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Simulation tools similar to SPICE are an important part of the design process for many systems because they reveal design problems before prototypes are built. SPICE, however, is not suitable to simulate inductive sensing systems because it does not include target interactions with the sensor coil. The open-source magnetics simulation software FEMM (Finite Element Method Magnetics) can provide information about these target interactions and the robustness of the system before production. Before using FEMM, we recommend using the LDC Tools Excel calculator to design the sensor coil, as well as reading LDC Sensor Design and LDC Target Design. For smooth targets that are at least as large as the sensor coil, TI also recommends using the Excel_FEMM tool (included in LDC Tools), which will automatically simulate custom sensors in FEMM. For smaller or irregularly shaped targets, the following guide will give the reader working knowledge of FEMM for inductive sensing simulations. The program can be downloaded from the FEMM home page.
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:
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.
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.
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.
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: