Adding an Equation to a Model Type

You can add an equation to a Model Type in the Model Editor.

Add an equation

1. Open the Model Editor for the Model Type.

2. Expand the Equations section.

3. Select the icon.

4. Rename the equation.

5. Select the Condition name, if applicable. See Add a condition to a Model Type for more information on conditions.

6. Enter the equation Name.

7. Define the equation Formula by using variables, real parameters, or integer parameters.

   The variables and parameters can be arrays or elements of arrays.

   Equations must use the internal units of measure for the variable's or real parameter's Type, which appears in the Units column in the Variables and Parameters sections, respectively. See Internal units of measure for more details.

8. If the equation formula uses an iterator, enter the Start and End values.

   See Example 1 in this section for details on how to use these values to add a series of equations to your Model Type.

9. Enter a Description of the equation that can help you diagnose the problem if the equation does not solve.

   When the expression syntax is valid, the equation Status changes from red to green. See Operators and functions for more information.

Example 1: Add a series of equations

You can use an iterator in your equation formula to add series of equations of the same type to your Model Type. For example, consider the following pressure equations from the Column in the Process library, which has an NStages number of stages. In Process mode for a Column with more than one stage, the Pressure_2 equation uses the i iterator to add an NStages-1 number of equations to the Column. These equations define the pressure at each stage in the Column.

Example 2: Add a time derivative equation

You can use the function der() to write differential equations. The example demonstrates how to add a time derivative equation to a Model Type.

10. Expand the Variables section, and add a variable; for example, X.

11. Expand the Equations section and enter the Formula der(X) = 1-X. After the equation is created, AVEVA Process Simulation automatically creates a dXdt variable in the Variables section.

12. Drag the Model Type onto the Canvas, expand the Simulation Manager view, and verify that the model is solved. X should equal 1 and dXdt should equal 0.