How to Define a Local Coordinate System

When setting up a model for analysis, it can be easier and more efficient to add loads and constraints in a coordinate system other than the default (particularly for off-axis loads or constraints or for curved geometry). The FEA Editor environment of FEMPRO provides the ability to define local coordinate systems (rectangular, cylindrical or spherical with the origin located anywhere in space) and then add loads and constraints relative to them.

For example, consider a model with a cylindrical surface. To add a force load to the nodes of the surface, you can define a local cylindrical coordinate system at the center of the cylinder as follows:

• In the FEA Editor environment tree view, right click on "Coordinate Systems" to access a pop-up menu of options and choose "New...". The "Creating Coordinate System Definition" dialog will appear (see Figure 1). This screen provides options for defining a new coordinate system.

  Figure 1: The "Creating Coordinate System Definition" dialog is used to define local coordinate systems.

• Click on the "Coordinate System Type" field and choose from the pull-down menu of options. For this example, "Cylindrical" was specified.
• Enter three sets of coordinates in the "Point A", "Point B" and "Point C" fields to define the local coordinate system per the diagram shown on the screen. For this example, the values were typed into the fields: Point A at (0,0,0); Point B at (0,0,1); and Point C at (0,1,0). Alternatively, you can define coordinates by clicking on points in the model as follows:
  • Click on the "Select A", "Select B" or "Select C" button. Then click on a vertex in the model to define the coordinate.
  • Click on the "Interactive" button. Then click on three vertices to define all three points sequentially. A graphical display of an axis will be drawn on the model to guide you as you click on the points.
• Click on the "OK" button to accept the coordinate system definition. It will then be listed in the model tree under "Coordinate Systems".

Now, you can conveniently add a force load to the model relative to the local coordinate system. For this example, select the vertices and right click anywhere in the working area to access a pop-up menu of options. Choose "Coordinate Systems" and then the local cylindrical coordinate system to apply it to the selected vertices (see Figure 2). Icons indicating the orientation of the local cylindrical coordinate system will appear on the vertices.

Figure 2: Apply the local cylindrical coordinate system to the selected vertices.

Right click in the working area and choose "Add" and "Nodal Forces..." to access the "Creating Nodal Force Objects" dialog. Notice that the active local cylindrical coordinate system is shown in the "Coordinate System" field (see Figure 3). The axis icons on the selected vertices will indicate the direction that you should define for the force load. In this case, the X direction was specified.

Figure 3: Add a force load to the selected vertices. The active local cylindrical coordinate system is shown in the "Coordinate System" field.

After adding the force load, the display will show the force arrows pointing in relation to the local cylindrical coordinate system (see Figure 4).

Figure 4: The force load is applied in relation to the local cylindrical coordinate system.

Thus, defining a local coordinate system in the FEA Editor environment can make it easier to apply loads and constraints. Local coordinate systems can either be reused in the Superview IV Results environment or defined there to display results in an arbitrary orientation relative to them (see How to Display Results in a Local Coordinate System).

For more information about defining local coordinate systems, see the ALGOR User's Guide.