Design and operation problems in a flexible manufacturing system with built in automated storage/retrieval system

Abstract

This thesis deals with design and operation problems of a Direct-Input-Output Manufacturing System(DIOMS) which has a built in automated storage/retrieval system(AS/RS). The storage/retrieval(S/R) machine not only stores and retrieves pallets in the rack openings but also loads and unloads pallets for the workstations. The workstations are placed on either one side of AS/RS or both sides of the AS/RS. The first part of this thesis investigates the workstation layout problem which has somewhat different problem characteristics compared to ordinary facility layout problem. In DIOMS, machine sizes are not necessarily equal, each machine may have clearance requirements on its side and candidate machine locations are finite and discrete. Considering these characteristics, a mathematical formulation is developed whose optimal solution is found by the Branch and Bound technique. Two heuristic algorithms are also proposed, one based on simulated annealing method and the other on pairwise interchange method. The validity of these heuristics is illustrated with example problems. Determination of a proper size of rack opening becomes the second subject of this study. Based on M/G/1 queueing theory, the average flow time of the system is expressed as a function of unit load size. Then a procedure to find an optimal solution is presented. With a simulation study, the validity of the objective function is shown. The third part is concerned with the dwell point strategies of S/R machine, when it becomes idle. For the unit-load AS/RS, two existing dwell point models are reformulated to show that one model can be transformed into a single facility location problem through two consecutive linear transformations whose solution is well known. Also, by utilizing a linear transformation and a minimax property, the other model is reformulated into such a form whose solution can be easily found. Through computational tests, the validity of the algorithms are illustrated in terms of...