1,720,985 research outputs found
Inventory Control for Multi-location Rental Systems
Get PDFVerhuurartikelen vervullen vraag naar producten die duur zijn in aanschaf en vaak alleen nodig zijn voor een beperkte periode. Voorbeelden van dit soort producten zijn auto’s, films, boeken, gereedschap, gelegenheidskleding en juwelen. Complexe voorraadproblemen komen voor bij verhuurbedrijven, vooral wanneer sprake is van meerdere vestigingslocaties. Bedrijven moeten bijvoorbeeld beslissen hoeveel artikelen toe te wijzen aan iedere locatie en hoe om te gaan met aanvragen voor artikelen die op dat moment aan andere klanten zijn verhuurd. Bij dergelijke aanvragen is het gebruikelijk om artikelen te verschepen vanuit andere locaties of vanuit een opslagdepot. Omdat de handelingskosten voor transporteren hoog kunnen zijn, is het belangrijk om de verschepingen goed te coördineren en tevens het proces van herpositioneren van artikelen over de locaties goed te regelen. Dit proefschrift presenteert nieuwe methoden voor voorraadbeheer en het organiseren van logistieke processen in verhuursystemen met meerdere locaties die onderling verhuurartikelen kunnen verschepen in reactie op de vraag. Voor verschillende veelvoorkomende types verhuursystemen optimaliseren we beslissingen voor voorraadniveaus, verschepingen en herpositionering. Aan de hand van wiskundige analyses leiden we theoretische eigenschappen van optimaal beleid af. Aan de hand van numerieke methoden bepalen we dit optimale beleid voor verhuursystemen met een gelimiteerd aantal locaties. Dit geeft belangrijke inzichten in effectieve strategieën voor het verschepen en herpositioneren van voorraad in verhuursystemen met meerdere locaties. Met deze inzichten ontwerpen we heuristieken die eenvoudig te implementeren zijn en die significante verbeteringen geven ten opzichte van beslissingsregels uit de praktijk
On optimal policies for production-inventory systems with compound Poisson demand and setup costs
Get PDFIn this paper, we consider a single-item, one-machine production-inventory system with compound Poisson demand. The production facility may be on or off. While on, the production rate is constant, and, while off, the production rate is zero. System costs consist of switching costs and inventory and backlogging costs. We provide conditions when (s, S)-policies are optimal under the long-run average expected cost criterion. These conditions are met in particular, but not necessarily, when the inventory costs are convex. The developed method in the proof is easy to apply to more general cases. Moreover, the method allows us to compute optimal policies very efficiently
Distributed Control of a Network with Multiple Electricity Producers and Consumers
Get PDFDit proefschrift onderzoekt gedistribueerde besluitvorming in een elektriciteitsnetwerk (Smart Grid) met meerdere electriciteitsproducenten en -consumenten. Het doel is om door middel van informatieuitwisseling de vraag naar en productie van elektriciteit door de producenten en consumenten te balanceren in onderdelen van het netwerk, bijvoorbeeld in een woonwijk. Hierbij nemen we aan dat de producenten en consumenten als actieve agents kunnen opereren en, binnen grenzen, kunnen beslissen wanneer zij energievragende of energieproducerende apparaten aan en uit zetten. Door deze beslissingen goed op elkaar af te stemmen, kan de belasting van, bijvoorbeeld, een bijbehorend transformatiestation verminderen en de energieconsumptie van het electriciteitsnetwerk als geheel afnemen.
Het is nodig om computer algoritmes te ontwikkelen die deze beslissingen coördineren, omdat het gedrag en de toestand van vele (honderden) apparaten deze beslissingen beïnvloeden. Een grote uitdaging binnen een dergelijk grootschalig optimalisatieprobleem is dan het vinden van schaalbare algoritmen.
Dit proefschrift onderzoekt hoe een bestaand gedistribueerd Model Predictive Control (MPC) algoritme aangepast en geïmplementeerd kan worden voor dergelijke Smart Grids. Deze methode lijkt op voorhaand goed schaalbaar, aangezien de beslissingen gedistribueerd worden berekend. Daarnaast maakt deze methode het mogelijk om te anticiperen op toekomstige situaties binnen het netwerk en rekening te houden met technische beperkingen van de agents in het netwerk. We onderzoeken de toepasbaarheid van deze MPC algoritmes voor de control van energieagents.
We concluderen in het algemeen dat onze modelering in combinatie met het gedistribueerde MPC algoritme goed gebruikt kan worden om lokale beslissingen in het Smart Grid te coördineren.
This dissertation investigates distributed decision making in an electricity network (Smart Grid) with multiple prosumers. Prosumers are both consumers and producers. The aim is to reach a global goal in the network based solely on local information. An example of such a goal is to steer the difference between electricity demand and production to a target value. We call the electricity prosumers agents, and these agents decide when to use their electrical demand and production devices. The optimal timing, for when and how much electric power to use or produce, depends on the overall power balance in the network and on the operational constraints from the devices themselves.
Algorithms are required to control the agents' decisions. One major challenge in a large-scale optimization-based control problem is to find a scalable algorithm. Therefore, this dissertation takes a distributed Model Predictive Control (MPC) algorithm and adapts it to the Smart Grid. We see in our simulations that this method scale well because computations are distributed between agents. In addition, the method anticipates future situations in the network and ensures that the operational constraints are not violated.
Controlling the order pool in make-to-order production systems
Get PDFVoor ‘Make-To-Order’ (MTO, oftewel klantordergestuurde) productiesystemen is de tijd die orders moeten wachten op beschikbare productiecapaciteit cruciaal. Het beheersen van die wachttijd is van groot belang om zowel korte als betrouwbare doorlooptijden te realiseren. Daarom analyseerde en ontwierp Remco Germs regels voor orderacceptatie en ordervrijgave, om daarmee de wachttijden te beheersen. Orderacceptatie en -vrijgave zijn de twee belangrijkste mechanismen om de lengte van wachttijden te beïnvloeden en zodoende de productie te sturen. De logistieke prestatie hangt in grote mate af van specifieke kenmerken van MTO-systemen, zoals routing variabiliteit, beperkte productiecapaciteit, omsteltijden, strikte leveringsvoorwaarden en onzekerheid in het aankomstpatroon van orders.
Om een beter begrip te krijgen van de afwegingen die MTO-bedrijven in dit opzicht moeten maken richt het proefschrift zich op de modellering van de belangrijkste kenmerken van MTO-systemen. De inzichten die dat oplevert worden vervolgens gebruikt om orderacceptatie- en ordervrijgaveregels te ontwikkelen die eenvoudig te begrijpen en daarom makkelijk in praktijksituaties te implementeren zijn. Deze relatief eenvoudige beslissingsregels kunnen al leiden tot significante verbeteringen in de logistieke prestaties van MTO-bedrijven.
The thesis of Remco Germs analyses and develops order acceptance and order release policies to control queues in make-to-order (MTO) production systems. Controlling the time orders spend waiting in queues is crucial for realizing short and reliable delivery times, two performance measures which are of strategic importance for many MTO com-panies. Order acceptance and order release are the two most important production con-trol mechanisms to influence the length of these queues. Their performance depends on typical characteristics of MTO systems, such as random (batch) order arrival, routing variability, fixed capacities, setup times and (strict) due-dates.
To better understand the underlying mechanisms of good order acceptance and order release policies the models in this thesis focus on the main characteristics of MTO systems. The insights obtained from these models are then used to develop order acceptance and order release policies that are easy to understand and thereby easy to implement in practice. The results show that these relatively simple policies may already lead to significant performance improvements for MTO companies.
On Proportionally Fair Solutions for the Divorced-Parents Problem
Get PDFWhen Dutch parents divorce, Dutch law dictates that the parental contributions to cover the financial needs ofthe children have to be proportionally fair. This rule is clear when parents only have common children. However,cases can be considerably more complicated, for example when parents have financial responsibilities to childrenfrom previous marriages. We show that, mathematically, this settlement problem can be modelled as a bipartiterationing problem for which a unique global proportionally fair solution exists. Moreover, we develop two efficientalgorithms for obtaining this proportionally fair solution, and we show numerically that both algorithms areconsiderably faster than standard convex optimization techniques. The first algorithm is a novel tailor-madefixed-point iteration algorithm, whereas the second algorithm only iteratively applies simple lawsuits involvinga single child and its parents. The inspiration for this latter algorithm comes from our main convergence proofin which we show that iteratively applying settlements on smaller subnetworks eventually leads to the samesettlement on the network as a whole. This has significant societal importance since in practice lawsuits areoften only held between two or a few parents. Moreover, our iterative algorithm is easy to understand, alsoby parents, legal counselors, and judges, which is crucial for its acceptance in practice. Finally, as the methodprovides a unique solution to any dispute, it removes the legal inequality perceived by parents. Consequently, itmay considerably reduce the workload of courts because parents and lawyers can compute the proportionally fairparental contributions before bringing their case to court
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Admission policies for the customized stochastic lot scheduling problem with strict due-dates
No full textThis papers considers admission control and scheduling of customer orders in a production system that produces different items on a single machine. Customer orders drive the production and belong to product families, and have family dependent due-date, size, and reward. When production changes from one family to another a setup time is incurred. Moreover, if an order cannot be accepted, it is considered lost upon arrival. The problem is to find a policy that accepts/rejects and schedules orders such that long run profit is maximized. This problem finds its motivation in batch industries in which suppliers have to realize high machine utilization while delivery times should be short and reliable and the production environment is subject to long setup times. We model the joint admission control/scheduling problem as a Markov decision process (MDP) to gain insight into the optimal control of the production system and use the MDP to benchmark the performance of a simple heuristic acceptance/scheduling policy. Numerical results show that the heuristic performs very well compared with the optimal policy for a wide range of parameter settings, including product family asymmetries in arrival rate, order size, and order reward.Scheduling Order acceptance Make-to-order production Stochastic lot scheduling problem Due-dates
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
