Expected waiting times in single-device trip-based material handling systems

In an earlier paper we presented an approximate analytical model to estimate the expected device utilization and the expected station cycle times (i.e. the average time between two successive arrivals of a device at each station) in a manufacturing system served by trip-based handling devices. Assuming that empty devices are dispatched according to the Modified First-Come-First-Served rule, the above model provides the expected device utilization, which the analyst can use to determine whether a proposed trip-based handling system is ‘stable’. In this paper we present an approximate analytical model to estimate the expected waiting times for move requests that occur in single-device trip-based handling systems such as cranes, vertical reciprocating conveyors, microload AS/RS, unit load Tandem AGVs, etc. The model represents a conceptual contribution, and it enhances the original model from a practitioner's viewpoint since expected waiting times (and the associated mean queue lengths) can play an important role in deciding whether the performance of a ‘stable’ system is actually ‘acceptable’.