An On-line Scheduling Method for Flexible Manufacturing Systems by a Hierarchical Decision Making System.

Abstract

With introduction of automated production lines, it is becoming more difficult to manage manufacturing systems. Many scheduling methods have been researched to improve the efficiency of manufacturing systems, yet they have not been utilized in large scale and complex systems because of many requirements and constraints in real manufacturing systems and lack of know-how for large scale and complex production control.This paper presents a hierarchical scheduling method for an on-line production line control, which consists of a strategic level scheduler (SLS) and operational level schedulers (OLS). The SLS aims at making a schedule referred by the OLS. The schedule solved by the SLS satisfies some constraints, and it is generated by a scheduling algorithm based on a human planning procedure and a temporal constraint propagation mechanism. To obtain a desired result (expected throughput), it is necessary to control the production line to obey the higher level decision. The OLS aims at the on-line control of the production line and is based on the know-how of the supervisor.This hierarchical method is evaluated by discrete event simulations before introducing it into real production lines. The results show that this method is more useful than the practical operation subject to minimizing of lateness and tardiness to dynamic environment such as failures of machines or AGVs.