• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 7
  • 1
  • Tagged with
  • 189
  • 45
  • 36
  • 35
  • 32
  • 29
  • 24
  • 24
  • 13
  • 13
  • 13
  • 11
  • 9
  • 8
  • 7
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Genetic algorithms in timetabling and scheduling

Fang, Hsiao-Lan January 1995 (has links)
This thesis investigates the use of genetic algorithms (GAs) for solving a range of timetabling and scheduling problems. Such problems are very hard in general, and GAs offer a useful and successful alternative to existing techniques. A framework is presented for GAs to solve modular timetabling problems in educational institutions. The approach involves three components: declaring problem-specific constraints, constructing a problem-specific evaluation function and using a problem-independent GA to attempt to solve the problem. Successful results are demonstrated and a general analysis of the reliability and robustness of the approach is conducted. The basic approach can readily handle a wide variety of general timetabling problem constraints, and is therefore likely to be of great practical usefulness (indeed, an earlier version is already in use). The approach relies for its success on the use of specially designed mutation operators which greatly improve upon the performance of a GA with standard operators. A framework for GAs in job-shop and open-shop scheduling is also presented. One of the key aspects of this approach is the use of specially designed representations for such scheduling problems. The representations implicitly encode a schedule by encoding instructions for a schedule builder. The general robustness of this approach is demonstrated with respect to experiments on a range of widely-used benchmark problems involving many different schedule quality criteria. When compared against a variety of common heuristic search approaches, the GA approach is clearly the most successful method overall. An extension to the representation, in which choices of heuristic for the schedule builder are also incorporated in the chromosome, is found to lead to new best results on the makespan for some well known benchmark open-shop scheduling problems. The general approach is also shown to be readily extendable to rescheduling and dynamic scheduling.
42

Tabu search for bus and train driver scheduling with time windows

Shen, Yindong January 2001 (has links)
The bus and train driver scheduling problem involves assigning bus or train work to drivers in such a way that all the bus or train work is covered and the number of drivers and duty costs are minimised. This is complicated by the fact that there are many restrictions on the duty generation. The generate-and-select approach is at present the most successful for bus and train driver scheduling. It involves generating a set of legal potential driver duties from which a minimal and most efficient subset is selected. Filtering rules are often applied so that the set of potential duties generated would not be prohibitively large. Moreover, windows of relief opportunities (WROs), which provide ranges of opportunities for relieving drivers, are beyond the capability of being handled by the existing systems. The usual practice is to consider one, sometimes two, discrete times within each time window. Optimality of solution is therefore compromised. The research presented in this thesis focuses on solving the driver scheduling problem with WROs using a constructive approach, which builds and refines a single schedule iteratively. Filtering rules are unnecessary under the approach. The 2-opt heuristic approach is first investigated, during which the potential of constructive heuristics is explored. Based on the experience, the Tabu Search meta-heuristic approach is then investigated. Multi-neighbourhoods and an appropriate memory scheme, which are essential elements of Tabu Search are designed and tailored for the driver scheduling problem with WROs. Alternative designs have been tested and compared with best known solutions drawn from real-life data sets. The tabu search approach is very fast, can handle WROs, and has achieved results comparable to those based on mathematical programming approaches. Taking advantage of WROs, it can improve best known solutions obtained by the existing systems. Consequently, it could be incorporated into existing systems to improve the solution by taking advantage of WROs.
43

Column generation approaches to bus driver scheduling

Fores, Sarah January 1996 (has links)
The bus driver scheduling problem involves assigning bus work to drivers in such a way that all the bus work is covered and the number of drivers and duty costs is minimised. This is complicated by the fact that there are restrictions on the formation of valid duties. A review of computerised scheduling systems is presented, along with a more detailed description of one such system which uses a set covering model to produce a schedule from a set of previously generated valid duties. This method first solves the Linear Programming relaxation, and then uses Branch and Bound techniques to search for a good integer solution. Improvements to this system are detailed. Most systems which use mathematical programming methods to solve the driver scheduling problem need heuristics to reduce the size of the problem since there are potentially many thousands of valid duties, even for small problems. Column generation is a technique which implicitly considers a much larger number of duties, whilst retaining a much smaller working duty subset. A specialised column generation method is implemented within the existing set covering system, and the results of tests on seven problems presented. Each problem instance is solved with two sizes of duty set, and timings compared to those tested on the set covering system. Results show an average reduction in execution time of 41% using column generation, and the larger data sets yield better schedules in terms of the number of duties and the overall cost.
44

Constraint satisfaction approaches to bus driver scheduling

Curtis, Suniel David January 2000 (has links)
The bus driver scheduling problem consists of assigning bus work to drivers so that all the bus work is covered and a combination of the number of drivers and associated costs is minimised. Restrictions imposed by logistic, legal and union agreements complicate the problem. Successful present day systems for computerised driver scheduling often use mathematical programming combined with heuristics. Purely heuristic approaches have found it very difficult to produce efficient driver schedules for large scheduling problems. Furthermore, some of these approaches may not be easily adaptable to different conditions. This thesis presents two new ways of using constraint satisfaction to form driver schedules. The two methods differ in their approach, one being a systematic constraint programming approach and the other being an adaptation of a local search method called GENET. The constraint programming approach uses a similar approach to mathematical programming systems in selecting the schedule from a large number of possible shifts, to allow adaptation to different regulations. In particular, a set partitioning formulation is used. It then makes use of the structure of the problem and the relaxed linear programming solution to the problem in producing a schedule. The GENET system has been adapted to cope with minimising the numbers of drivers in a schedule and with the memory problems caused by the huge number of constraints involved in the set partitioning model. The constraint programming approach has been shown to solve successfully several small scheduling problems from different companies using varying regulations. Local search procedures have hitherto not had great success on driver scheduling problems. GENET has been adapted to solve some of the small schedules from its initial state where it could not solve any. Features of the adaptation may be of interest to researchers using GENET on similar problems.
45

Supporting decision analysis : a pragmatic approach

Simpson, Lisa January 1998 (has links)
Decision making is a practical task. Clearly, all aspects of any approach to decision analysis and decision support should be considered with respect to their ease of application as well as their value. The work presented in this thesis is motivated by such a viewpoint. The field of decision analysis is broad, and this is reflected by the consideration of four linked aspects. Cross-sections have been taken through the research literature in an attempt to consider the most important aspects of decision analysis and decision support. These research findings are examined with respect to how things might function in practice. Specifically the aspects which I consider are: a comparison of underlying mathematical theories; the elicitation and application of preference data; facilitation as a group decision support tool; and the development of hypothetical scenarios. The purpose of decision analysis and decision support is to improve problem solving. With a pragmatic approach in mind, two normative models are compared on the basis of the assumptions they make about a decision maker. Further, how these alternative techniques have worked in practice is discussed. In order to make use of such methods, particular data are required. Perhaps a major criticism of decision analysis concerns its use of subjective preference data. Therefore, an investigation of the ease with which these data can be elicited is conducted. The nature of the data is considered via an application. Having concentrated upon decision analysis, i.e. the examination of a formulated problem, I take a broader view of the field by considering decision support. Supporting decision making requires the setting of aims and objectives in addition to establishing a problem model from a problem mess. Supporting a group adds complexity to the analysis's role. Group decision making and group decision support are examined. Particular attention is given to the technique of group facilitation and some pertinent issues for successful decision support are established. In order to strengthen these findings a further study of group decision making is made. Case study work provides a more realistic view of supporting an actual group in a live setting. In addition, I am able to describe the development and use of hypothetical scenarios to promote decision analysis and decision support. Decision analysis and decision support is no different from any other technology in that it is not a 'quick fix'. Users are faced with a learning curve as they are required to approach their problem in a novel way. From an analysis's perspective, the needs of each decision maker may be different, so any technique must be flexible. This thesis demonstrates the ability of both decision makers and analysts to rise to such challenges, resulting in successful applications of decision analysis and support. It also reinforces the value of employing these techniques. Further, I identify aspects which can make this undertaking easier.
46

Risk assessment models for resource failure in grid computing

Alsoghayer, Raid Abdullah January 2011 (has links)
Service Level Agreements (SLAs) are introduced in order to overcome the limitations associated with the best-effort approach in Grid computing, and to accordingly make Grid computing more attractive for commercial uses. However, commercial Grid providers are not keen to adopt SLAs since there is a risk of SLA violation as a result of resource failure, which will result in a penalty fee; therefore, the need to model the resources risk of failure is critical to Grid resource providers. Essentially, moving from the best-effort approach for accepting SLAs to a risk-aware approach assists the Grid resource provider to provide a high-level Quality of Service (QoS). Moreover, risk is an important factor in establishing the resource price and penalty fee in the case of resource failure. In light of this, we propose a mathematical model to predict the risk of failure of a Grid resource using a discrete-time analytical model driven by reliability functions fitted to observed data. The model relies on the resource historical information so as to predict the probability of the resource failure (risk of failure) for a given time interval. The model was evaluated by comparing the predicted risk of failure with the observed risk of failure using availability data gathered from Grids resources. The risk of failure is an important property of a Grid resource, especially when scheduling jobs optimally in relation to resources so as to achieve a business objective. However, in Grid computing, user-centric scheduling algorithms ignore the risk factor and mostly address the minimisation of the cost of the resource allocation, or the overall deadline by which the job must be executed completely. Therefore, we propose a novel user-centric scheduling algorithm for scheduling Bag of Tasks (BoT) applications. The algorithm, which aims to meet user requirements, takes into account the risk of failure, the cost of resources and the job deadline. With this in mind, through simulation, we demonstrate that the algorithm provides a near-optimal solution for minimizing the cost of executing BoT jobs. Also, we show that the execution time of the proposed algorithm is very low, and is therefore suitable for solving scheduling problems in real-time. Risk assessment benefits the resource provider by providing methods to either support accepting or rejecting an SLA. Moreover, it will enable the resource provider to understand the capacity of the infrastructure and to thereby plan future investment. Scheduling algorithms will benefit the resource provider by providing methods to meet user requirements and the better utilisation of resources. The ability to adopt a risk assessment method and user-centric algorithms makes the exploitation of Grid systems more realistic.
47

Modelling contracts and workflows for verification and enactment

Farrell, Andrew D. H. January 2008 (has links)
The work presented in this thesis concerns some aspects related to the Modelling of Contracts and Workflows for Verification and Enactment. We have sought to gain some insight into the nature of contracts and workflows. in order that we may model them. primarily, for the purposes of verifying certain properties and for enacting them. Workflows help coordinate the enactment of business processes. A notable aspect of workflow technologies is the lack of formal semantics for workflow models. In this thesis, we consider the characterisation of workflow using a number of formal tools, viz. Milner's CCS, Cleaveland et ai's Prioritised CCS (which we abbreviate to PCCS) and the Situation Calculus (thanks mainly to Reiter), which is based on First-Order Logic. Using these, we provide formalisations of production workflows, which are somewhat rigid, inflexible structures, akin to production lines. We do so, in order that we may fiJo: their operational meaning for the purposes of verification and enactment. We define the Liesbet meta-model for production workflow to provide a reference ontology for the task of formalisation. We have also implemented a framework for the verification and enactment of Liesbet workflow models. Regarding verification, we are particularly interested in the key property of soundness, which is concerned with an absence of locking and redundant tasks in a workflow model. Our framework is capable of verifying this property of workflow models, as well as arbitrary temporally-extended constraints', which are constraints whose satisfaction is determined over successive states of enactment of a model. We also consider the definition of more flexible workflows, including collaborative workflows, using an approach that we have conceived called Institutional Workflow Modelling (IWM). The essence of IWM lies (in part) in the identification that the structure of a workflow model necessarily entails the existence of counts as relations. These relations prescribe how the occurrence of certain actions, in the context of a particular workflow model. count as the occurrence of other actions. We have also been interested in the modelling of contracts; and have found IWM to be useful as a foundational basis for contract modelling. Another fundamental aspect of our IWM-based approach is a correspondence, which we have identified, between counts as relations and methods in Hierarchical Task Network (HTN)-based planning. Thus, we are able to advocate the use of an HTN-based planning framework for the verification of flexible workflows and contracts. We have implemented such a framework, whose planner is called Theodore. We define a sjmilar notion of soundness for flexible workflows and contracts, which the Theodore-based framework is able to verify, along with arbitrary temporally extended constraints.
48

Methods and algorithms for integrated multi-scale optimisation of production planning and scheduling

Steffansson, Hlynur January 2007 (has links)
No description available.
49

Risk assessment and the management of environmentally hazardous substances in electrical and electronic equipment

Martin, Alexander Dennis January 2008 (has links)
Producers that distribute and/or sell electrical and electronic equipment in the EU must ensure compliance with a raft of legislation that restricts environmentally hazardous substances in such equipment. Producers must also address the concerns of their stakeholders (e.g. consumers and pressure groups) regarding environmentally hazardous substances in their products. Producers are at risk of losing sales, profitability and market share as well as diminishing the value of their bands and tarnishing their reputations if they do not comply with legislation or address stakeholder expectations. For example, an enforcement official could detect non-compliance with the law and impose a distribution block that stopped a producer from supplying products and subsequently bring an abrupt end to its trading. Ensuring compliance with the law involves identifying and understanding the requirements of relevant legislation and devising and implementing plans to meet these requirements. This is challenging when the legislation restricts substances in individual product parts that, in the case of electronic products, are numerous as well as sourced by producers from hundreds of suppliers. Hence producers must manage their supply chains to manage the risks that they are exposed to. While producers use risk assessment in product design and manufacture, few producers have assessed risks in their supply chains. A tool was developed for use by producers to assess their exposure to such risks. This tool was tested at Sony Computer Entertainment Europe and proved a powerful method for identifying and prioritising key risks in the management of environmentally hazardous substances in products. The tool could be readily adapted and used by other producers that wished to assess the same or similar risks in their supply chains and complements existing management practices (e.g. auditing, product testing).
50

Management and process optimisation : a wireless terrestrial navigation system application

Bravo-Chacon, Jorge January 2010 (has links)
No description available.

Page generated in 0.0949 seconds