Dynamic OHT allocation and dispatching in large-scaled 300 mm AMHS management

This paper presents a two-phased approach for effective dynamic overhead hoist transport (OHT) allocation and dispatching in large-scaled, 300 mm automatic material handling system (AMHS) management. The OHT dispatching problem is first explored with the help of simulation models of each OHT loop. The best OHT dispatching policy that efficiently controls OHT transports is then selected among several OHT dispatching rules. For each OHT loop, its required number of OHT vehicles can be calculated from the simulation results on the given service requirements to this loop. However, as the total number of OHT vehicles in the fab is limited, the allocation of OHT vehicles for this loop is determined by considering the total requirements on OHT services from all the other loops. Our objective of OHT allocation and dispatching has two folds: (1) to meet the transport requirements of throughput; and (2) to minimize the carrier delivery times. Numerical results based on realistic data from a local 300 mm mass production fab demonstrate that this two-phased approach performs well both in minimizing the averages and variances of carrier delivery times as well as in achieving the target requirements.