Browsing by Author "Khayyati, Siamak"

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    Production and energy mode control of a production-inventory system
    (ELSEVIER, 2023) Baris Tan; Oktay Karabag; Siamak Khayyati; Khayyati, Siamak; Tan, Baris; Karabag, Oktay
    Energy efficiency in manufacturing can be improved by controlling energy modes and production dy-namically. We examine a production-inventory system that can operate in Working Idle and Off energy modes with mode-dependent energy costs. There can be a warm-up delay to switch between one mode to another. With random inter-arrival production and warm-up times we formulate the problem of de-termining in which mode the production resource should operate at a given time depending on the state of the system as a stochastic control problem under the long-run average profit criterion considering the sales revenue together with energy inventory holding and backlog costs. The optimal solution of the problem for the exponential inter-arrival production and warm-up times is determined by solving the Markov Decision Process with a linear programming approach. The structure of the optimal policy for the exponential case uses two thresholds to switch between the Working and Idle or Working and Off modes. We use the two-threshold policy as an approximate policy to control a system with correlated inter-event times with general distributions. This system is modelled as a Quasi Birth and Death Process and analyzed by using a matrix-geometric method. Our numerical experiments show that the joint pro-duction and energy control policy performs better compared to the pure production and energy control policies depending on the system parameters. In summary we propose a joint energy and production control policy that improves energy efficiency by controlling the energy modes depending on the state of the system. (c) 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )