Representing workload control of manufacturing systems as a dynamic model

Abstract

Workload control (WLC) is a methodology that aims to regulate the total workload in the production system by controlling the input and output of orders. It leads to more predictable throughput times and more accurate delivery date promising. In this paper, we propose a representation of the workload control principles by means of a dynamic model based on bond graphs, and present the modeling of this system. According to the bond graph methodology, the manufacturing entities can be associated to constitutive equations, which can be combined to generate a state model. The presented state model provides informations about instantaneous levels of work in process of the system, and can show the effect of release policies (input control) and capacity adjustments (output control) into these levels of work in process. Exploratory simulations were carried out in Simulink. The existing simulation of WLC systems usually employs open-loop descriptive models based on discrete events simulation, and the parameters of input and output control are empirically/experimentally defined. A gap was identified in the literature, specially concerning the parameter setting for output control. In the proposed model, input and output control are performed automatically, by the controllers implemented to the dynamic model (closed-loop system). Therefore, the proposed approach can bring prescriptive directions to the parameter setting of WLC, and we believe it can also bring future comparative insights to the existing simulations of this methodology.