Conventional electromagnetic testing methods based on inducing alternating current (AC) are confined to detect facial defects because of the skin effect. Aiming at nonferrous metal detection, this paper proposes an injected direct current field measurement (IDCFM) based on a tunnel magneto resistance (TMR) sensor. By injecting a direct current (DC) to nonferrous metal, a constant magnetic field will be excited around the surface of nonferrous metal. As cracks exist inside or near the surface of the nonferrous metal, the current distribution will be disturbed, which causes the change of external magnetic field. Therefore tiny cracks on the surface and inside defects can be detected and located by using a TMR sensor with high sensitivity. A three-dimensional finite-element method was employed to model the magnetic field produced by the DC current. The difference in magnetic field under different magnitudes of direct current was discussed. Simulation results showed a linearity between the external magnetic field intensity and the injected DC intensity. Contrast experiments with coils, Hall elements, GMR sensors and TMR sensors were also carried out. IDCFM signals with different sensors were compared under the same current intensity. The performance of four different sensors under different injected direct current was also compared through experiments. Experiments indicated that TMR sensors have many advantages over other sensors. Applying TMR sensors in IDCFM method, the precision could be 10 times greater than that of GMR sensors, and hundreds times greater than that of Hall elements. Also, the current intensity in IDCFM could be reduced from 100 A to less than 1 A utilizing TMR sensors, which allows IDCFM method to operate continuously for a long time without damaging the workpiece.
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