| Summary: | 碩士 === 國立臺灣大學 === 電機工程學研究所 === 91 === Among the proposed automatic material handling systems (AMHS) solutions, OHT (Overhead Hoist Transport) is a promising technology to fulfill the requirements of transportation automation in intrabays and to realize tool-to-tool delivery in a 300mm fab. Limited by the hardware mechanisms, an intrabay OHT system is usually configured as a circular loop where one or more OHT vehicles run around the loop. OHT dispatching deals with the assignment of an empty OHT to a transport job
There are usually multiple objectives or requirements to OHT dispatching. It is difficult to solve OHT dispatching problems with simple heuristic rules or with linear programming techniques for small-scaled problems. Heuristic rules are efficient in generating a solution and its results are much closer to those from human decision-makers. However, its optimality is not guaranteed and it is unable to fulfill the requirements of multiple objectives. The linear programming approach is more time-consuming and its applicability is limited by the problem sizes.
We propose, in this thesis, a knowledge-based solution methodology to the OHT dispatching problems in an intrabay loop. This methodology starts with the analysis of atomic patterns of transport jobs. The optimal dispatching can be achieved by enumerating all the atomic patterns of two jobs. Based on these patterns, we can extend to the dispatching problems of three or more jobs. Each time when we extend the problem size, solutions to new job patterns are found and incorporated as the atomic patterns. During the derivation of solutions, either mathematic inductions or human judgments are used, based on the previously derived solutions to smaller sizes as well as on the human expertise to the problems. All the derivations of solutions are off-line conducted. During on-line applications, the solutions are generated immediately by matching the given job patterns to the atomic patterns.
We adopt the conceptual graphs for knowledge representation of atomic patterns. Induction logics are developed to derive the new atomic patterns by decomposing the problem into smaller ones to which the existing atomic patterns can be applied. For problems that are new or without atomic patterns for their decomposed problems, further analysis or human problem-solvers are needed. The solutions to them render themselves new atomic patterns in our knowledge- based system.
We implement a knowledge-based OHT dispatching system with PROLOG+CG. The dispatching problems of (2 vehicles, 2 jobs) and (3 vehicles, 3 jobs) pairs are tested to assess the feasibility of the proposed methodology. Numerical results demonstrate that this knowledge-based system generates a feasible solution in seconds.
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