Non-destructive detection of metallic objects under AC magnetic field illumination

We have investigated the detectability of objects in AC magnetic fields using Finite Element Analysis (FEA) simulation. A vertical AC magnetic field of 1mT was used with different frequencies. Magnetic images were obtained by observing the Bz component at different stand-off heights. In all cases, the measurements were taken vertically above the objects. Non-magnetic (Al) and magnetic (Steel 416 and 1010) cylindrical objects were investigated. In vertical field illumination configuration, three characteristic frequencies were identified. At 50 Hz, magnetic and non-magnetic objects became distinguishable based on their magnetic responses. At 1 kHz, it was possible to differentiate between different magnetic materials. Similarly, at 5 kHz it was possible to distinguish between stainless Steel 416 and other magnetic materials. It was also observed that magnetic materials always produced higher magnetic values/contrasts at lower frequencies compared to those of non-magnetic materials. As the frequency increased, a field inversion was observed for the magnetic material with lower conductivity. Since this field inversion frequency is related to the conductivity of the magnetic material, it is therefore identified as a characteristic frequency of that particular magnetic material. This field inversion is due to the Eddy current produced within the magnetic material. It is observed that for the simulation setting used, a stand-off height between 1 to 2 cm is essential to obtain well-resolved magnetic images. Experiments performed using a Quantum Well Hall Effect (QWHE) sensor-based 2D AC/DC camera validated the simulated outcomes.