2 Inductive Sensor Simulation

1. Open FEMM
2. Click File → New
3. In the Create a new problem dialog box, select Magnetics Problem and click OK.

Figure 2-1 Create a New Problem Dialog Box

When choosing between the two problem types, consider the geometry of the coil. For both planar and axisymmetric problems, the FEMM workspace forms a cross section of the coil and target. The workspace is a slice of the PCB coil such that all layers and traces of the PCB are visible. Keep this in mind while constructing the coil. It is important to construct the cross section such that the simulation will create a coil shape. See how each problem type turns the 2D workspace into a 3D solution:

1. Planar problems:

   Planar problems expand along the axis perpendicular to the plane. The user must specify the distance of this expansion. This type of problem is most often used to simulate racetrack coils. However, planar problems cut off the rounded edges of the coils, leaving only the long parallel section. This makes the simulation much less accurate than spiral coil simulations using axisymmetric problems.

   Figure 2-2 Example Planar Problem Workspace With Labeled Traces and Target
2. Axisymmetric problems:

   Axisymmetric problems rotate around a user-specified axis. This type of problem is the most useful for inductive sensing simulations and should be used for all circular coils. Note that coils simulated with this problem type will not be true coils, but will be a series of concentric circles. Even so, the coils are fairly accurate simulations.

   Figure 2-3 Example Axisymmetric Problem Workspace With Labeled Traces and Target

1. Navigate to the Problem menu shown in Figure 2-4.
2. In the Problem Definition dialog box, choose planar or axisymmetric for the Problem Type. If “planar” is selected, enter the depth of the problem expansion in the Depth field.
3. Enter the appropriate length units for the coil design in the Length Units field. Typically mm or mils are most useful.
4. Define a Frequency (Hz). All magnetic problems must have a defined frequency. For inductive sensing problems, this is comparable to choosing a capacitor for a known inductor.

   Use the Spiral Inductor Designer in LDC Tools to calculate a frequency and enter that frequency in the problem Frequency (Hz) field. If the FEMM simulation includes a target, use the sensor frequency with target interaction.

5. In the AC Solver field, select Succ. Approx. Ignore the other fields and click OK.

Figure 2-4 Problem Definition Dialogue Box

1. Use this button to place nodes. Use either segments or arcs to connect them. At least two nodes must be placed before placing segments. These will form the boundaries of your shape(s).

   Use the trace width, trace spacing, and trace thickness from the Spiral Inductor Designer to create a cross section of each trace. Here are key tips for placing objects:

   1. It can be challenging to place nodes exactly in-line with each other. It may be easier to place one node, copy it, and specify X- and Y- translations for the new nodes. Note that the units are specified in the Problem Definition menu.
   2. Click close to the desired nodes to place segments or arcs on the node.
   3. Though the coils in this tutorial will use rectangular traces, the best approximation of real traces is a trapezoidal shape.
2. Other helpful tips when moving objects:
   1. Right-click near an object to select it.
   2. Double-right-click the object to show the X,Y position of the object.
   3. Navigate with keyboard arrows.
   4. Click the Page Up and Page Down keys to zoom in and out.
   5. Undo only works ONCE.
   6. Click the button to move objects.
   7. Click the button to copy objects.
   8. Click the button to select a group of objects. Then click the button to highlight objects in a rectangular area or the button to highlight objects in a circular area. Press the space bar to join the objects into a group.
   9. Click the button to scale
3. For axisymmetric problems only: Mouse over the leftmost segment or arc or double-right-click the arc and look at the bottom left corner of the screen to determine the X,Y location of the object. Select all drawn objects and translate them left so that the leftmost segment or arc now has an x-value equal to the inner radius of the sensor coil. The axis of rotation will be the y-axis, so offsets in the x-direction will form gaps or holes in the object.

1. Click the button to open the Boundary Builder.
2. The Boundary Builder builds the model, shows the axis of symmetry, and defines several boundaries that form a gradient to infinity.
3. Leave the values in the Open Boundary Builder dialog box unchanged and click OK.
   Figure 2-5 Boundary Builder Dialogue Box
4. The workspace should now look similar to Figure 2-6 for axisymmetric problems or similar to Figure 2-7 for planar problems:
   Figure 2-6 Axisymmetric Problem With Boundaries
   Figure 2-7 Planar Problem With Boundaries

1. Navigate to the Properties → Circuits menu. In the Property Definition dialog dox, click Add Property.
2. In the Circuit Property dialog box, name the circuit and specify an operating current.
   1. The current value is not especially important. A value anywhere from 1 mA to 1 A is acceptable.
3. Make sure the Series option is checked and click OK.
   Figure 2-14 Circuit Property Menu
4. For planar problems only:
   1. For racetrack coils and similar circuits, the parallel traces are not automatically connected by the planar expansion. At least two separate circuits are required to correctly model the current flow.
   2. To model the current flow, define two circuits with the same current magnitude. One should be positive and the other should be negative. Assign one to each side of the racetrack coil. Your workspace should look similar to Figure 2-15:
      Figure 2-15 Example Planar Problem Workspace With Defined Circuits for Racetrack Coil
5. Right-click the material nodes again and click the space bar to return to the Properties for selected block menu.
   Figure 2-16 Properties for Selected Block Menu
6. 1. Select the circuit that you just created in the In Circuit field.
   2. Specify the number of turns in the coil. This will not change the size of the coil and only change the generated magnetic field. The simulation will yield the most realistic results if only one turn is used per coil boundary and the number of coil boundaries is equal to the number of turns in the real coil.
   3. Leave the Let Triangle choose Mesh Size box selected.
7. Repeat this process for all boundaries in the coil. The workspace should now look similar to Figure 2-17:
   Figure 2-17 Axisymmetric Problem Workspace With Defined Circuits

1. Click the Solve button to generate the answer file.
2. Answer files will be saved in the .ans file format in the current working directory.