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  • 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.
1

Cyclic Scheduling Problems

Kampmeyer, Thomas 13 July 2006 (has links)
For classical non-cyclic scheduling problems, we are given a set of operations,each of which has to be processed exactly once. The aim is to minimize or maximizea given objective function such as makespan or sum of all (weighted) completion times for a given set of constraints. The set of constraints is usually given by precedence constraints between the operations. In contrast to these problems, for cyclic scheduling problems we are given a set of operations, each of which has to be processed infinitely often. Such types of scheduling problems arise in different application areas like compiler design, manufacturing, digital signal processing, railway scheduling, timetabling, etc. The problem is to find a periodic schedule which minimizes a given objective function. There exist two objective functions which are important in this area of cyclic scheduling. The objective which is considered throughout this work is to minimize the time difference between two succeeding occurrences of one operation for a given set of constraints. This time difference is called cycle time. In this thesis, we develop a general framework to model and to describe cyclic scheduling problems with resource constraints. Furthermore, we extend the model to describe blocking constraints for cyclic scheduling problems. In order to solve the problem, we develop a local search approach.
2

Job-shop scheduling with limited buffer capacities

Heitmann, Silvia 18 July 2007 (has links)
In this work, we investigate job-shop problems where limited capacity buffers to store jobs in non-processing periods are present. In such a problem setting, after finishing processing on a machine, a job either directly has to be processed on the following machine or it has to be stored in a prespecified buffer. If the buffer is completely occupied the job may wait on its current machine but blocks this machine for other jobs. Besides a general buffer model,also specific configurations are considered.The key issue to develop fast heuristics for the job-shop problem with buffers is to find a compact representation of solutions. In contrast to the classical job-shop problem,where a solution may be given by the sequences of the jobs on the machines, now also the buffers have to be incorporated in the solution representation. In this work, we propose two solution representations for the job-shop problem with buffers. Furthermore, we investigate whether the given solution representations can be simplified for specific buffer configurations. For the general buffer configuration it is shown that an incorporation of the buffers in the solution representation is necessary, whereas for specific buffer configurations possible simplifications are presented. Based on the given solution representations we develop local search heuristics in the second part of this work. Therefore, the well-known block approach for the classical job-shop problem is generalized to the job-shop problem with specific buffer configurations.
3

Shop-Scheduling Problems with Transportation

Knust, Sigrid 26 September 2000 (has links)
In this thesis scheduling problems with transportation aspects are studied. Classical scheduling models for problems with multiple operations are the so-called shop-scheduling models. In these models jobs consisting of different operations have to be planned on certain machines in such a way that a given objective function is minimized. Each machine may process at most one operation at a time and operations belonging to the same job cannot be processed simultaneously. We generalize these classical shop-scheduling problems by assuming that the jobs additionally have to be transported between the machines. This transportation has to be done by robots which can handle at most one job at a time. Besides transportation times which occur for the jobs during their transport, also empty moving times are considered which arise when a robot moves empty from one machine to another. Two types of problems are distinguished: on the one hand, problems without transportation conflicts (i.e. each transportation can be performed without delay), and on the other hand, problems where transportation conflicts may arise due to a limited capacity of transport robots. In the first part of this thesis several new complexity results are derived for flow-shop problems with a single robot. Since very special cases of these problems are already NP-hard, in the second part of this thesis some techniques are developed for dealing with these hard problems in practice. We concentrate on the job-shop problem with a single robot and the makespan objective. At first we study the subproblem which arises for the robot when some scheduling decisions for the machines have already been made. The resulting single-machine problem can be regarded as a generalization of the traveling salesman problem with time windows where additionally minimal time-lags between certain jobs have to be respected and the makespan has to be minimized. For this single-machine problem we adapt immediate selection techniques used for other scheduling problems and calculate lower bounds based on linear programming and the technique of column generation. On the other hand, to determine upper bounds for the single-machine problem we develop an efficient local search algorithm which finds good solutions in reasonable time. This algorithm is integrated into a local search algorithm for the job-shop problem with a single robot. Finally, the proposed algorithms are tested on different test data and computational results are presented.

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