A Combined Scheduling and Simulation Method to Analyze the Performance of the Dual-Robot In-Line Stocker

The in-line stocker is a new type of the automated material handling system that has begun to be used in the display industry. This not only moves unit loads under processing across different locations within manufacturing facilities, similar to the automated guided vehicle (AGV), but also stores them on its shelves, similar to the automated storage and retrieval system (ASRS), which can significantly reduce space for material handling. However, traffic rates are generally very high inside the in-line stocker and two robots serve along the single lane, which is the dual-robot in-line stocker (DRIS). One difficulty in applying the DRIS to shop floors is that the exact transport capacity of the unit DRIS is not known. This paper develops an analytical model to estimate the capacity of the DRIS based on a combined scheduling and simulation method. It calculates movements of two robots over time in the space consisting of time and location and precisely measure necessary time for waiting or backtracking to avoid collision of two robots. An experimental analysis was conducted to validate the correctness and usefulness of the model based on the data used in an actual manufacturing site. The analysis result illustrates that the average processing capacity of the DRIS increases compared to the SRIS (single robot in-line stocker) as the length of the DIRS increases, which is consistent with the expectation of practitioners in the industry. The paper also verifies that it is necessary to carefully determine the operating specifications in actual uses since the transport capacity of the DIRS varies considerably by its operating parameters, which can be optimized by the analytical model.