Scheduling Heuristics in Practice — Flexible Flow Shops and Flow Shops with Reentry

Efficient scheduling of industrial systems typically has a major impact on productivity levels.

In this seminar we focus on the applications of scheduling heuristics in two important industries,   namely steelmaking and microelectronics.

In steel production the steelmaking-continuous casting (SCC) process is often a bottleneck and its scheduling has become more challenging over the years. We first describe the modeling of the essential features of an SCC process as a flexible flow shop with unrelated parallel machine environments, stage skipping, and maximum waiting time limits in between successive stages. The objective is the minimisation of the total weighted waiting time, total earliness, and total tardiness. The problem can be formulated as a mixed integer program and we present an iterated greedy matheuristic that solves its subproblems to find a near-optimal solution. Through numerical experiments, we show the effectiveness of such an algorithm in a practical setting.

The microelectronics industry is conceptually very different from the steel making industry. The manufacturing processes in a wafer fab can be modeled as flow shops with re-entry, which are special cases of job shops with recirculation. The re-entries of the orders make the associated scheduling problems conceptually difficult. We discuss the properties of the optimal schedules for various different objective functions.

We conclude this presentation with various other scheduling applications in industry that deserve more research attention in the future.