A handshake operation can mitigate workload imbalance and interference between twin transporters ... more A handshake operation can mitigate workload imbalance and interference between twin transporters in a material handling system. Terminal operators in a rail-based automated container terminal can employ the handshake operation to twin overhead shuttle cranes (OSs) under maritime demand uncertainty. Since a handshake location is critical to collaboration performance, terminal operators often rely on simulation experiments with a manual iterative design to determine optimal handshake locations. However, the simulation optimization is still challenging when a simulation execution is computationally expensive. This study proposes a Bayesian optimization-based approach to expedite the decision-making process. The approach infers the conditional outcomes of a simulation and actively searches optimal handshake locations. Our optimization results show that the proposed approach maximizes the collaborations between the twin OSs within fewer simulation runs. This study also provides extensive simulation analysis of the handshake locations. The experiments indicate that a handshake location has a significant influence on the required space for handshake operations and the workloads of the twin OSs.
Journal of Society of Korea Industrial and Systems Engineering
An automated material handling system (AMHS) has been emerging as an important factor in the semi... more An automated material handling system (AMHS) has been emerging as an important factor in the semiconductor wafer manufacturing industry. In general, an automated guided vehicle (AGV) in the Fab's AMHS travels hundreds of miles on guided paths to transport a lot through hundreds of operations. The AMHS aims to transfer wafers while ensuring a short delivery time and high operational reliability. Many linear and analytic approaches have evaluated and improved the performance of the AMHS under a deterministic environment. However, the analytic approaches cannot consider a non-linear, non-convex, and black-box performance measurement of the AMHS owing to the AMHS's complexity and uncertainty. Unexpected vehicle congestion increases the delivery time and deteriorates the Fab's production efficiency. In this study, we propose a Q-Learning based dynamic routing algorithm considering vehicle congestion to reduce the delivery time. The proposed algorithm captures time-variant vehicle traffic and decreases vehicle congestion. Through simulation experiments, we confirm that the proposed algorithm finds an efficient path for the vehicles compared to benchmark algorithms with a reduced mean and decreased standard deviation of the delivery time in the Fab's AMHS.
Cooperation between pickers occurs when many picking assignments are needed to fill large orders.... more Cooperation between pickers occurs when many picking assignments are needed to fill large orders. This study discusses two strategies for use in zone picking systems with pick-to-light (PTL) technology: along-a-line collaboration, i.e., collaboration along a zone line, and across-a-line asynchronization, i.e., relaxation of synchronization across a conveyor line. Queueing models, Continuous-timed Markov chains (CTMC), and simulations were used to determine if the strategies could improve throughput and cycle time. In simulations with industry cases and data, the along-a-line collaboration showed a 26.7∼53.2% reduction, the across-a-line asynchronization showed an 8.2∼21.0% reduction, and combining the strategies showed a 50.9∼66.3% (on average 57.4%) reduction in cycle times compared to regular zone pickings without collaboration. An analytical comparison confirmed that using either strategy, or combining them, for zone picking systems with PTL technology can improve throughput and reduce work-in-process, particularly for large orders. INDEX TERMS Order picking, zone picking, pick-to-light Technology, queueing model, collaboration.
A commitment of delivery time is critical in some online businesses (De Koster, 2003). An importa... more A commitment of delivery time is critical in some online businesses (De Koster, 2003). An important challenge to meeting customers’ needs is timely order picking which is also relevant to worker safety, item freshness, overall operational synchronization, and reduced overtime. We analyze an order batch picking situation where a trip is constrained by vehicle capacity and must be completed within a specified time. We develop a model which partitions orders to batches to minimize the total travel time such that each trip meets the orders’ time constraints and capacity limit, and also determines a suitable operational policy for each batch. Each policy is characterized by routing method, travel speed, capacity, and pick time. The proposed batching model can simultaneously group orders and can select a best policy among possible policy choices
Industrial Engineering and Management Systems, 2015
This paper analyses the best batch formations for order picking throughput in narrow-aisle order ... more This paper analyses the best batch formations for order picking throughput in narrow-aisle order picking systems. Our analytical comparison finds that a high pick density variation leads to a heavy picker blocking. Simulation experiments show that a distance-based batching algorithm reduces picker blocking by decreasing the number of aisles visited and stabilizing the variation in number of picks per aisle by packing orders tightly, and that the solution quality and mechanism for determining the batch size dictated by the sorting strategy causes varying amounts of blocking. We conclude that combining a distance-based batching method with an appropriate batch sizing strategy will reduce picker blocking and shorten travel in narrow-aisle picking systems.
Pluripotency, the ability of embryonic stem cells to differentiate into specialized cell types, i... more Pluripotency, the ability of embryonic stem cells to differentiate into specialized cell types, is determined by ESC-specific gene regulators such as transcription factors and chromatin modification factors. It is not well understood how ESCs are poised for differentiation, however, and methods are needed for prognosis of the molecular changes in the differentiation of ESCs into specific organs. We describe a new approach to infer cell-type specific gene regulatory programs based on gene regulatory interactions in ESCs. Our method infers the molecular logic of gene regulatory mechanisms by mapping the position-specific combinatory patterns of numerous regulators in ESCs into cell-type specific gene regulations. We validate the proposed approach by recapitulating the RNA-seq and microarray data of neuronal progenitor cells, adult liver cells, and ESCs from the integrated patterns of diverse gene regulators in ESCs. We find that the collective functions of diverse gene regulators in ESCs represent distinct gene regulatory programs in specialized cell types. Our new approach expands our understanding of the differential gene regulatory information in developments encoded in regulatory networks of ESCs.
Understanding gene regulatory information in DNA remains a significant challenge in biomedical re... more Understanding gene regulatory information in DNA remains a significant challenge in biomedical research. This study presents a computational approach to infer gene regulatory programs from primary DNA sequences. Using DNA around transcription start sites as attributes, our model predicts gene regulation in the gene. We find that H3K27ac around TSS is an informative descriptor of the transcription program in mouse embryonic stem cells. We build a computational model inferring the cell-type-specific H3K27ac signatures in the DNA around TSS. A comparison of embryonic stem cell and liver cell-specific H3K27ac signatures in DNA shows that the H3K27ac signatures in DNA around TSS efficiently distinguish the cell-type specific H3K27ac peaks and the gene regulation. The arrangement of the H3K27ac signatures inferred from the DNA represents the transcription regulation of the gene in mESC. We show that the DNA around transcription start sites is associated with the gene regulatory program by...
Bucket brigade order picking improves operational productivity by balancing workloads among picke... more Bucket brigade order picking improves operational productivity by balancing workloads among pickers with a minimal level of managerial planning and oversight. However, due to variability and uncertainty of the pick locations within a particular order or batch, pickers can encounter blocking delays and thus lose productivity. This study formulates a model to quantify blocking delays and develops a control model to reduce blocking in bucket brigade order picking systems. The Indexed Batching Model for Bucket brigades (IBMB) has indexed batching constraints for generating batch alternatives, bucket brigade picker blocking constraints for quantifying blocking delay, and release-time updating constraints for progressively connecting the batching results with blocking quantification. The IBMB minimizes total retrieval time and improves picker utilization from 2 to 9 % across diverse and practical order picking situations while maintaining the static Work-In-Process. We note that modeling the separation of retrieved batches into orders still remains a challenge.
International Journal of Production Economics, 2015
Bucket brigade is a linear order-picking process with one loading station and one unloading stati... more Bucket brigade is a linear order-picking process with one loading station and one unloading station. Here we model and quantify picker blocking in bucket brigade order picking systems (OPSs). A bucket brigade improves throughput and reduces variability in OPSs. However, each order picking trip fills different orders and creates workload variation per order. We show that bucket brigade order picking experiences picker blocking when there is a workload imbalance per pick face. We derive a closed-form solution to quantify the level of blocking for two extreme walk speed cases. Additional simulation comparisons validate the picker blocking model which includes backward walk and hand-off delays. We identify the relationship between picker blocking in bucket brigade OPSs and picker blocking in a circular-aisle abstraction of OPSs in which backward walk and hand-off delays as well as forward walk speed are considered. Our analytical model and simulations show that aggregating orders into batches smoothes the workload variation by pooling the randomness of picks in each order and that slowest-to-fastest picker sequencing modulates picker blocking between two pickers, i.e., the interaction between neighboring pickers.
Order picking operations play a critical role in the order fulfillment process of distribution ce... more Order picking operations play a critical role in the order fulfillment process of distribution centers (DCs). Picking a batch of orders is often favored when customers' demands create a large number of small orders, since the traditional single-order picking process results in low utilization of order pickers and significant operational costs. Specifically, batch picking improves order picking performance by consolidating multiple orders in a-batch‖ to reduce the number of trips and total travel distance required to retrieve the items. As more pickers are added to meet increased demand, order picking performance is likely to decline due to significant picker blocking. However, in batch picking, the process of assigning orders to particular batches allows additional flexibility to reduce picker blocking. This dissertation aims to identify, analyze, and control, or mitigate, picker blocking while batch picking in picker-to-part systems. We first develop a large-scale proximity-batching procedure that can enhance the solution quality of traditional batching models to near-optimality as measured by travel distance. Through simulation studies, picker blocking is quantified. The results illustrate: a) a complex relationship iv between picker blocking and batch formation; and b) a significant productivity loss due to picker blocking. Based on our analysis, we develop additional analytical and simulation models to investigate the effects of picker blocking in batch picking and to identify the picking, batching, and sorting strategies that reduce congestion. A new batching model (called Indexed order Batching Model (IBM)) is proposed to consider both order proximity and picker blocking to optimize the total order picking time. We also apply the proposed approach to bucket brigade picking systems where hand-off delay as well as picker blocking must be considered. The research offers new insights about picker blocking in batch picking operations, develops batch picking models, and provides complete control procedures for large-scale, dynamic batch picking situations. The twin goals of added flexibility and reduced costs are highlighted throughout the analysis.
This study proposes a job scheduling model and its heuristics for an automated container terminal... more This study proposes a job scheduling model and its heuristics for an automated container terminal with an overhead shuttle crane (OS) to reduce the total tardiness time of flatcars and external trucks by considering the separation of each job into a main job, and a premarshaling or remarshaling job. The OS is busy or idle according to the fluctuations in the processing times of different pieces of equipment. We identify the OS job sequencing problem considering job separation (OSJSPS) as a mixed-integer programming (MIP) model, which simultaneously sequences a set of jobs and searches for their possible separation into premarshaling and remarshaling jobs. We present a two-stage genetic algorithm (TGA) based on two local improvement procedures: an iterative local search procedure and an opportunistic job separation procedure. We conclude that the two-stage genetic algorithm reduces the total tardiness time of the container terminal's flatcars and external trucks as the number of OS jobs increases. INDEX TERMS Automated container terminal, job sequencing, job separation, overhead shuttle crane.
This paper studies an order assorting system (OAS) composed of multiple-parallel order assorting ... more This paper studies an order assorting system (OAS) composed of multiple-parallel order assorting aisles (OAAs) in a retailer's order fulfillment center. We evaluate two worker-assignment policies for a worker-to-part OAA: a dedicated-assignment policy and a shared-assignment policy. Our analytical models evaluate the mean value and variance of worker process time including assorting time, walk time, empty walk time, and blocking time when single-line products arrive randomly. From the worker process time, we identify the system flow time of product lines per each worker-assignment policy using G/G/1 and G/G/2 queuing models. We find that the shared-assignment policy performs better compared to the dedicated-assignment policy in spite of workers' productivity losses incurred by blocking delays over high sort probability, slow walk speed, and high magnitude and variations of inter-arrival times. At the worker-centric break-even point of worker process time, where worker process time suggests selecting either policy, system flow time always suggests selecting the shared-assignment policy.
Motivation: Histone modifications regulate chromatin structure and gene expression. Although nucl... more Motivation: Histone modifications regulate chromatin structure and gene expression. Although nucleosome formation is known to be affected by primary DNA sequence composition, no sequence signature has been identified for histone modifications. It is known that dense H3K4me3 nucleosome sites are accompanied by a low density of other nucleosomes and are associated with gene activation. This observation suggests a different sequence composition of H3K4me3 from other nucleosomes. Approach: To understand the relationship between genome sequence and chromatin structure, we studied DNA sequences at histone modification sites in various human cell types. We found sequence specificity for H3K4me3, but not for other histone modifications. Using the sequence specificities of H3 and H3K4me3 nucleosomes, we developed a model that computes the probability of H3K4me3 occupation at each base pair from the genome sequence context. Results: A comparison of our predictions with experimental data suggests a high performance of our method, revealing a strong association between H3K4me3 and specific genomic DNA context. The high probability of H3K4me3 occupation occurs at transcription start and termination sites, exon boundaries and binding sites of transcription regulators involved in chromatin modification activities, including histone acetylases and enhancer-and insulator-associated factors. Thus, the human genome sequence contains signatures for chromatin modifications essential for gene regulation and development. Our method may be applied to find new sequence elements functioning by chromatin modulation.
This research presents several heuristics to optimise the location of crossovers in a conveyor-ba... more This research presents several heuristics to optimise the location of crossovers in a conveyor-based automated material handling system (AMHS) for a semiconductor wafer fabrication facility. The objective is to determine the location of crossovers that minimises the total cost of the expected work-in-process on the conveyor and the cost of installing and operating the AMHS with the crossovers. The proposed heuristics are integrated with a queuing-based analytical model incorporating practical hardware considerations of the AMHS, such as turntables and crossovers. To illustrate the proposed heuristics' practical application they are applied to SEMATECH's virtual wafer fabrication facility. Experimental results demonstrate that under a wide variety of operating conditions and cost scenarios the local improvement heuristic is able to identify the optimal solution and outperform other commonly used heuristics for layout design such as genetic algorithms.
Large-scale order batching in parallel-aisle picking systems
This article discusses an order batching formulation and heuristic solution procedure suitable fo... more This article discusses an order batching formulation and heuristic solution procedure suitable for a large-scale order picking situation in parallel-aisle picking systems. Order batching can decrease the total travel distance of pickers not only through reducing the number of trips but also by shortening the length of each trip. In practice, some order picking systems retrieve 500–2000 orders per hour
Analysis of picker blocking in narrow-aisle batch picking
A note on picker blocking models in a parallel-aisle order picking system
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