Conflict between Energy, Stability and Robustness in Production Schedules

Paper in proceedings, 2017

A systematic method to evaluate the conflict between robustness, stability and energy consumption is proposed in this paper. Energy optimization is combined with robust scheduling techniques to analyze the trade-off. In rescheduling, slack is often used to protect a schedule from disruptions. However, results from the literature on energy minimization show that a reduction in energy consumption is achieved by extending the execution time of operations. Thus, slack in schedules is diminished on behalf of longer execution times. The proposed method, which quantitatively shows this conflict, is based on a multi-objective optimization formulation where efficient computation of the involved criteria are developed. This includes a convex surrogate stability measure that makes it possible to evaluate different operation sequences by a mixed-integer nonlinear programming formulation. Previous work connecting the two research fields use simulation for analyzing the impact of disruptions in order to generate robust production schedules. Our results show that an increase in energy efficiency comes at a cost of reducing stability and robustness and hence becoming more sensitive to disruptions.