New strategies for device dispatching in trip-based material handling systems.

Abstract

It has been shown in the literature that the manner in which empty devices in a trip-based material handling system are dispatched affects the overall productivity of the handling system. Existing dispatching rules: (1) do not assign a move request until a device deposits a load and becomes empty; (2) with the exception of one rule, do not allow an empty device to be reassigned to another move request; (3) do not consider the status and current assignment(s) of other devices before assigning a move request; (4) do not take advantage of available information such as processing times; (5) may lead to considerable empty travel (also known in industry as "dead heading") which reduces the efficiency of the system. In this dissertation we present new device dispatching rules to address the above shortcomings and take advantage of information that should be readily available in centrally controlled systems. We first show that a cyclic-schedule approach (which is based on "look-ahead" and optimization techniques) may not be acceptable due to an elevated expected work-in-process (WIP) level, although under a cyclic schedule empty travel is considerably reduced compared to existing rules. We then present two new dispatching rules, namely, modified shortest-travel-time-first (MOD STTF) and bidding-based-dynamic-dispatching (B$\sp2$D$\sp2$) which are aimed at minimizing both the expected load waiting times and empty device travel. The latter rule is a novel application of the bidding concept to device dispatching in that all the devices (empty or otherwise) are allowed to "bid" on incoming move requests. Simulation results indicate that, compared to existing rules, both MOD STTF and B$\sp2$D$\sp2$ improve system performance considerably. Extensions of both rules which allow systems to look ahead are also investigated. Simulation results show that the MOD STTF rule with look-ahead can further improve system performance if the coefficient of variation associated with processing times is small and the "signaled" move requests are based on higher processing time estimates. Lastly, we show alternative priority-based dispatching strategies for systems with loads having unequal priorities. Empirical results indicate that MOD STTF-based rules perform better than STTF-based rules, and MOD STTF-based rules move loads with higher priorities sooner than other rules. In addition, within the assumptions of our study and the parameter ranges we tested, the performance of the device dispatching rules does not seem to be affected by the production scheduling rules used for load selection at the processor input queues.