Scheduling ASC and AGV considering direct, buffer, and hybrid modes for transferring containers

Abstract

At automated container terminals (ACTs), automated stacking cranes (ASCs) and automated guided vehicles (AGVs) perform yard vertical loading/unloading and horizontal transport operations, respectively. The two operating devices transfer containers at the seaside. This paper studies an integrated scheduling problem of ASCs and AGVs considering direct, buffer, and hybrid modes for transferring containers. In direct mode, the two devices are synchronized at the pad to transfer containers from one device to the other. Buffer mode decouples the synchronized operations between ASCs and AGVs, allowing one device to temporarily store the container at buffer sites (i.e., AGV partners) for subsequent operations by the other. Supported by advanced positioning technology, direct mode speeds up the transferring operations and helps to reduce partner overflow. Therefore, the hybrid mode with a combination of pads and partners makes operations more flexible. We formulate the integrated scheduling problem as a multi-layer vehicle routing problem and connect them via precedence relationships and transfer constraints. Three continuous time integer programming models are developed to sequence the handling tasks under the minimization of the makespan. A relaxation model and the cutting-plane approach are proposed to derive a lower bound. To enhance the search capability, a genetic algorithm embedded with several neighborhood generation operators is developed to solve the problem. In addition, a flexible strategy with a task pool and a transfer mode selection mechanism is proposed for decoding individuals in the genetic algorithms into handling sequences. Numerical experiments show that the hybrid mode outperforms the direct and buffer modes by an average of 12.35% and 8.26%, respectively. The makespan gap between the lower bound and the proposed algorithm is 6.5% on average. In addition, AGV utilization is highest in the hybrid mode, and the fluctuations in the container arrival rate have minimal effect on the hybrid mode. Moreover, based on the results, the opportunity and risk of configuring a hybrid mode through technological upgrading are discussed.