Dynamic Modelling and Analysis of Information Flows in Production-Inventory and Supply Chain Systems
Sammanfattning: System dynamics is the fundamental concept of this academic dissertation. The focus is on analysis and modelling of properties representative for production-inventory and supply chain systems. The modelling approach is based on Industrial Dynamics but employs a hybrid approach of Control Theory and simulation in the analysis. Simplification based on a structural approach is performed on block diagrams of complex systems as a prerequisite of analysis. In general, there are three main activities of interest when modelling manufacturing systems from a system dynamics point of view. Thus forecasting, lead times and inventory replenishment rules are each analysed and the properties of the suggested models discussed. Analogies between delays and density functions are identified introducing the concept of expected dynamic behaviour. Inventory replenishment rules based on inventory position enables a decomposition of production-inventory systems, thus simplifying the inclusion of trigger decision rule models. A production-inventory system and the Forrester supply chain are analysed based on a system simplification approach. It is shown that the amplification of demand fluctuations of the original Forrester model is not due to the much quoted feedback effect. Instead it is shown that no net feedback exists within the echelons due to cancellations of feedback information. Hence, the amplification is due to the design of the feedforward of orders through the supply chain. A difficult problem is the performance evaluation of dynamic systems. It is known that the net present value of a system can easily be obtained if a Laplace or z-transform model of the system and its cash flows exists. This theory is expanded also to cover general state space models and an application to a simple production-inventory model is given. In most cases, however, the models do not capture all aspects of economic relevance. Instead, as a complement to the cash flow based analysis, a multiple criteria approach for evaluating complex systems is suggested.
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