This study is concerned with multi-aisle S/R machine system (MASS) in which a storage and retrieval (S/R) machine serves storage racks in more than one aisle through a transfer called traverser, and is divided into the following two parts:
i) Operating policies of MASS are considered.
With randomized storage assignment rule, the average travel time of the S/R machine is determined under single and dual commands. Based on the travel time models, the average performance of S/R machine is evaluated. Also, the expected travel times of S/R machine are derived with two kinds of alternative I/O locations. Then, we propose rack-class-based storage assignment rule (RCB rule) and class selection procedure to find the minimum number of S/R machines required and identify the number of aisles each S/R machine serves. Other storage assignment rule is suggested to reduce the risk of the failure of either S/R machine or other handling device.
ii) Design aspects of MASS are studied
A mathematical model is developed from which the system parameters, such as length and height of the system, the number of S/R machines, traversers and aisles are determined which provide a minimum initial investment and operating cost. Through experiments, sensitivity analysis is made for throughput and storage volume. Then, we suggest an approach which determines rack storage volume under randomized, class-based and turnover-based storage assignment rules in typical unit-load automated storage and retrieval systems (unit-load AS/RS). Trade-off analysis is made between the storage volume and the expected travel time of S/R machine under each storage rule.
