VEM2.0 Instruction Manual

July 2001 - David Voltmer

OVERVIEW
VEM2.0 is designed to solve two dimensional electromagnetic problems quickly and accurately in a Windows environment. VEM is a set of MATLAB files that features:

Easy entry of geometry, material, and excitation information:
Quick, accurate solution of Poisson’s equation; and
Wide variety of graphical modes in which the solution can be viewed.

The standard mouse, menu, and button features of a Windows environment are used throughout the application.

INSTALLING VEM

Since VEM2.0 runs under MATLAB 5.2 (and later versions) copy the set of .m and .p files to a MATLAB sub-directory named vem. Add the path c:\MATLAB\vem with the MATLAB Path Browser.

RUNNING VEM

Start MATLAB; in the Command Window type vem at the prompt; VEM opens with the Drawing Window shown in Figure 1. Following computations, VEM opens the Solution Window shown later in Figure 2.

Figure 1 - VEM Drawing Window.

ENTERING OBJECTS INTO THE DRAWING WINDOW
The electromagnetic structure is composed of point, line, or rectangular objects of various materials activated by sources. These elements are entered in the Drawing Window as follows.

GEOMETRY PULL-DOWN MENU

Select either Cartesian (z-invariant) or Cylindrical (rotationally symmetric) geometry from the pull-down menu.

SIMULATION TYPE PULL-DOWN MENU

Select Electrostatic-Dielectric, Electrostatic-Conductor or Magnetostatic simulation type from the pull-down menu.

OBJECT TYPE TOOLBAR

For Electrostatic simulation three different buttons are available for constructing an electromagnetic structure. select either Perfect Electric Conductor (PEC) (left button), Dielectric material (center button), or Charge (right button) by clicking the mouse on the appropriate button of the object type toolbar. For Magnetostatic simulation type select either Perfect Magnetic Conductor (PMC) (left button), Magnetic material (center button), or Current (right button).

OBJECT SHAPE & SELECTION TOOLBAR
There are three shapes of objects—points, lines, and rectangular solids. A unique button is designated for each of these shapes regardless of the object type or material. Not all shapes are available for all types of objects.

POINTS

Points are a valid entry for charges or currents only. They are activated by clicking on the Point Button shown at right. They are positioned in the Drawing Window by moving the cursor to the desired location and clicking the left mouse button. For Cartesian geometry the point represents a infinitely long filament perpendicular to the plane of the screen. For Cylindrical geometry the point represents a loop which encircles the z-axis on the left edge of the drawing window.

LINES

Lines are a valid entry for PECs and PMCs only. They are activated by clicking on the Line Button shown at right. They are inserted in the Drawing Window by clicking at one end of the line and dragging the cursor to the other end of the line. For Cartesian geometry the line represents an infinite plane perpendicular to the screen. For Cylindrical geometry the line represents a disc perpendicular to both the z-axis and the screen when the line is drawn in the radial direction; it represents a cylindrical surface with its axis coincident with the z-axis when the line is drawn in the z-direction.

SOLIDS

Solids are a valid entry for PECs, PMCs, Dielectrics, and Magnetics. They are activated by clicking on the Solid Button shown at right. They are inserted in the Drawing Window by the clicking the left mouse button at one corner and dragging the mouse to the opposite corner. For Cartesian geometry the solid represents an infinitely-long, rectangular bar of material with its axis perpendicular to the screen. For Cylindrical geometry the solid represents a cylindrical shell of material with its axis coincident with the z-axis.

SELECTION

The Selection Button enables the selection of an object of the currently-defined material type. To select an object, move the cursor to the object and click the mouse. Upon selection an object’s properties are displayed in the Material Properties and Excitation Editor. A selected object is signified by black, square dots at its corners. The dielectric object of Figure 1 is selected. A selected object can be

edited (via the Material Properties Editor),
copied (CTRL-C),
pasted (CTRL-V),
moved (depress and hold down the left mouse button, drag the object to the new location, and release the mouse button) or,
deleted (CTRL-X).

MATERIAL PROPERTIES & EXCITATION EDITOR - The material properties and the state of attached sources are entered with the Material Properties & Excitation Editor. The objects are described in pairs since the Electrostatic and Magnetostatic simulations are duals of each other. PERFECT ELECTRIC/MAGNETIC CONDUCTORS

PEC or PMC objects are selected by clicking the mouse over the PEC or PMC Button of the Object Type Toolbar which in turn activates the PEC/PMC Object Shape & Selection Toolbar. PEC/PMC objects are shown as black lines or rectangles. PEC/PMC elements can be assigned three states:

float, i.e., have no sources or charges/currents attached,
set voltage/magnetic potential, i.e., a source of fixed voltage/magnetic potential is attached, or
set charge/current, i.e., a charge/current source of fixed charge/current is attached. The charge and current are assumed to be uniformly distributed on the surface of the object. The current flows out of the plane of the window, i.e., the +z direction.

Click the mouse in the desired radio button (float, set voltage/potential, or set charge/current) in the Material Properties & Excitation Editor. For the set voltage/potential and set charge/current buttons, click the mouse in the corresponding window and enter the voltage/potential or charge/current per meter, respectively, followed by RETURN or ENTER.

DIELECTRIC - CONDUCTOR - MAGNETIC MATERIAL

Dielectric, Conductor, or Magnetic material objects are selected by their respective button of the Object Type Toolbar that in turn activates the Dielectric/Conductor/Magnetic Object Shape & Selection Toolbar. Dielectric/Conductor/Magnetic objects can be entered only as solids. The color of Dielectric/Conductor/Magnetic objects depends upon the material selected in the Material Properties & Excitation Editor. Upon entry of a Dielectric/Conductor/Magnetic object the default color and relative permittivity/conductivity/permeability are displayed in the Material Properties & Excitation Editor. Different material is selected from the pull-down menu that is activated by clicking the down arrow. Move the cursor through the list of materials until the desired material is highlighted and click to select a new material. The Edit List button can be used to change the default list of Dielectric/Conductor/Magnetic materials.

The Edit Materials List window opens when the Edit List button is clicked. The default material can be edited or deleted; a new material can be added with the appropriate button. This opens the Edit Material Specification window; enter desired properties in the appropriate boxes and click the OK button. Note that the new material is inserted into the material list immediately following the highlighted material of the original list.

CHARGE/CURRENT

Charge/Current objects are selected by the Charge/Current Button of the Object Type Toolbar that in turn activates the Charge/Current Object Shape & Selections Toolbar. Charge/Current objects can be entered as points only. These points can be within Dielectric/Conductor/Magnetic material, but not within PEC/PMC material. The magnitude and sign of the Charge/Current object are entered by clicking the mouse in the entry window, entering the value from the keyboard, and pressing ENTER or RETURN. Charge polarity is indicated by +/-; the direction of the current is the usual arrow convention--out is a dot in a circle (the head of the current arrow) and in is an x in a circle (the tail of the current arrow).

SOLVE BUTTON

Once all the objects are entered into the Drawing Window, the problem can be solved by clicking on the Solve Button. This activates the matrix solving routines. A dialog box will appear requesting a choice of solution method. The Direct method is applicable to most problems; the Iterative method is a vestige of earlier versions before sparse matrix routines were available. A second dialog box appears with generation of the system matrix; a third dialog box appear to note that the solution is in progress. For a 500 MHz machine, most problems are completed in less than 30 seconds. But for a higher resolution or a slower machine the solution time will be longer.

When the Solve Button is clicked and no non-zero potentials have been specified in the Drawing Window, an error dialog box appears. To enable a solution, just click OK and use the Select Button for PEC/PMC to specify a potential or the Charge/Current Button to add sources. Then the Solve Button will properly execute a solution.

Upon completion of a solution, VEM will open the Solution Window as shown in Figure 2.

DISPLAYING RESULTS IN THE SOLUTIONS WINDOW

The default mode for the Solutions Window displays the 2 dimentional equipotential lines superimposed upon the electromagnetic structure as constructed in the Drawing Window. The toolbars of the Solutions Window differ significantly from those of the Drawing Window. The Geometry Pull-down Menu and the Simulation Type Pull-dowm Menu are non-functional in the Solutions Window since they are fixed in the Drawing Window when laying out a problem. The Drawing Button replaces the Solve Button. (These two buttons are the means of switching from Draw mode to Plot mode.) The Plot Type Toolbar replaces the Object Type Toolbar. The Plot Properties Editor provides a wide variety of potential, field, and flux solution plots. The Contours Editor provides control of the contours plots. The Cursor Position & Potential Indicator displays voltage/potential at a particular cursor location. The View Angle Editor provides control of the viewing angles of the surface plots.

2D PLOT TOOLBAR

A variety of two-dimesional displays of the solution are available through the 2D Plot Toolbar—the electromagnetic structure, the flux, the vector fields, the contours of equipotential, and a 3-dimensional surface plot of the potential. The default mode which appears immediately after the solution has been computed is the equipotentials superimposed upon the structure. The individual buttons toggle on and off each graphical feature. Any combination of structure, flux, fields, and contours can be displayed. When the 3D potential surface is activated a 3D window with its own toolbar is opened. The various buttons on the 2D Plot Toolbar function as follows.

STRUCTURE

The Electromagnetic Structure Button toggles the display of the electromagnetic structure in the Solutions Window. The structure is superimposed with equipotential contours or with certain properties of fields (as set by the Plot Properties Editor).

CONTOURS

The Contours Button activates the display of contours of equipotential for the electromagnetic structure. In the default mode, nine (9) differently-colored contours equally divide the voltage difference between the highest and lowest voltages. The potential limits and the number of contours displayed are selected in the Contours Editor. The value of voltage/potential of each contour can be labeled by clicking the labels radio button. The voltage/potential at the cursor location is displayed by the Cursor Position & Potential Indicator.

Figure 2 - VEM 2D Solution Window in default mode.

FIELDS

The Fields Button activates a display of the field vectors at all grid points. From the Plot Properties Editor a wide selection of display parameters is available by clicking on the Prop button to activate a dialog box with radio button choices of:

field parameter to display as a vector (field intensity or flux density);
vector length calculations (linearly, logarithmically, or uniform scaled);
vector color (magnitude of field parameter, white, or black);
background shading (no shading, color proportional to field intensity or to voltage/potential); and
background rendering (interpolated, faceted, or flat).

The spatial distribution of the vectors is set by clicking in the entry window and entering the desired density; the default setting is a vector in every cell. The vector properties are saved for future fields displays by clicking on the STORE Button.

Figure 3 shows the VEM 2D Solution Window with vectors activated.

Figure 3 - VEM 2D Solution Window with vectors display activated.

FLUX

The Flux Button enables visualization of the flux associated with the electromagnetic structure. Clicking on the Flux Button when in the default display window will plot the flux lines superimposed upon the structure and equipotential displays. In addition the Flux Editor is opened directly above the Contours Editor on the left side of the display window. The Flux Editor provides a number of options and information.

The flux within a flux tube is displayed in the upper box.
The number of flux lines can be adjusted with the slider immediately below the flux density box.
A single flux line can be added to the display by clicking in the Add Line radio button, moving the cursor to the point at which the flux line is to begin, and clicking the mouse. The additional flux line is drawn from the selected point in the direction of decreasing potential. If the selected location is at the lowest potential in the display window, not flux line is drawn. Flux lines can be added as long as the Add Line radio button is checked. To deactivate this feature, toggle the Add Line radio button by clicking it.
All flux lines can be cleared with the Clear button. To redisplay flux lines, move the slider to a new position.
The flux crossing a horizontal or vertical line can be displayed by clicking in the A Line radio button, moving the cursor to the desired location in the solutions window, clicking at the starting point of the line, and clicking at the ending point of the line. The total flux crossing this line is displayed in the correspoinding window. Vectors on the line indicate the relative strength of the normal component of the flux along the line. The length of the vectors is normalized to the peak value along the line. New lines can replace the original line by drawing a new line with the mouse as long as the A Line radio button is checked. To uncheck this feature, toggle the A Line radio button by clicking it.
The flux crossing a rectangular surface can be displayed by clicking in the A Surface radio button, moving the cursor to the desired location, clicking at one corner, and clicking at a diagonally opposite corner of the desired surface. The total flux crossing this surface is displayed in the corresponding window. Vectors on the surface indicate the relative strength of the normal component of the flux along the surface. The length of the vectors is normalized to the peak value along the surface. New surfaces can replace the original surface by drawing a new surface as long as the A Surface radio button is checked. To uncheck this feature, toggle the A Surface radio button by clicking it. The sign of the flux crossing the surface (but not the orientation of the vector display) by checking the appropriate Flux Leaving or Flux Entering radio button. The default choice is Flux Leaving.

Figure 4 shows the VEM 2D Solution Window with flux, contours, and structure activated.

Figure 4 - VEM 2D Solution Window with flux, contours, and structure displays activated.

3D PLOTS

Three dimensional surface plots of the potential are obtained with the 3D Plots Button. The features of this display mode are described below.

3D PLOT TOOLBAR

A variety of three-dimesional displays of the solution are available through the 3D Plot Toolbar—the electromagnetic structure, the contours, and the surface plot of the potential. The default mode which appears immediately after the 3D Plots Button is clicked is the surface and the structure displays superimposed. Any combination of the three plot modes can be used together. In addition, the 2D Plots Button activates the 2D plot modes. The features of these button are as follows. STRUCTURE

The electromagnetic Structure Button enables the structure to be seen with the surface plot. However, in the 3D mode the structure is below the potential surface plot and may not be visible from all aspects. Usually it is visible by "turning over" the surface plot to see the bottom side.

SURFACE PLOT

The Surface Button is used to display the 3D potential surface. In the 3D mode the surface can be viewed from any aspect angle by virture of the Aspect Editor that appears in the upper left side of the Solutions Window. The default aspect is -38^oazimuth and 30^o elevation, the default parameters of MATLAB. The sliders of the Aspect Editor can be used to adjust these viewing parameters followed by clicking the Apply Button. The Prop Button is not functional. An alternate means of varying the viewing aspect is to click within the 3D plot and while holding down the mouse button to move the mouse till the desired view is obtained. The sliders and the current values of azimuth and elevation change with this mouse motion. For both methods, clicking the Restore Defaults radio button followed by the Apply Button will restore the default viewing aspect.

CONTOURS

The Contours Button is used to display the contours of equipotential on the 3D potential surface. The contours are not in a plane, but lie on the surface plot. All of the contours are black so that they are visible on the surface plot.

The potential at a particular X-Y coordinate can be viewed in the Cursor Position & Potential Indicator. The coordinate position is the cursor position on the TOP surface of the 3D plot cube.

2D PLOTS

Return to the 2D plot mode is enabled by clicking the 2D Plots Button.

Figure 5 shows the VEM Solution Window with surface potential, contours, and structure activated.

Figure 5 - VEM 3D Solution Window with surface, contours, and structure displays activated.

DRAWING BUTTON

To make modifications to the electromagnetic structure, click on Structure Button. A dialog box appears as a caution that the solution data already calculated will be erased in switching from the Solution Window to the Drawing Window. The Drawing Button can be activated from both the 2D and 3D plot modes.

ADDITIONAL GEOMETRY AND SIMULATION MODES

Figure 6 shows the VEM Solution Window for cartesian geometry, magnetostatic simulation. Note that for magnetostatic simulation the Flux Button is non-functional since the magnetic vector equipotentials are equivalent to the magnetic flux for z-directed currents.

Figure 6 - VEM 2D Solution Window for cartesian, magnetostatic problem.

Figure 6 shows the VEM Solution Window for cylindrical geometry, electrostatic conductor simulation.

Figure 7 - VEM 2D Solution Window for cylindrical, electrostatic conductor problem.

SAVING/OPENING STRUCTURE FILE
The structure data is saved by use of the normal pull-down File menu of Windows. The default location for structure files is c:\MATLAB\vem\My VEM Files. Files can be saved or opened from the File menu. If the solve function was previously executed before saving a file, when it is opened it will automatically open and display the solution window.

EXITING VEM

To exit VEM the usual Windows procedures are used in the VEM window or the MATLAB window. With this action a dialog box will appear with prompts for saving the current structure file.

HINTS AND SUGGESTIONS

Maximize the VEM window for best results.
Use slow, deliberate clicks of the mouse when activating buttons. VEM lacks the robustness of a true Windows program due to underlying MATLAB functions.
Use the cursor position indicator for accurate positioning of objects in the Drawing Window.
The grid can be toggled on and off from the View menu or with CTRL-G.
The resolution of the Drawing and Solution Windows (and the corresponding grid spacing) can be adjusted from the Change Grid Spacing option of the Draw menu. The default resolution is .02 meters or 2% of the default solution region. Coarser resolutions give less accurate solutions, but are solve more quickly; finer resolutions are more accurate but take longer.
The range of the x or y axis can be changed by the Scale/Move Drawing option of the Draw menu. The default setting is 0 < x,y < 1m. Note that the range of x and the range of y are set equal in VEM.
When Dielectric/Magnetic objects overlap, VEM considers the material entered last as the proper material in the region of overlap and "overrides" the other Dielectric/Magnetic objects.
When PEC/PMC objects overlap, they are combined into a single object. PEC/PMC objects always appear above Dielectric/Magnetic objects.
When Charge/Current objects overlap, the object entered last overrides other Charge/Current objects. When Charge/Current overlaps a PEC/PMC object, the charge or current value is assigned to the PEC/PMC object and the Charge/Current object is deleted. Charge/Current objects always appear above Dielectric/Magnetic objects.