4.2.6 Create Modelling And Simulation Project

Now that vital preparatory work has been completed it is appropriate to switch on the computer and enter details needed to create the modelling project in Powersim™ Studio.

Enter Project Information. Project information by accessing the menus:
<Project>
<Project Settings>
<Time Measurement>
<Available Calendars>, where there is a choice of:
<Bank>
<Fiscal>
<Gregorian>
<Time has unit>:
<Second>
<Minute>
<Hour>
<Day>
<Week>
<Month>
<Quarter>
<Year>
<Presentation Mode Setup>—here, the option of opening the model in presentation mode can be selected. Presentation mode is selected when a model had been developed for demonstration purposes or for experimentation where access to other than selected controls is blocked.
Enter Simulation Setup Details. Simulation setup details are entered by accessing the menus:
<Simulation and Settings>
<Simulation>
<Time Settings>
<Calendar>
<Timestep>—a very wide range of values can be selected for simulation timestep. Timestep value selected must be small compared to the time horizon (the elapsed time between ‘Start Time’ and ‘Stop Time’). For example if a model runs for several years, say 10, it would be appropriate to select ‘timestep’ of one quarter or a month. A rule of thumb used for selection of timestep is that the timestep selected should be less than, or equal to, one fifth of the smallest time interval expected to be encountered in the simulation.
<Start Time>
<Stop Time>
<Simulation State>
<Time>
<Simulation Speed>
<Maximum Speed>—maximum speed is generally selected where plots of simulation results or tables are generated for subsequent analysis. This is particularly useful where simulations might otherwise run over long periods and simulating in real time or near real time makes little practical sense.
<Real Time>—some simulations that might run for very short periods if allowed to be simulated as quickly as the computer can make calculations may be completed before observations of behaviour can be made. In these situations it can be helpful to run the simulation in real time.
<Custom>—custom speed settings are used when a simulation is conducted to give a sense of the extent to which the simulation represents accelerated time. For example one second of simulated time might be used represent one day of real time.
<Integration>—there are four selected options for the method of numerical approximation to be used by Powersim™ Studio. The choice is made on the extent to which, and how rapidly, time-dependent functions within the model being simulated change. If that change is relatively slow then 1^st Order Euler (fixed step) integration will be sufficient. If that change can be dramatic varying from slow to rapid with large and frequent changes in slope of the time-dependent functions being integrated, then 4^th Order Runge-Kutta (fixed step) integration will be needed. 1^st order numerical methods can be calculated quickly whilst 4^th Order methods are time consuming. Regardless of the integration method chosen, it is essential to test the sensitivity of the simulation to changes in the method of integration.
Create Units. Units can be created for local or global use. The following example demonstrates how to create a new global unit for our Population Model. This new unit will be called individuals (plural) and individual (singular):
<Project> (located in the left pane of the screen)
<Global Units> Double click on the <Global Units> icon and the <Global Units> dialogue box will appear in the right pane of the Powersim™ Studio screen. Right click anywhere in that dialogue box to launch the <Add Unit> dialogue box. Left click on the <Add Unit> icon and <Unit 1> <Atomic Unit> will appear in the <Global Units> dialogue box. Double click on <Unit 1> to launch <Unit 1 Properties> dialogue box. Enter details where necessary and permitted:
<Long name>—enter
<Plural name>—enter
<Definition Text>—automatically set as ‘Atomic’
<Normal Unit> or <Point Unit>—select using appropriate radio button. Point units are used to explicitly define a unique point on a scale. The boiling point of water 100°C, a date in history such as 1 January 2004 are examples of such unique points that can be represented by point units in Powersim™ Studio. Point units are prefixed with ‘@’. A point unit is defined by two parts. The first defines the scale of the unit, while the other defines the offset. A derived point unit is derived from SI base units or from atomic units you have created. Normal units are defined by the scale of the unit alone.
Commence Building the Model. Provided we have a good understanding of the problem to be modelled, building the model should now be a relatively straight forward task. This task is made even less troublesome through the use of the modules as defined here. The general strategy for assembling the model is:
Models should contain no more than four or five system dynamics modelling icons (stocks, rates or auxiliaries) before conducting comprehensive tests to ensure the model actually works as intended, that is, it is verified.
Focus on one stock (level or accumulator) at a time.
Add one flow and rate variable at a time, testing as you go to ensure the model actually behaves as intended. In most cases this will involve generating dummy input flows and observing the consequences of those flows over time.
Create and progressively connect auxiliaries.
Progressively assign units to each stock, rate and auxiliary (noting that through a top-down approach most units were created at the outset).
Repeatedly check that units are consistent.
At each stage, and before the model exceeds four or five icons, conduct logical, extreme value and mass balance checks as described here.
Always remember that it is most important to build a working model in which you have complete confidence (and can demonstrate to others—regardless of how simple it might seem) before adding functionality. Adding functionality to more closely represent real-world behaviour can follow.