The curse of reality - why process scheduling optimization problems are difficult in practice

Two process scheduling examples from consumer goods industries are presented. These problems contain several features, which in practice tend to make problems difficult to solve and present barriers to regular use of scheduling technology. Several instances of the main challenge to obtaining a solution, the underlying process physics, are highlighted. Computational issues can be traced to operations being strongly interrelated by resource sharing, inventory constraints, manpower availability, and management policies. The question of what defines optimality arises when demands, process rates, yields, and batch cycle times are variable over time. Ultimately, it becomes necessary to tradeoff between optimality with respect to a given set of process parameter estimates and robustness to a set of realistic scenarios. Although the examples presented are relatively concise due to simplifying assumptions, implementation and representation of large problems are themselves challenges. If an enormous amount of resources are required to build and maintain models, even the highest quality methodology is in trouble. (C) 2000 Elsevier Science Ltd. All rights reserved.