Geometry Editor Tool 

Highlights

Geometry Editor Session Showing Microstrip Low Pass Filter

Introduction 

The geometry editor allows geometric structures to be built and meshed ready for solution by the Celia FDTD solver

The functionality is based on the Constructive Solid Geometry (CSG) solid modelling paradigm.  This supports a Regularised Boolean Set of geometric constructs that can be defined and used to perform Boolean shape combinations using the difference, intersection or union operators. (see technical geometry)

When the geometry editor is started the project management screen is replaced by the geometry editor screen, shown in Figure 1. 

Figure 1  Geometry Editor Window

This window contains a plotting surface, which by default starts up with a single plotting window, the display dialog which controls what is shown in the plotting window and finally the toolbar on the right. 

All construction and meshing operations are invoked using the toolbar.   

Geometric Primitive Creation

The building blocks of the model are geometric primitives.  These are basic shapes that can be generated in the editor or imported from other CAD packages.

Primitive generation within the interface is carried out through input dialogs for primitive specification.  Primitives available through the interface are the sphere, cylinder, cone, conic frustum and linear and rotational extrusions.  Additionally, primitives can be imported in STL format.

An example of a primitive creation dialog is shown in Figure 2.

Figure 2 Create Sphere Dialog

The primitive is defined by entering the appropriate values into the edit boxes.  The material specifier is a combo box from which can be selected any of the materials in the global material database.

All primitives can be edited following creation.

More complex primitives can be created using the extrusion function.  Figure 3 shows a monofilar helical antenna generated as a combination of linear and rotational extrusions.

Figure 3 Monofilar helical Antenna 

Part Creation and the CSG Representation 

In addition to simple primitives, more complex components can be constructed by using the boolean combinatorial logic of the CSG representation.  In other words, simple primitives can be combined to form more complicated shapes using logical operations, union, difference and intersection.

In CSG object representation is stored in the form of an attribute tree.  The leaves of the tree contain the primitives and the nodes contain the operations.  As an example consider a sphere with a cylindrical hole through it.  The geometry in question is shown in Figure 4.

Figure 4 Sphere with Cylindrical Hole

 

CSG tree for sphere with hole

A dialog is provided for building CSG trees (named parts in the editor).  More complex trees can be constructed by adding trees (other parts) to the terminals of the branches using the part creation dialog.  Construction of CSG trees is dealt with in standard CAD and Graphics texts. 

Model and Mesh Display  

A model, in the context of the geometry editor, is a construction of geometric objects that are to be meshed for FDTD solution together with all relevant solver directives such as excitation points and output points etc.  Figure 5 shows a model plot of a sphere with a cylindrical hole.   

Figure 5 CSG Difference Product Between Sphere and Cylinder

The model plot shows the geometry and in addition to this, the geometry is placed in context within the other attributes of the FDTD model such as mesh extent, excitation type and location, output point location, wires etc.

In the case shown in Figure 5, the model is configured as a scattering case in which a Huygens surface has been defined to apply the plane wave excitation.  The Huygens surface is shown as the magenta wire frame cube.  The black wire frame cube is the mesh extent.

The image is taken from an interactive session on the geometry editor and is shown as it would be seen in the plotting window of the editor.  The editor has interactive mouse manipulations for pan, rotate and zoom to allow quick and simple browsing of models.

Also shown on this figure (although at too low a resolution to see properly) is an output point to the left of the sphere (looks like a small speck but is in fact an arrow at the field node location requested).

Figure 6 shows a close up of the output request glyphs, blue for an electric field node and red for a magnetic field node output.  This facility allows confirmation of correct location of the output data request.

Figure 6 Close up of Output Glyphs

In order to mesh the model a mesh creation tool is provided.  Once meshed, the geometry can be shown in the meshed state.  The mesh plot for the sphere with hole is shown in Figure 7.

Figure 7 3D Mesh Plot of 'Sphere-Difference-Cylinder' Product

The mesh plot is identical to the model plot however the pre-meshed geometry is replaced by the stair stepped FDTD representation of the geometry.

As well as a 3D mesh plot, a 2D mesh plot facility is provided to allow a scan through each of the mesh planes.  This display function shows each individual electric field node material for detailed mesh diagnostics prior to solution. 

Arrow glyphs are used to show the nodes in the appropriate colour for the material at that node.  Figure 8 shows a cut through the XY plane of a meshed sphere.  A dialog is provided to allow simple mesh traversal in this mode.

Figure 8 2D Mesh Plot