تجزیه و تحلیل استراتژی کنترل پویا از FMS تحت حالات مختلف

This paper presents a simulation study aimed at evaluating the performances of a flexible manufacturing system (FMS) in terms of makespan, average flow time, average delay time at local buffers and average machine utilization, subject to different control strategies which include routing flexibilities and dispatching rules. The routing strategies under evaluation are ‘no alternative routings’; ‘alternative routings dynamic’; and ‘alternative routings planned’. Above routing strategies are combined with seven dispatching rules, and studied in different production volume which varies from 50 to 500 parts. In addition, impacts of both infinite and finite local buffer capacities are analyzed. Since an FMS usually deals with a variety of products, effects of changing the part mix ratio are also discussed. Finally, machine failure is also introduced in this research to study the effects of machine reliability on the system. Simulation results indicate that the ‘alternative routings planned’ strategy outperforms other routing strategies if the local buffer size is infinity. However, there is no particular dispatching rule that performs well in all buffer size settings but infinity buffer size is not the best choice with respect to the four performance measures. In addition, the four performance measures, except machine utilization, under different control strategies seem quite insensitive to the variation in part mix ratios.

Flexible Manufacturing System (FMS) is an integrated computer controlled system that consists of, but not restricted to, computer numerical controlled (CNC) machine tools, and automated material and tool handling devices. An FMS is capable of simultaneously handling a variety of product types in small to medium-sized batches and at a high rate comparable to that of traditional assembly lines which is designed for high-volume or low-variety manufacturing system. This system can process any products that belong to a certain number of families within its stated capacity according to a predetermined schedule. Usually, the system is designed such that manual intervention and change over time are minimized. Over the past years, increase in customer expectation affected mainly the assembly phrase of production, which assembles mass-produced parts into various end-products. However, customer's expectation now requires suppliers to produce quality individual parts that make up the end-products. In order to stay competitive; it is becoming more imperative to fulfill their expectations by adapting FMSs.

This study shows that the performances of the FMS under study can be improved considerably by using appropriate routing policies and dispatching rules. Several observations can be stated as follows: (I) For the infinite buffer capacity case, ARP routing policy performs better in all seven dispatching rules as compared to the ARD and NAR routing policies. NAR routing policy combined with the seven dispatching rules performs the worst. (II) For the finite buffer capacity case, it is evident that there is no single criterion that can always dominate others. There is no particular dispatching rule that performs well in all buffer sizes capacities. It is important for the scheduler to set their objective for the production such as flow time, machine utilization and delay time at local buffer, etc., before selecting the control rules. In addition, the results showed that ARD routing policy combined with different dispatching rules tends to perform better in small to medium buffer sizes capacities compare to the large buffer sizes capacity. Overall, the buffer size of 8 gives the best results under the seven dispatching rules for most of the performances measures. (III) Changing of part mix ratios had little impact on the results. There is no particular part mix ratio that performed well under all FMS situation. (IV) For the machine failure case, the results showed that the four performance measures are not as good as the reliable cases. It was shown that it is important to consider uncertainty.