Breaking the Limit on the Number of Robots Through the Conflict-Free Scheduling in Robotic Mobile Fulfillment Systems

Abstract

The robotic mobile fulfillment system (RMFS) utilizes robots to automate logistics handling. Although robots can replace workers, there are serious limitations and conflicts that make the system inefficient, resulting in a limited number of robots in the system. A conflict-free scheduling approach is studied, demonstrating better system efficiency, breaking the limit on the number of robots, and accommodating more robots in operation compared to the traditional strategy. A mathematical model aiming to minimize the total completion time is presented. An A* algorithm is modified for path planning, and an approach with two avoidance rules and six priority rules is proposed for conflict-free scheduling. Based on a case study involving four layout scales, the conflict-free scheduling is suitable for layouts with a higher number of robots, pillars, and a unidirectional path networks. More robots can be accommodated within the system, and even a threefold increase (45 to 150) will not result in a decrease in system efficiency, reducing the total completion time by 49.3%. In the largest layout system, the number of robots doubled (160 to 320) and the total completion time was reduced by 26.1%. This indicates the great potential for the conflict-free scheduling approach in future logistic applications, which could reduce resource consumption for scheduling and increase system efficiency. Note to Practitioners—This paper was motivated by the problem of multi-robot scheduling in robotic mobile fulfillment systems. It can be scalable to other layouts of multi-robot systems. The maximum permissible floor space for fire zoning in the fire regulations. Conflicts can easily occur if a large number of robots are operating in a limited space. Resulting in a limited number of robots in the system. This paper proposes a novel conflict-free scheduling to break the limit on the number of robots. The approach includes how to plan the path, which avoidance rules and priority rules to utilize. We mathematically model the path planning and conflict-free for multi-robot complete all task. We show the effectiveness of conflict-free scheduling in breaking the limit on the number of robots under different layouts. We define the ratio of the number of QR code squares to the number of robots, which is used to evaluate scheduling methods in various layout scenarios. The largest one has 3,880 QR codes, which is the maximum allowable floor space for single-story e-commerce warehouses. We doubled the number of robots and reduced the total completion time by 26.1%. In future research, we will consider more rules and several expansions of the layout.