A study on the control problem of stacker cranes in the in-line stocker of a TFT-LCD bay

In-line stockers are the choice of automated material handling systems in many modern TFT-LCD (Thin Film Transistor-Liquid Crystal Display) bays. The performance of an in-line stocker depends much on the performance of its stacker cranes. In this paper, the aim is to study three stacker-crane control problems, i.e., task-determination, port-selection, and cassette-selection, which affect the performance of stacker cranes. This study represents a significant contribution to the field, providing new insights into these critical control problems. The purpose of task-determination problem is to determine whether the next task of a stacker crane is a port-clearing task or a cassette-pickup-and-delivery task. The port-selection problem will appear if the next task of the stacker crane is port-clearing. Its purpose is to determine which port the stacker crane should visit to perform port-clearing task. On the other hand, if the stacker crane’s next task is cassette-pickup-and-delivery, the cassette-selection problem will appear. In this problem, the stacker crane needs to determine which cassette it should pick up and deliver first. Rules with different control logics are proposed for each problem. To understand the performance of the proposed rules, computer simulations were conducted. Simulation results in three performance measures—throughput, the mean flow time of cassettes and the mean tardiness of cassettes—that reflect the need to assess both the efficiency and effectiveness of the in-line stocker system, as well as its ability to meet customer demands and deadlines were collected. The analysis of the results led to the identification of the best rules, longest starvation time rule and shortest distance from the cassette’s current position to its destination port rule, for each problem and the best combination of rules for achieving the best overall performance, resulting in a more effective and efficient solution for solving stacker-crane control problems. The research findings have practical implications for improving the efficiency and productivity of stacker cranes in various industrial settings. It is believed this knowledge can benefit TFT-LCD manufacturers in improving the performance of their in-line stockers and increase their competitiveness as a result.