Hybrid FMS scheduling using T-TPN and Beam Search in uncertain environments

Abstract

This paper is about the incremental computation of control sequences for complex discrete event systems (DES) in uncertain environments. Transition-timed Petri nets (T-TPN) that behave under earliest firing policy are used to model a class of flexible manufacturing systems (FMS) where operations are proceeded with partial routing flexibility (namely hybrid FMS). Uncertainties are assumed to occur due to interruption of operations or unreliable resources. The objective is to find a control sequence from an initial state to a reference one with a trade-off between performance and robustness. For that, a systematic T-TPN-based multi-level formalism is used for the modelling. Then a new cost function is introduced to estimate the time and risk to reach the reference. A modified beam search algorithm is also proposed to selectively explore the PN state space and search for the best control sequence.