3 BUILDING A BASIC POWERSIM™ STUDIO MODEL

3.1 BACKGROUND

In the most general situation encountered in system dynamics modelling we analyse influences that flow- (rate-) controlling variables have on state variables, the latter being depicted as stocks (levels or accumulators). In this chapter, we demonstrate how such influences are calculated through detailed steps of building a model followed by stepwise manual simulation. The equivalent computer simulation is developed. The results of the two simulations are compared.

The example model contains a single state variable (stock) under the influence of two rate-controlling variables, one inflow and one outflow. The model is constructed as two simple modules. Firstly, these modules are considered separately. They are then combined and their combined influence on the state variable is analysed once the model has been assembled.

The example given in this chapter does not contain any feedback mechanism and, therefore, could be modelled using a spreadsheet. However, as is invariably the case in system dynamics modelling, simple modules are combined. Complexity is progressively added. Once this happens spreadsheet calculations are no longer possible. This is certainly the case where feedback mechanisms and delays are present in our models.

This chapter also serves to explain the foundations of the mathematical method used by Powersim™ Studio to perform integration calculations. Practical steps are also explained to keep errors inherent in numerical of approximation methods to a minimum.