To load the STELLA™ software, click on Start--Programs--STELLA . (Note: You may find this application listed under Concept Mapping) A window in the
Map Level is shown below.
You will see an earth icon on the left indicating the Global Diagram Level (see below).
Click on the earth icon on the left of the window so that the equation icon X2appears. In this mode, we can construct the diagram model with equations. The most important icons for building a model appear on the top left of this level's window.
Stock--noun, something that accumulates. Represented by a square icon. They collect whatever flows into them. The default stock is a Reservoir that is
depicted by a rectangle. At any instant, the magnitudes of the stocks give us a snapshot of the system. The most common type of stock is a reservoir, which is
represented by a plain rectangle. If you are modeling the population of a country, for example, a stock would represent the population
Flow--verb, activity that changes magnitude of stock. Represented by an icon that looks like a directed pipe with a spigot. A flow fills and drains
accumulations. It changes the magnitude of the stock. The default flow is a uniflow that flows in one direction only. The white arrowhead indicates the
direction of positive flow. Be careful to draw your flow in the correct direction (either into or out of a stock). To create a biflow (a flow which flows in both
directions, click and hold the biflow icon and select the bi-directional flow.) Some examples of such activities are births in a population, decay of radioactivity,
formation of an enzyme, improvement of self-esteem, or growth of money. The flow icon represents a directed pipe with a spigot and flow regulator. With a
population model, flows would represent the births (in) and deaths (out).
Converter--converts, stores equation or constant, does not accumulate. Represented by a small circle. This multi-purpose building block holds values for
constants, calculates algebraic relationships, and stores graphical functions. In general, it converts inputs into outputs. We can use a converter to modify an
activity. A converter can store an equation or a constant. For example, with the population model a converter might store the constant growth rate, say 10% =
0.1
Connector--transmits inputs and information. Represented by red arrow. As its name implies, this building block serves to connect model elements. For
example, in a population model, a connector can transmit the growth rate value from the growth rate converter to the growth flow. Because they transmit data,
connectors do not have numerical values. Also, a flow can go into a stock but a connector cannot.
Dynamite--To remove a component from the diagram, we use dynamite. Using the delete key does not completely eliminate the item from the model. At the
top of menu select the dynamite icon, which is the next-to-the-last icon. When we remove an item with dynamite, the process eliminates the item and all
connected connectors and flows.
Click once on the appropriate icon in the Building Block palette. Move the mouse to the desired location and click once more to place the item.
Once you place the item, its name will be highlighted. Give the item an appropriate name by typing over the highlighting.
Complete a STELLA™ model to represent the position of an object that is falling with constant velocity with respect to time.
Load the STELLA™ software. Go to Start-Programs-STELLA (Remember: The software may be listed under Concept Mapping)
Check to see that the earth icon is on the left.
We need a stock to represent Position. Create a stock and name it "Position".
Now create a flow into position. This will be a biflow. We want the positive direction going into "Position", so begin your flow outside the stock and drag it into the stock. Name your flow "Change in Position".
We want to indicate that change in position can flow into or out of the stock (can be positive or negative). To do this, we want our flow to be a biflow. Double
click on the flow "Change in Position". You should see the screen below. Change the indicator from UNIFLOW to BIFLOW.
Click "OK". You should notice that your flow now has a black arrow on one end (to show negative flow direction) and a white arrow on the other (positive
flow) end.
Now create a Converter for velocity and name it "Velocity".
We need to connect this converter to our flow with a Connector. Choose the connector icon and click on "Velocity". Hold and drag the "Change in Position" flow and release when the flow becomes shaded (this indicates the connection was made).
Now add another converter for "Initial Position". A connector cannot be drawn from a converter directly to a stock. We will link these with another method
below.
Now hit the Map/Model toggle (Globe icon) for the X2 screen. Notice question marks on the building blocks. This indicates that these building blocks need to
have additional information filled in.
Double click on the "Change in Position" flow. Notice that the BIFLOW indicator is marked. We need to complete the flow equation down in the large white
box. Notice that because "Velocity" is attached to the flow with a connector, that "Velocity" is listed as a required input. Click in the large white box and then
click "Velocity" under required inputs. This completes the flow equation. Click OK to return to the model.
Now we will complete the inputs for the other building blocks.
Double click on the "Velocity" converter. For now, we will set an initial value of 0 in the large white box. Click OK to return to the model.
Double click on the "Initial Position" converter. For now, we will set an initial value of 1 in the large white box. Click OK to return to the model.
Double click on the "Position" stock. You see several building blocks that could be used as Allowable Inputs. We want "Position" to be a function of "Initial
Position", so choose that by clicking on it once. Click OK to return to the model.
You should notice that all question marks have now been removed from the building blocks in the model.
You have now completed the basics of the model. But how do you know if it "works"?
Before you run the model, take some time to sketch the following graphs on paper.
Consider an object which is falling with a constant velocity of 10 starting at position = 0.
On a sheet of paper, sketch two graphs:
We will run the model and look at a graph of position and velocity with respect to time.
For our test run, set an initial velocity of 10. Do that by clicking on the "Velocity" converter and changing the value from 0 to 10. Click OK to return to the
model.
Choose the graph icon from the tool bar. Click anywhere on your model page to place the graph. You may drag the graph to another part of the screen if you choose.
Double click on the graph. Since we would like to graph position and velocity vs. time, you need to choose both of those building blocks by double clicking on
them. You should notice they have moved from the Allowable to the Selected column. We will let the computer use default values for the scale and display.
Click OK to return to the graph.
You are now ready to run your graph. You may do so either by going up to the RUN pull down menu or by using CTRL-R.
Take a moment to compare the model-generated graphs with the ones you drew. How are they similar? How are the different? Notice the scale along the
y-axis of the STELLA™ graph. Both position and velocity have a different numeric scale. You will learn later how to set the graph so that both factors are
graphed on the same scale of numbers.
The Slider Input Device allows you to adjust constant values. On the left side of your document, click the up arrow to enter Interface mode. Click the slider
icon once to select it and then move your cursor into place on the page and click again to deposit it. An unassigned slider is shown below.
Open the slider by double clicking it. You will see the slider's dialog box as shown below. We will create this slider for "Velocity" so that we may adjust the
velocity for values from 0 to 15.
To move a variable from Allowable to Selected, double click it. You may then input your minimum and maximum values. Finally, decide if you want your values to have floating point decimals or be integers only. For this example, select an increment of 1. You may then return to the interface page by clicking on OK.
You have created a slider that lets you adjust Velocity by clicking and dragging the knob. Now create a slider for Initial Position so that it will allow integer
values from 1 to 50. Return to the interface level and your screen should show the following.
Click the down arrow on the left side to return to map/model mode. Notice converter icons have been changed to indicate they are now sliders.
Go back up to Interface Level. Create another graph in this level for Position and Velocity.
Adjust the sliders for a different initial position and velocity and rerun the graphs. Notice how the graphs are affected.
You may have noticed that you have to bring your graph back to the screen by clicking on it after each time that you adjust the sliders. You can eliminate this
by "pinning down" the graph. Click on the pushpin icon in the upper left corner.
Continue to run your model for different values for Position and Velocity. Describe ….
Create the following, which models free fall.
Use the following "values" to initialize each building block:
| Variable | Value |
| Position | 2 |
| Velocity | Initial_Velocity |
| Change in position | Velocity |
| Change in velocity | Acceleration |
| Acceleration | 9.8 |
| Mass | 1 |
| Kinetic_Energy | 0.5 * Mass * Velocity ^2 (see below) |
Note that in this model acceleration, mass and initial velocity are converters with sliders.
Go up to interface level and set your sliders as follows:
For further information, consult the on-line tutorial found under the Help Menu in the Stella™ software (press F1 or click on Help).
Last updated 17 July 2002