Fast estimation of coil factor and orthogonality errors in tri-axial magnetic coil systems

We investigate methods used for calibrating magnetic sensors and tri-axial magnetic coil systems that are used to generate the calibration field for magnetometer sensor calibration. Since existing magnetic coil calibration techniques found in the literature either did not fully account for secondary errors such as orthogonality, did not account for time-varying noise levels during measurement, or were too time-consuming, existing magnetometer calibration algorithms were evaluated for use in calibrating coils. The TWOSTEP and Geometric Approach algorithms were extensively tested using synthetic data. The TWOSTEP algorithm was selected to be adapted to enable its use to rapidly calibrate tri-axial magnetic coils using an automated procedure to determine coil factors (i.e. the proportionality constants relating the excitation currents to the induced magnetic fields) and coil orthogonality alignment errors. Experimental measurements on an 8 m length, 2 m diameter tri-axial coil system, consisting of two orthogonal sets of cos(θ) coils and a solenoidal coil, show that the automated procedure enables an accurate tri-axial coil calibration to be conducted with 90 s of measurement data, sampled at 6.25 kHz. Furthermore, this method was shown to significantly reduce the difference between the predicted and generated magnetic field—as measured by a calibrated reference magnetometer, compared to using coil factors derived theoretically from geometric coil parameters. (paper)