Two Years On-line: A Dynamic Scheduler for a Hot Steel Mill

This paper describes the development ,and installation of the 5 Mill Scheduler, a dynamic scheduling application for a hot steel mill. The main objective of the scheduler is to dynamically generate ,and revise schedules, ,and make effective use of the resources of the phant, while satisfying a complex set of constraints dealing with the dynamic behavior of the resources and the material processing specifications. The design of the system evolved from a knowledge-based approach, ,as suggested by the user, to a constraint-based reasoning approach embedding user supplied heuristics to prune the search space. The constraint-based approach proved to significantly outperform a pure knowledge-based approach. The goal of the scheduler was to operate on-line, closely linked to monitoring devices. The installation of the scheduler required some refinements of the design to cope with imprecise data and with the need to provide greater flexibility to the operators in dealing with resource constraints. The scheduler was installed in February 1992 and has been continuously operating on-line since that date. 1. The application domain. Number 5 Mill is a hot rolling mill. It heats billets and ingots to required temperatures, according to specific heating cycles, and then runs them through rolls to reduce the cross-section ,areas of the material to customer specification. The Number 5 Mill is a multi-faceted operation with a wide product range: 50 different heating cycles covering 350 grades of steel, with processing temperatures ranging from