Simulation Integrated Design for Logistics

The design of an innovative logistic system is a complex problem in which many different disciplines are involved. Each discipline developed its own way of conceptual modeling for a logistic system based on a mono disciplinary perception. It usually leads to a communication problem between the different disciplines and consequently to expectations of the performance that don't correspond with reality.

In this publication a basic systems approach is used to define a conceptual model of a logistic system that can be used by all disciplines involved as a common reference of the design. A combination of a soft and a hard systems approach leads to a conceptual model in which the problem is formulated in terms of required performances and process structures. The logistic system is modelled as a structure of functions around three flows: orders, products and resources. The model evolves during the design project and is an enduring supporting tool for decision making with a clear relation to the systems objectives. This PROcess-PERformance model (PROPER) model is formulated in interdisciplinary terms and thereby enables the communication between different disciplines.

The PROPER model only reflects the structure of a system; it does not model the time dependent behavior of the system. This behavior is essential for correct decision making and usually this behavior is simulated on a computer. In practice simulation is only used during the final stages of a design project and then a correction of objectives and/or decisions is impossible or very expensive.

In this book the use of simulation is recommended for decision making from the very start. To achieve the description of time dependent behavior is also defined at an interdisciplinary level. Natural language is used to describe the processes as defined in the PROPER model at each aggregation stratum. These descriptions enrich the problem formulation phase by expressing the parallel and stochastic aspects of the system.

Like the other disciplines, simulation evolved as a specialist discipline. In order to preserve a direct connection with the process descriptions of the PROPER model, these natural language process descriptions are translated into an object oriented Process Description Language PDL. This language can be implemented in any object oriented software environment. It is here implemented in the Borland Delphi platform that is based on the programming language Pascal. The implementation is called TOMAS: Tool for Object oriented Modeling And Simulation . TOMAS is completely object oriented and fully complies with the Process Interaction implementation of the Discrete Event System Specification method (DEVS). In order to support the growing level of detail of the PROPER model during a design project, TOMAS also supports distributed simulation by offering an open event scheduling mechanism and communication between models at different aggregation strata.

Finally the use of PROPER, PDL and TOMAS is illustrated with an already finished complex project: the design of the first automated container terminal in Rotterdam. It is shown that the use of this approach would have led to a clear and complete objective definition and would have warned the project participants in an early stage for a mismatch between expected and real operational performance.

This approach will therefore not automatically lead to improved logistic designs, but it does contribute to a better correspondence between expectations and reality.