Freescale Sensor Fusion Library for Kinetis MCUs, Rev. 0.7, 9/2015

Freescale Semiconductor, Inc.

Figure 12. Distorted (left) and Corrected (right) Magnetic Field Data (simulated)

Both hard- and soft-iron interferences are a function of the sensor environment, and not the sensor

itself. Each product design will inevitably result in different distortions.

Engineers assigned the task of physically designing PCBs and housings should pay careful attention to

sources of magnetic interference early in the design phase.

Inductive charging films found in some portable devices exhibit a significant amount of magnetic

hysteresis. This is a nonlinear phenomena and cannot be fully corrected by the Freescale magnetic

calibration library.

4.7.2

The Magnetic Buffer

Freescale’s magnetic calibration library performs a total least squares fit of a number of data points to

map the measured ellipsoid of measurements back into the ideal sphere. Quality of that fit improves as

number and spacing of samples across the ellipsoid surface increases. There is a tradeoff in terms of

data set size used for calibration versus CPU resources versus quality of fit.

Figure 13 shows improvement in standard deviation of computed results (for a uniform field) versus

constellation size.