Cross-level model of a transfer machine energy demand using a two-machine generalized threshold representation

The paper proposes an energy consumption oriented optimization scheme for a demand driven machine tool working in a system context. The aim is optimize performance of manufacturing system engaging more than one organizational level, in this case the machine and the system levels. The paper exploits a former study on Pareto optimal Minimum Energy-Time functions, representing the best possible machine setups under varying cycle-time. The functions are used to model the productive state of the machine, whereas stochastic behaviours (failures, blocking, starvation) and inter-machine interaction are described using a Markovian General Threshold Model. Additionally, instead of using a constant power per machine state assumption, a functional relationship between processing rate and machine demand is used. A stand-by energy-saving policy, which exploits the flexibility in adopting a variable processing rate and a settable threshold level of the interoperational parts buffer as a trigger, is developed and benchmarked. The model is used to analyse an industrial case study of a three-station transfer machine. The study takes into account a number of cost contributing factors, apart from sole energy demand, such as tooling cost, operator cost, inventory and costs related to potentially undelivered throughput. The application of the proposed cross-level optimization scheme showed an improvement both in energy-efficiency and in profitability of the production system under investigation.