Our research focuses on the storage decision in a semi‐automated storage system, where the inventory is stored on mobile storage pods. In a typical system, each storage pod carries a mixture of ...items, and the inventory of each item is spread over multiple storage pods. These pods are transported by robotic drives to stationary stations on the boundary of the storage zone where associates conduct pick or stow operations. The storage decision is to decide to which storage location within the storage zone to return a pod upon the completion of a pick or stow operation. The storage decision has a direct impact on the total travel time and hence the workload of the robotic drives. We develop a fluid model to analyze the performance of velocity‐based storage policies. We characterize the maximum possible improvement from applying a velocity‐based storage policy in comparison to the random storage policy. We show that class‐based storage with two or three classes can achieve most of the potential benefits and that these benefits increase with greater variation in the pod velocities. To validate the findings, we build a discrete‐time simulator with real industry data. We observe an 8% to 10% reduction in the travel distance with a 2‐class or 3‐class storage policy, depending on the parameter settings. From a sensitivity analysis, we establish the robustness of the class‐based storage policies as they continue to perform well under a broad range of warehouse settings, including different zoning strategies, resource utilization, and space utilization levels.
How to think about planned lead times Graves, Stephen C.
International journal of production research,
01/2022, Letnik:
60, Številka:
1
Journal Article
Recenzirano
A fundamental construct of most planning systems is the planned lead time. A planned lead time is a control parameter that refers to the planned amount of time for a production, transportation or ...replenishment activity. The intent of this article is to provide a brief, critical examination of planned lead times. We first discuss why planned lead times are used and why they are important. We then discuss the trade-offs to be considered in setting the planned lead times. We follow this with a review of the research literature on how to specify the planned lead times. Finally, we offer a simple model to provide some insight on how the planned lead time for an activity depends on the stochastic variability of the resource requirements for the activity, and on the flexibility and utilisation of the resource associated with the activity.
Remanufactured products that can substitute for new products are generally claimed to save energy. These claims are made from studies that look mainly at the differences in materials production and ...manufacturing. However, when the use phase is included, the situation can change radically. In this Article, 25 case studies for eight different product categories were studied, including: (1) furniture, (2) clothing, (3) computers, (4) electric motors, (5) tires, (6) appliances, (7) engines, and (8) toner cartridges. For most of these products, the use phase energy dominates that for materials production and manufacturing combined. As a result, small changes in use phase efficiency can overwhelm the claimed savings from materials production and manufacturing. These use phase energy changes are primarily due to efficiency improvements in new products, and efficiency degradation in remanufactured products. For those products with no, or an unchanging, use phase energy requirement, remanufacturing can save energy. For the 25 cases, we found that 8 cases clearly saved energy, 6 did not, and 11 were too close to call. In some cases, we could examine how the energy savings potential of remanufacturing has changed over time. Specifically, during times of significant improvements in energy efficiency, remanufacturing would often not save energy. A general design trend seems to be to add power to a previously unpowered product, and then to improve on the energy efficiency of the product over time. These trends tend to undermine the energy savings potential of remanufacturing.
Production systems have to be adapted continuously to changing circumstances. This means that the responsible production planner has to frequently make decisions about reconfiguring complex systems ...under uncertainty whose outcome greatly affects the company's business success. Discrete-event simulation is one powerful tool to support the necessary analysis and scenario evaluation, but still remains time-consuming and tricky to set up and to maintain. When implemented as a digital twin of the production system, the simulation model can maintain a high degree of accuracy over a long time period. An approach to realise this potential is presented and illustrated in this paper with a use case from the automotive industry. This paper contributes several new methods and findings to the development of digital twins of production systems: Firstly, it demonstrates how exceptional events in the validation of the digital twin can be handled. Secondly, it shows how structural changes in the system can be discovered using data on machine activity and process mining. Thirdly, the paper introduces a possibility on how to assess the accuracy of the digital twin. Furthermore, it demonstrates how to assess the robustness of the digital twin to estimation errors in machine processing times.
Stowage decisions in multi-zone storage systems Yuan, Rong; Cezik, Tolga; Graves, Stephen C.
International journal of production research,
01/2018, Letnik:
56, Številka:
1-2
Journal Article
Recenzirano
Odprti dostop
The stowage decision determines how arriving products are distributed in a storage system or warehouse. In particular, we consider the zone-stowage decision for large warehouses that are organised ...into distinct storage zones. An example would be a multi-floor warehouse where each floor is a storage zone. Each storage zone has limited picking capacity; we want to stow the product inventory across the storage zones so as to be able to meet uncertain demand requirements with the limited picking capacity in each zone. Determining how to spread the inventory across the storage zones is the zone-stowage decision that we consider in this paper. With a simulation study, we identify two zone-stowage policies that are effective in balancing the picking workload across different storage zones. The first zone-stowage policy achieves a chaining-inspired allocation by splitting the received quantity for each product across two storage zones; the second zone-stowage policy explicitly tracks the expected workload for each storage zone, termed the velocity of the zone, and then stows arriving products to the storage zone with the smallest velocity.
•Develops a network flow model for evaluation of performance of a multi-material separation system.•Provides an optimization-based method for estimating separation efficiency parameters for each ...sorting unit from measurement data sets.•Validates the model with data from an existing material recovery facility (MRF).•Illustrates how to use model for uncertainty analysis and sensitivity analysis.
In this paper, we model the recycling process for solid waste as performed in a material recovery facility. The intent is to inform the design and evaluation of a material recovery facility (MRF) in order to increase its profit, efficiency and recovery rate. We model the MRF as a multi-stage material separation process and develop a network flow model that evaluates the performance of the MRF through a system of linear equations. We estimate the parameters of the network flow model from historical data to find the best fit. We validate the model using a case-study of a light-packaging recovery section of an MRF in Spain. Additionally, we examine how uncertainty in the input material composition propagates through the system, and conduct a sensitivity analysis on the model parameters.
The ETS gene family is frequently involved in chromosome translocations that cause human cancer, including prostate cancer, leukemia, and sarcoma. However, the mechanisms by which oncogenic ETS ...proteins, which are DNA-binding transcription factors, target genes necessary for tumorigenesis is not well understood. Ewing's sarcoma serves as a paradigm for the entire class of ETS-associated tumors because nearly all cases harbor recurrent chromosomal translocations involving ETS genes. The most common translocation in Ewing's sarcoma encodes the EWS/FLI oncogenic transcription factor. We used whole genome localization (ChIP-chip) to identify target genes that are directly bound by EWS/FLI. Analysis of the promoters of these genes demonstrated a significant over-representation of highly repetitive GGAA-containing elements (microsatellites). In a parallel approach, we found that EWS/FLI uses GGAA microsatellites to regulate the expression of some of its target genes including NR0B1, a gene required for Ewing's sarcoma oncogenesis. The microsatellite in the NR0B1 promoter bound EWS/FLI in vitro and in vivo and was both necessary and sufficient to confer EWS/FLI regulation to a reporter gene. Genome wide computational studies demonstrated that GGAA microsatellites were enriched close to EWS/FLI-up-regulated genes but not down-regulated genes. Mechanistic studies demonstrated that the ability of EWS/FLI to bind DNA and modulate gene expression through these repetitive elements depended on the number of consecutive GGAA motifs. These findings illustrate an unprecedented route to specificity for ETS proteins and use of microsatellites in tumorigenesis.
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