is likely to be too important to be overlooked
- Literature review on autonomous production control methodsPublication . Martins, Luis; Varela, Leonilde; Fernandes, Nuno O.; Silva, Sílvio do Carmo; Machado, JoséProduction environments are becoming more complex and dynamics. This is influenced by external factors related with products’ characteristics and costumers’ requirements and internal factors related with processing times variability, machine failures, setup times, between others. To face this increasing complexity and dynamics, it is crucial to have effective production control methods, considering Interoperability Enablers for Cyber-Physical Systems. However, production control methods most in used today, are focused on centralised decision-making and planning, and considered inadequate to deal with the increasing dynamics of these systems. Autonomous Production Control (APC) may be an adequate alternative to face this complexity, allowing flexible and rapid reaction to possible disturbances that may occur in the production system. However, as APC is the relatively new concept, there are no existing surveys. Therefore, we review and discuss the literature on APC methods to bring more attention to this promising topic of research, highlighting future research directions.
- Material flow control in make-to-stock production systems: an assessment of order generation, order release and production authorization by simulationPublication . Thürer, Matthias; Fernandes, Nuno O.; Lödding, Hermann; Stevenson, MarkMaterial Flow Control (MFC) is a key element of production planning and control. The literature typically categorizes different MFC methods according to how material flow control is realized. This distinction overlooks that MFC decisions can be subdivided into three independent tasks that are executed as orders progress through the system: (i) order generation, (ii) order release, and (iii) production authorization. MFC methods are typically designed for only one of these three tasks, which leaves a large part of the order flow uncontrolled. This study therefore not only provides a new categorization of MFC methods, but also argues for the simultaneous application (or the combining) of three different MFC methods for order generation, order release, and production authorization. To support this argument, the performance effects of an integrated MFC approach are evaluated. Findings show that each individual MFC method impacts different performance metrics, which can be explained by the presence of a hierarchy of workloads, where each workload level constrains the succeeding hierarchical level. That is, each MFC method has a main impact on a different workload. This has important implications for the design of MFC methods and extends recent literature on hierarchical production planning and control systems.
- DDMRP relative priority for production execution: an assessment by simulationPublication . Fernandes, Nuno O.; Thürer, Matthias; Silva, Sílvio do CarmoDemand-Driven Material Requirements Planning (DDMRP) was designed to improve supply chain performance in complex and uncertain environments. Literature on the topic suggests that production replenishment orders should be dispatched for execution based on the buffers’ penetration ratio of the products ordered, which is a measure of protection against stock depletion. However, the actual performance impact of this dispatching rule remains largely unknown as is the impact of different lot transfer policies. A simulation analysis was carried out to compare the performance of the lowest net flow position, the highest buffer penetration ratio, earliest operation due date and first-come first-served rules under synchronized and unsynchronized lot transfer policies. Results of our study show that the choice of dispatching rules is contingent on the setting of top-of-yellow and top-of-green, which determine the re-order quantity, and on the demand mix of products. The earliest operation due date rule shows great potential to outperform the rule typically applied in a DDMRP context specifically for a high demand mix. These findings provide important insights for improving industrial practice and for guiding future research on DDMRP.
- Inspecção visual automática em produtos cerâmicosPublication . Gonçalves, Paulo; Fernandes, Nuno O.; Fernandes, António MarquesA aplicação de um sistema de visão por computador é representada no presente artigo, com o objectivo de automatizar o posto de trabalho de inspecção de produtos cerâmicos, no subsector de pavimentos e revestimentos. O sistema de inspecção apresentado é composto por um sistema de iluminação para garantir luminosidade constante no produto cerâmico a inspeccionar e por um sistema de captura e processamento de imagens a cores. Os algoritmos desenvolvidos para a detecção de defeitos incluem algoritmos de binarização, baseados em análise estatística e morfologia matemática. São apresentados resultados bastante satisfatórios na identificação do defeito grânulo. O sistema realiza ainda o controlo dimensional do mosaico/azulejo, através dos descritores área, largura e comprimento, com uma precisão de décimas de milímetro. O sistema inspecciona de forma automática os defeitos descritos em produtos cerâmicos de padrão uniforme e de uma só cor, tendo sido obtidos resultados bastante satisfatórios para o número de produtos cerâmicos disponibilizados pelas empresas do subsector de pavimentos e revestimentos.
- Lot splitting under load-limiting order release in high-variety shops: an assessment by simulationPublication . Thürer, M.; Fernandes, Nuno O.; Silva, Sílvio do Carmo; Stevenson, M.Lot splitting is an important approach for shops that compete on short delivery times. Similarly, such shops can benefit from load-limiting order release mechanisms that balance workloads and regulate throughput times. Yet few studies have examined the combined effect of lot splitting and load-limiting order release. We use simulation to assess the combined effect of lot splitting and Paired-cell Overlapping Loops of Cards with Authorization (POLCA), an important load-limiting order release mechanism in the context of time-based competition. The experimental design includes different lot sizes, lot transfer policies, and POLCA quanta, i.e. the limit on the size of jobs represented by a single POLCA card. Lot splitting improves performance if lots can proceed independently as this ensures the quick replenishment of queues at downstream stations. However, we find that enforcing the synchronization of all lots that make up a job at every routing step leads to a deterioration in performance. This extends previous research, which appears to have overemphasized the positive effects of lot splitting. Meanwhile, although POLCA cards were originally used to represent lots, we demonstrate that using cards to represent a certain amount of workload can improve percentage tardy performance. This may also have resonance with other card-based solutions, including kanban.
- POLC-A: an assessment of POLCA’s authorization elementPublication . Thürer, Matthias; Fernandes, Nuno O.; Stevenson, Mark; Silva, Cristóvão; Silva, Sílvio do CarmoPOLCA (i.e. Paired-cell Overlapping Loops of Cards with Authorization) is a card-based production control approach developed to support the adoption of Quick Response Manufacturing. POLCA’s control mechanism is unique since it combines a card-based element (the paired cell overlapping loops of cards) with a higher-level Material Requirements Planning system for release authorization. POLCA has been applied in practice and evaluated in research, but the loops of cards element (POLC) has been adopted without the authorization element (A). In response, we use simulation to evaluate the effect of POLCA’s authorization element. We show that this element has a direct detrimental effect on percentage tardy and mean tardiness performance. While the literature argues that the authorization element should be an integral part of POLCA, our results suggest the opposite. This has important implications for research and practice. Instead of using POLCA with its authorization element, it is preferable to combine POLC—the card-based element—with a shop floor dispatching rule.
- On the meaning of ConWIP cards: an assessment by simulationPublication . Thürer, M.; Fernandes, Nuno O.; Ziengs, N.; Stevenson, M.The simplicity of Constant Work-In-Process (ConWIP) makes it one of the most widely adopted card-based production control solutions. Its simplicity, however, also limits the opportunities that are available to improve the concept. There are arguably only two major search directions: (i) to alter the meaning of cards away from controlling jobs; and (ii) to adopt alternative, more sophisticated backlog sequencing rules. In this study, we outline a simple, practical load-based ConWIP system that changes the meaning of cards. Rather than controlling the number of jobs, cards are associated with a certain amount of workload. Simulation results demonstrate the positive performance impact of limiting the total shop load. The Workload Control literature advocates the use of a corrected load measure as it better represents the direct load queuing at a station; but this worsens performance when compared to a shop load measure in the context of ConWIP.
- POLCA control in two-stage production systemsPublication . Fernandes, Nuno O.; Thürer, Matthias; Mirzaei, Nima; Ferreira, Luís Pinto; Silva, Francisco J.G.; Silva, Sílvio do CarmoPOLCA (Paired-cell Overlapping Loops of Cards with Authorization) is a decision support system for material flow control under Quick Response Manufacturing. It operates in the context of low-volume, high-mix, and cellular manufacturing. While there is an increasing literature on POLCA performance, current studies usually assume full availability of components (or parts) at assembly stations, neglecting parts manufacturing and feeding. Therefore, this study uses simulation to assess POLCA performance in a two-stage production system, where at the first stage parts are manufactured and at the second, they are assembled into end-products. The study demonstrates that using POLCA to control both production stages, manufacturing and assembly, significantly outperforms the use of POLCA at the assembly stage only, leading to important reductions of the total throughput time of orders and on the percentage of tardy orders. Statistical analysis of our results was conducted using ANOVA.