# Using Magnum to calculate flux coupling

A customer working with RFID devices asked me about finding magnetic flux inside a detector coil surrounded by an array of drive coils. The drive coils may move around or rotate. There are no iron structures. The calculations are fast and easy using the free-space mode of Magnum. In this post, I’ll review the required techniques.

Detector coil position and current elements of the drive coil

The first figure shows a test geometry. The detector coil has a rectangular cross section (?x = 6.0 cm, ?y = 3.5 cm). For convenience, I located it in the plane z = 0.00 cm. I included one circular drive coil of radius 5.0 cm with center at (x = 5.0 cm, y = 5.0 cm and z = 5.0). To illustrate parameter variations, I rotated it 45° around the x axis. Here is the coil definition file created in MagWinder:

GLOBAL
DUnit:   1.0000E+02
END
COIL
Name: RotatingCoil
Current:   1.0000E+00
Ds:   2.5000E-01
Shift:   5.0000E+00  5.0000E+00  5.0000E+00
Part
Name: RotatingCoil
Type: Circle
Fab:   5.0000E+00
* Change ThetaX to rotate the coil
Rotate:    45.000    0.000    0.000  XYZ
End
END
ENDFILE

The coil can be rotated by editing the part in Magwinder or simply changing the value 45.000 with a text editor. The coil definition file is processed with Magwinder to create a list of 126 short current elements to calculate the applied field at the position of the diagnostic coil.

In a Magnum free space solution, the computational mesh defines a set of node points to calculate the applied field. The field at other points is determined by interpolation. All elements have ?r = 1.0. Usually, the mesh is a single-region box that surrounds the volume of interest. In this case, I used a special region division to enable automatic flux calculations. The figure below shows the mesh, extending over a range that encloses the detector coil (-5.0 ? x ? 5.0, -5.0 ? y ? 5.0, -1.0 ? z ? 1.0). The upper and lower spaces in z are divided into Region 1 and 2. Region 3 occupies the lower space in z and has the cross-section dimensions of the detector coil (?x = 6.0 cm, ?y = 3.5 cm). The important point is that the surface between Region 3 and 1 corresponds to the area enclosed by the detector coil.

Computational mesh with special diagnostic regions

The Magnum solution takes only a few seconds. MagView is then used to analyze the solution. To calculate the flux integral, click on Analysis/Surface integral to show the dialog illustrated below. We want to find the flux out of Region 3 into Region 1 (i.e., a positive value corresponds to Bz > 0.0). Therefore, we set the dialog so that Region 3 is internal and Region 1 is external. The program determines integrals over the defined surface and records the results in a data file (shown below):

Dialog to define surface integrals

———- Surface Integrals ———-
Region status
RegNo   Status    Name
===================================
1  External   AIRUPPER
3  Internal   COILOUTLINE
Surface area of region set (m2):   2.100000E-03
MagFlux:  -2.537678E-09

As a check, we confirm that the area equals 0.060 m × 0.035 m. The flux implies an average flux density <Bz> = 1.208E-6 tesla. Inspection of a plot of Bz in the plane z = 0.0 cm shows that this is a reasonable value.

Plot of Bz over the plane z = 0.0 cm