Motion Planning Method for Two Stacker Cranes in an Automated Storage and Retrieval System

In this paper, a motion planning method for two stacker cranes in an Automated Storage and Retrieval System (AS/RS) is proposed. For the cranes to operate cooperatively, they must perform tasks while avoiding collisions. In addition, the requirements, which include fast operation and short calculation time, must be satisfied, along with a specific mechanical constraint on the motion of the stacker cranes. For these problems, an approach is proposed in which a motion is generated on two levels. On the first, collision is avoided by using constraint on trajectories. A trajectory generated on this level ensures the shortest travel time. If a collision cannot be avoided on the first level, the system shifts to the second, in which heuristics are used for collision avoidance. The proposal is for highspeed heuristics based on a binary search. The effectiveness of the proposed algorithm is shown through simulations. The simulation results indicate that, in a layout of 60 racks in the horizontal direction and 10 in the vertical direction under standard task conditions, the method has an efficiency of 1.91 with respect to a single crane system and 1.66 seconds for the motion planning of one task when a computer with a 3.0 GHz CPU is used.