Defining units and values for systems engineering

You can use the Unit property to define the concept of a Dimension in a systems engineering design.

About this task

The SysML units define the dimensions used in projects. For example, meter, inch, kilometer, mile, and light_year refer to the concept of length. Therefore, they are all associated to the dimension Length. Every unit must have a SysML valueType to express a given quantity in a standard way so that other quantities having the same unit can be compared. The unit is typically taken from a standard library of units, such as SI or NIST that are part of the SysML profile.

You might define units and value types for your project from diagrams, or the browser. To define units in a diagram, select the Unit button Unit button, and place the new unit into the diagram. To use the browser, expand the TypesPkg, and highlight Units.

To create extensions of the UML data types in block definition diagrams, use a ValueType, as a data type, such as a type for a value property for a block. A UML data type typically expresses a quantity in a software implementation type, such as a float or double. In SysML, the valueType expresses a quantity in a standard unit, such as milliampere. The valueType also includes a placeholder for the quantity plus the unit. Placeholder for the quantity plus the unit

In SysML, you can define a valueType without a unit. For example, when the valueType expresses a ratio, the valueType expresses a quantity only.

Procedure

  1. Right-click and select Add New Unit.
  2. Type the name of the Unit into the browser field, and double-click the Unit name and select the Tags tab from the Features window.
  3. In the Dimensions field, click the Selection button to display the Select Value window, and select the Dimension from either the Dimensions list in the profile, or in the TypesPkg.
  4. Click OK.
  5. Highlight ValueTypes in the TypesPkg in the browser, right-click, and select Add New ValueType.
  6. Type the ValueType code into the browser field, double-click the code and select the Details tab.
  7. Select the required Basic Type, and click OK.
    Note: Units are not used as types for value properties since the unit concept in SysML does not include the concept of quantity.

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