Browsing by Author "Stevenson, M."
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- Load-Based POLCA: an assessment of the load accounting approachPublication . Fernandes, Nuno O.; Thürer, M.; Stevenson, M.; 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. 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.
- 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.
- 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.