Modeling Discrete and Continuous Processes

by Magne Myrtveit Normally, we try to aggregate discreteness in System Dynamics (SD) models by making continuous flows. This is a good modeling practice, but in some cases discreteness may have a significant influence on system behavior (see note). Consequently, we need to model such discreteness to explore its impact on behavior. The System Dynamics (SD) technology was originally designed with support only for systems where the states change in a continuous manner over time. A SD model typically aggregates discrete events of the underlying system to a level where the events can be considered to be part of a continuous flow. As an example, the manufacture of individual products will be treated as a flow of products over time. Continuous simulations use integration with respect to time to represent the development of systems. With Studio 2001 Powersim introduces accumulation in addition to integration. In contrast, accumulation is used exclusively in spreadsheet models, as spreadsheets cannot integrate. Accumulation means that values are added to or subtracted from state variables. Below we will first deal with continuous systems, and how they can be used to represent discrete processes through a number of workarounds. Then we will describe how discrete parts of a system can be modeled using flows that accumulate as opposed to integrate. Finally, we will take a look at logical states and how they can be set, cleared and remembered using logical levels and flows. Table of