Order picking optimisation on a unidirectional cyclical picking line

Thesis (PhD)--Stellenbosch University, 2020.ENGLISH SUMMARY : The order picking system in a company's distribution centre is the biggest contributor to the operational cost within the DC. Optimisation should thus aim at running this activity as effciently as possible. The order picking process consists of three main activities, namely walking to the stock, picking stock in fullment of a customer order and handling the picked stock for further processing. While the total amount of work for the picking and handling activities remain constant, the minimisation of walking distance becomes the main objective when minimising the total picking effort. The minimisation of walking distance can be translated into a reduced overall picking time which can lead to a decrease in the total cost of operating the picking system. The main objective of this dissertation is to optimise the order picking system on a unidirectional cyclical picking line. Order batching is introduced to the picking system, since it is an effective methodology that minimises walking distance in operations research literature. Order batching has been introduced to the standard single block parallel-aisle warehouse layout, but not to the specic layout of a unidirectional cyclical picking line. Additionally, the unidirectional cyclical picking line can offer two conguration options that change the physical set up and thereby inffuence the way in which pickers walk during the order picking process. Order batching is introduced to the unidirectional cyclical picking line through picking location based order-to-route closeness metrics. These metrics are further extended by taking the characteristics of the layout into account. The distribution centre of a prominent South African retailer provides real life test instances. Introducing the layout specic stops non-identical spans metric in combination with the greedy smallest entry heuristic results in a reduction of 48:3% in walking distance. Order batching increases the pick density which may lead to higher levels in picker congestion. In a discrete event simulation, the reduction of the overall picking time through a decrease in walking distance is thus conrmed. On tested sample picking waves, the overall picking time can be reduced by up to 21% per wave. A good number of pickers in the picking system is dependent on the pick density. The pick density, amongst other explanatory variables, can also be used to predict the reduction in picking time. The effects of different structural options of the unidirectional cyclical picking line, namely the U- and Z-conguration, are investigated. This results in four decision tiers that have to be addressed while optimising the order picking system. The rst decision tier assigns stock to picking lines, the second arranges stock around a picking line, the third chooses the conguration and the last sequences the orders to be picked. Order batching is added as an additional layer. An increase in pick density benets the reduction of walking distance throughout the decision tiers and supports the choice of the U-conguration after evaluating different test instances. The total completion time of a picking wave can thus be reduced by up to 28% when compared to benchmark instances. The dissertation is concluded by suggesting further research directions.AFRIKAANSE OPSOMMING : Die opmaak van bestellings op 'n uitsoeklyn in 'n onderneming se distribusiesentrum is die grootste bydraer tot die bedryfskoste van 'n distribusiesentrum. Dit is dus belangrik om hierdie aktiwiteit so doeltreffend moontlik te maak. Die proses om bestellings op te maak bestaan uit drie hoofaktiwiteite, naamlik stap na die voorraad, uitsoek (kies en bymekaarsit) van die voorraad vir 'n bestelling en die pak van die gekose voorraad in kartonne vir verdere verwerking en verspreiding. Omdat die totale hoeveelheid werk vir die uitsoek- en hanteringsaktiwiteite konstant bly, word die vermindering van loopafstand die hoofdoelwit om die totale koste van hierdie proses te minimeer. Die minimering van loopafstand lei tot 'n vermindering in totale tyd om bestellings op te maak, wat op sy beurt weer lei tot 'n afname in die totale koste van die stelsel om bestellings op te maak. Die hoofdoel van hierdie proefskrif is om die stelsel vir die uitsoek van bestellings op 'n eenrigting sikliese uitsoeklyn te optimeer. Metodes vir die samevoeging of groepering (Eng.: batching) van bestellings (om gelyktydig opgemaak te word) word ontwikkel vir hierdie uitsoekstelsel aangesien operasionelenavorsingsliteratuur aantoon dat groepering van bestellings 'n effektiewe metode is om loopafstand te verminder. Groepering van bestellings is reeds gedoen vir die standaard blokuitleg van distribusiesentra, maar nie vir hierdie spesieke uitleg van 'n eenrigting sikliese uitsoeklyn nie. Daarbenewens het die eenrigting sikliese uitsoeklyn twee kongurasie-opsies wat die siese opstelling verander en sodoende die manier beinvloed waarop werkers tydens die uitsoekproses loop. Die groepering van bestellings word ontwikkel vir 'n eenrigting sikliese uitsoeklyn deur middel van 'n plek-gebaseerde maatstaf wat die nabyheid van bestellings se roetes meet. Hierdie maatstaf word verder uitgebrei deur die eienskappe van die uitleg in ag te neem. Regte voorbeelde van die probleem uit 'n distribusiesentrum van 'n prominente Suid-Afrikaanse kleinhandelaar word gebruik vir toetsing. Die ontwikkeling en implementering van 'n uitlegspesieke stop-nie identiese-strek-maatstaf in kombinasie met die gulsige kleinste-invoegingsheuristiek lei tot 'n vermindering van 48:3% in stapafstand. Die groepering van bestellings verhoog die digtheid van plekke waar werkers stop vir voorraad, wat kan lei tot ho er vlakke van kongestie vir werkers. 'n Diskrete-gebeurtenis-simulasie bevestig dat 'n afname in loopafstand ook 'n vermindering van die totale voltooiingstyd tot gevolg het. Met behulp van werklike historiese data kon die totale tyd vir die uitsoek van bestellings met tot 21% per golf verminder word. 'n Goeie aantal werkers in die uitsoekstelsel is afhanklik van die uitsoekdigtheid. Die uitsoekdigtheid en andere verklarende veranderlikes, kan ook gebruik word om die vermindering in totale tyd om bestellings op te maak, te voorspel. Die invloed van verskillende strukturele opsies van die eenrigting sikliese uitsoeklyn, naamlik die U- en Z-kongurasie, word ook ondersoek. Dit het tot gevolg dat vier besluitnemingsvlakke aangespreek moet word om die uitsoekstelsel te optimeer. Die eerste besluitnemingsvlak ken voorraad aan die uitsoeklyne toe, die tweede rangskik voorraad binne die uitsoeklyn, die derde kies die kongurasie van die lyn en die laaste kies die volgorde waarin die bestellings uitgesoek word. Groepering van bestellings word bygevoeg as 'n addisionele vlak. 'n Toename in werksdigtheid bevoordeel die vermindering van loopafstand deur die besluitvlakke en bevoordeel die U-kongurasie na evaluering van verskillende toetsdata. Die totale voltooiingstyd van 'n uitsoekgolf kan dus verminder word met tot 28% in vergelyking met eweknie voorbeelde. Die studie word afgesluit deur verdere navorsingsmoontlikhede voor te stel.Doctora

Microguards and micromessengers of the genome

The regulation of gene expression is of fundamental importance to maintain organismal function and integrity and requires a multifaceted and highly ordered sequence of events. The cyclic nature of gene expression is known as ‘transcription dynamics’. Disruption or perturbation of these dynamics can result in significant fitness costs arising from genome instability, accelerated ageing and disease. We review recent research that supports the idea that an important new role for small RNAs, particularly microRNAs (miRNAs), is in protecting the genome against short-term transcriptional fluctuations, in a process we term ‘microguarding’. An additional emerging role for miRNAs is as ‘micromessengers’—through alteration of gene expression in target cells to which they are trafficked within microvesicles. We describe the scant but emerging evidence that miRNAs can be moved between different cells, individuals and even species, to exert biologically significant responses. With these two new roles, miRNAs have the potential to protect against deleterious gene expression variation from perturbation and to themselves perturb the expression of genes in target cells. These interactions between cells will frequently be subject to conflicts of interest when they occur between unrelated cells that lack a coincidence of fitness interests. Hence, there is the potential for miRNAs to represent both a means to resolve conflicts of interest, as well as instigate them. We conclude by exploring this conflict hypothesis, by describing some of the initial evidence consistent with it and proposing new ideas for future research into this exciting topic

Order picking in parallel-aisle warehouses with multiple blocks::complexity and a graph theory-based heuristic

In this paper, we consider the order picking problem (OPP), which constitutes one of the special cases of the Steiner travelling salesperson problem and addresses the costliest operation in a warehouse. Given a list of items to be picked and their locations in the warehouse layout, the OPP aims to find the shortest route that starts from a depot point, picks all the items in the list, and returns to the depot. This paper fills two important gaps regarding the OPP. First, to the best of our knowledge, we present the first complexity results on the problem. Second, we propose a heuristic approach that makes use of its graph-theoretic properties. Computational experiments on randomly generated instances show that the heuristic not only outperforms its state-of-the-art counterparts in the literature, but it is also robust in terms of changing problem parameters

Establishing a foundation for large DNA transfer to artificial minichromosomes and B insert platforms in maize

In plants, conventional genetic engineering methods limit the number of available traits that could potentially improve the quality of agriculture. Agrobacterium-mediated transformation and biolistic bombardment are tools used in transferring genes into plant cells, both of which result in random integrations into host genomes. The absence of targeting machinery, together with low DNA carrying capacity on most plasmid vectors, limit researchers to a few genes in a single modification experiment, a process that takes [about]1 year in most plant species. While stacking traits from independent genetic modifications allow for an increase in the number of transgenes in a single plant, recovery of all genes in subsequent generations becomes increasingly difficult due to independent segregation in meiosis. Alternatively, the use of binary bacterial artificial chromosomes (BiBACs), large insert cloning vectors, can maintain and transfer up to 300 kps, but are also subject to random integrations. Therefore, establishment of a BiBAC targeting system would be advantageous for researchers focusing on creating plant lines that contain several genes that work together to express complex traits, such as disease resistance clusters or whole biosynthetic pathways. Additionally, BiBAC targeting to a location outside the native chromosomal sets, such as an artificial minichromosome or B chromosome platform, would enable researchers to stack traits without disrupting endogenous sequences.Includes bibliographical reference

Decoding the heterogeneity of skin in homeostasis and regeneration at single-cell resolution

The skin plays a critical role in securing homeostasis in the mammalian body. Its epidermis forms a tight barrier, which separates the internal from the external environment, thereby shielding the body from physical and chemical insult. Due to the exposed position of skin as the outermost organ of the body, skin cells need to be replaced continuously. Cellular maintenance and regeneration of the skin and its associated hair follicles is orchestrated by a variety of stem cell populations. Because of its regenerative properties, the mouse skin is one of the most important model organs in stem cell research and regenerative medicine. The skin is a complex multicellular system composed of a large variety of molecularly and functionally distinct cell populations. The physiology of the skin is a result of the intricate interplay of these diverse cell types. Accordingly, knowledge about the cellular composition of the skin is an essential step in understanding its biology. For a long time, cell populations in the skin were defined based on the expression of individual molecular markers, thus making a comprehensive analysis of cellular heterogeneity impossible. In this thesis, I describe how we used single-cell transcriptomics to create systematic cell type maps of the skin in order to analyze complex molecular processes at single-cell resolution. In the first part of this thesis, I provide an overview of the morphology, function and cellular heterogeneity of the skin. I put particular emphasis on the skin as a self-maintaining tissue and model organ for stem cell research, describing regenerative process such as skin barrier maintenance, cyclical regeneration of hair follicles and cutaneous wound healing in great detail. Then, I introduce single-cell RNA-sequencing as a technique, which has revolutionized the way we analyze and conceptualize cellular heterogeneity in complex tissues. Next, I portray how we championed the application of single-cell transcriptomics in skin biology with three key papers. In Paper I, we used single-cell RNA-sequencing to analyze the mouse epidermis including hair follicles during its resting stage (telogen). We discovered previously unknown cellular heterogeneity in the epidermis and demonstrated that the complexity of this tissue is the result of just two vectors of variation: differentiation stage and spatial position. In Paper II, we analyzed the complete mouse skin, including both epidermal and stromal cells, during hair growth (anagen) and rest (telogen). In addition to describing novel cell types in the stromal part of the skin, we model cellular differentiation and lineage specification in the growing hair follicle at unprecedented resolution. In Paper III, we use single-cell transcriptomics to track molecular changes in different stem cell populations during wound healing and answer several key questions related to stem cell identity and plasticity during regenerative processes. In the last section of this thesis, I demonstrate that our studies have not just allowed us to analyze the cellular heterogeneity of the mouse skin at unprecedented detail, but have also enabled us to address a variety of critical questions such as how stem cell identity is shaped and how regenerative processes are orchestrated in the skin. I thus outline how our endeavors mark the first step towards a systems biology of the skin

Cultural Revolution as Method

This paper treats the Chinese Cultural Revolution as a means by which to open on to a more affective approach to the question of the political. It examines one piece of art-technology of that period and shows the way it intuitively worked within the fluidity of power to produce political intensity. This one technology is a microcosm of the Cultural Revolution notion of the political that was built around an attempt to channel and harness affective power towards revolutionary ends. Both because it attempts to direct the political through the affective dimension and because its methods of doing so resembled contemporary art practices, this paper opens on to the possibilities of a method based on an art rather than a science of the political

Skin cell heterogeneity and dynamics during morphogenesis, tissue homeostasis, and regeneration

Skin is our protective barrier against various environmental harms. For the skin to fulfill its crucial function, it relies on multiple cell types working in concert; but most importantly it relies on skin-resident epithelial stem cells. These cells ensure an intact barrier through constant replacement of the epidermis and they ensure proper hair production through cyclical regeneration of hair follicles. This combination of constant and cyclical renewal within one tissue makes skin a prime model system for the study of adult tissue stem cells. The overall aim of this thesis was to transcriptionally dissect this well-established model system in a systematic and unbiased way. The majority of data presented in this thesis is based on the combination of single-cell RNA sequencing and in situ stainings of mRNA. This combination allows us to appreciate the genome-wide transcriptional heterogeneity while still being able to place the identified cell populations in their spatial tissue context. In Paper I, the first whole-transcriptome study of skin at the single-cell level, we examined the vectors describing cellular heterogeneity within the epidermal compartment of mouse skin during its resting stage (telogen). In Paper II, we expanded on this analysis by including full-thickness skin during rest (telogen) and growth (anagen). This allowed for an unbiased census of all major cell types contained in the skin, and it furthermore enabled us to study how skin achieves and accommodates hair growth. In Paper III, we studied the role of dermal fibroblasts in early embryonic skin development. We uncovered unexpected heterogeneity among embryonic fibroblasts and explored their supportive functions for skin maturation. Moreover, we identified novel keratinocyte subpopulations and closely analyzed epidermal fate decisions. In Paper IV, we monitored transcriptional adaptations of two distinct epidermal stem cell populations during their contribution to wound healing. This allowed us to answer fundamental questions about stem cell plasticity and the dynamics of cell adaptations following injury. In sum, this thesis uncovers the dynamic and heterogeneous nature of mouse skin during adult tissue homeostasis, embryonic development, and tissue regeneration after injury. Most importantly, we provide new insights into how stem cell identity is shaped and how developmental as well as regenerative processes are orchestrated

Operations Research Modeling of Cyclic Train Timetabling, Cyclic Train Platforming, and Bus Routing Problems

Public transportation or mass transit involves the movement of large numbers of people between a given numbers of locations. The services provided by this system can be classified into three groups: (i) short haul: a low-speed service within small areas with high population; (ii) city transit: transporting people within a city; and (iii) long haul: a service with long trips, few stops, and high speed (Khisty and Lall, 2003). It can be also classified based on local and express services. The public transportation planning includes five consecutive steps: (i) the network design and route design; (ii) the setting frequencies or line plan; (iii) the timetabling; (iv) the vehicle scheduling; and (v) the crew scheduling and rostering (Guihaire and Hao, 2008; Schöbel, 2012). The first part of this dissertation considers three problems in passenger railway transportation. It has been observed that the demand for rail travel has grown rapidly over the last decades and it is expected that the growth continues in the future. High quality railway services are needed to accommodate increasing numbers of passengers and goods. This is one of the key factors for economic growth. The high costs of railway infrastructure ask for an increased utilization of the existing infrastructure. Attractive railway services can only be offered with more reliable rolling stock and a more reliable infrastructure. However, to keep a high quality standard of operations, smarter methods of timetable construction are indispensable, since existing methods have major shortcomings. The first part of this dissertation, comprising Chapters 1-6, aims at developing a cyclic (or periodic) timetable for a passenger railway system. Three different scenarios are considered and three mixed integer linear programs, combined with heuristics for calculating upper and lower bounds on the optimal value for each scenario, will be developed. The reason of considering a periodic timetable is that it is easy to remember for passengers. The main inputs are the line plan and travel time between and minimum dwell time at each station. The output of each model is an optimal periodic timetable. We try to optimize the quality of service for the railway system by minimizing the length of cycle by which trains are dispatched from their origin. Hence, we consider the cycle length as the primary objective function. Since minimizing travel time is a key factor in measuring service quality, another criterion--total dwell time of the trains--is considered and added to the objective function. The first problem, presented in Chapter 3, has already been published in a scholarly journal (Heydar et al., 2013). This chapter is an extension of the work of Bergmann (1975) and is the simplest part of this research. In this problem, we consider a single-track unidirectional railway line between two major stations with a number of stations in between. Two train types--express and local--are dispatched from the first station in an alternate fashion. The express train stops at no intermediate station, while the local train should make a stop at every intermediate station for a minimum amount of dwell time. A mixed integer linear program is developed in order to minimize the length of the dispatching cycle and minimize the total dwell time of the local train at all stations combined. Constraints include a minimum dwell time for the local train at each station, a maximum total dwell time for the local train, and headway considerations on the main line an in stations. Hundreds of randomly generated problem instances with up to 70 stations are considered and solved to optimality in a reasonable amount of time. Instances of this problem typically have multiple optimal solutions, so we develop a procedure for finding all optimal solutions of this problem. In the second problem, presented in Chapter 4, we present the literature\u27s first mixed integer linear programming model of a cyclic, combined train timetabling and platforming problem which is an extension of the model presented in Chapter 3 and Heydar et al. (2013). The work on this problem has been submitted to a leading transportation journal (Petering et al., 2012). From another perspective, this work can be seen as investigating the capacity of a single track, unidirectional rail line that adheres to a cyclic timetable. In this problem, a set of intermediate stations lies between an origin and destination with one or more parallel sidings at each station. A total of T train types--each with a given starting and finishing point and a set of known intermediate station stops--are dispatched from their respective starting points in cyclic fashion, with one train of each type dispatched per cycle. A mixed integer linear program is developed in order to schedule the train arrivals and departures at the stations and assign trains to tracks (platforms) in the stations so as to minimize the length of the dispatching cycle and/or minimize the total stopping (dwell) time of all train types at all stations combined. Constraints include a minimum dwell time for each train type in each of the stations in which it stops, a maximum total dwell time for each train type, and headway considerations on the main line and on the tracks in the stations. This problem belongs to the class of NP-hard problems. Hundreds of randomly generated and real-world problem instances with 4-35 intermediate stations and 2-11 train types are considered and solved to optimality in a reasonable amount of time using IBM ILOG CPLEX. Chapter 5 expands upon the work in Chapter 4. Here, we present a mixed integer linear program for cyclic train timetabling and routing on a single track, bi-directional rail line. There are T train types and one train of each type is dispatched per cycle. The decisions include the sequencing of the train types on the main line and the assignment of train types to station platforms. Two conflicting objectives--(1) minimizing cycle length (primary objective) and (2) minimizing total train journey time (secondary objective)--are combined into a single weighted sum objective to generate Pareto optimal solutions. Constraints include a minimum stopping time for each train type in each station, a maximum allowed journey time for each train type, and a minimum headway on the main line and on platforms in stations. The MILP considers five aspects of the railway system: (1) bi-directional train travel between stations, (2) trains moving at different speeds on the main line, (3) trains having the option to stop at stations even if they are not required to, (4) more than one siding or platform at a station, and (5) any number of train types. In order to solve large scale instances, various heuristics and exact methods are employed for computing secondary parameters and for finding lower and upper bounds on the primary objective. These heuristics and exact methods are combined with the math model to allow CPLEX 12.4 to find optimal solutions to large problem instances in a reasonable amount of time. The results show that it is sometimes necessary for (1) a train type to stop at a station where stopping is not required or (2) a train type to travel slower than its normal speed in order to minimize timetable cycle time. In the second part of this dissertation, comprising Chapters 7-9, we study a transit-based evacuation problem which is an extension of bus routing problem. This work has been already submitted to a leading transportation journal (Heydar et al., 2014). This paper presents a mathematical model to plan emergencies in a highly populated urban zone where a certain numbers of pedestrians depend on transit for evacuation. The proposed model features a two-level operational framework. The first level operation guides evacuees through urban streets and crosswalks (referred to as the pedestrian network ) to designated pick-up points (e.g., bus stops), and the second level operation properly dispatches and routes a fleet of buses at different depots to those pick-up points and transports evacuees to their destinations or safe places. In this level, the buses are routed through the so-called vehicular network. An integrated mixed integer linear program that can effectively take into account the interactions between the aforementioned two networks is formulated to find the maximal evacuation efficiency in the two networks. Since the large instances of the proposed model are mathematically difficult to solve to optimality, a two-stage heuristic is developed to solve larger instances of the model. Over one hundred numerical examples and runs solved by the heuristic illustrate the effectiveness of the proposed solution method in handling large-scale real-world instances