Throughput analysis of multi-device trip-based material handling systems operating under the modified-FCFS dispatching rule

We present an analytic model to assess the performance of a trip-based material handling system with multiple devices dispatched according to the modified-first-come-first-served rule. A key element of the model is the analytic treatment of empty trips based on whether they are device- or station-initiated. The model is evaluated against simulation results using multiple layout configurations and devices at different levels of utilisation. In developing the analytic model, we also investigate the role and impact of device-initiated dispatching vs. station-initiated dispatching, and show results that shed light on conflicting views presented in the literature on this subject. The proposed analytic model can be used to rapidly evaluate alternative handling systems and to conduct various ‘what if’ analyses; it performs reasonably well across a range of systems that were tested via simulation. In evaluating the accuracy of the model, we introduce a unique and more effective technique based on the allocation of empty trips out of each station. In addition, we propose an approximate but simple method to check for system stability.