High-Resolution Magnetic Imaging Probe With Staggered Sensor Arrays for Small Defects Inspection

The earlier deficiencies are detected, the better health and safety of an industrial structure can be maintained. To this aim, tools that can detect small defects are required. However, it is still a challenging problem to detect small defects economically and reliably. This article presents a novel high-resolution magnetic field imaging probe with two rows of tunneling magnetoresistance (TMR) array sensors. The sensors are integrated on a printed circuit board (PCB) by golden wire bonding technology. The probe outputs absolute and differential signals simultaneously. The distance between the centers of each of two adjacent sensors in one row is 0.5 mm resulting in high spatial resolution magnetic field images. The two rows of sensors are placed with a 0.25-mm offset and the differential outputs of the sensors are merged to further increase the image pixel pitch to 0.25 mm. The operating principle of the probe is studied based on a 3-D finite element method (FEM) model, and the feasibility of detecting small defects using the probe is tested experimentally. The experimental results show that a small defect with dimensions 1 mm (length)
$\times0.1$
mm (width)
$\times0.1$
mm (depth) can be detected by the probe. In addition, an image processing algorithm that employs the gray-level co-occurrence matrix (GLCM) method is developed to reduce the noise in the image, which retains the large connection area of the texture features to extract the indications of defects. It is found that the probe has outstanding detection ability for small defects compared with current eddy current testing (ECT) array probes.