Principle of eddy current testing:

Eddy current detection is the application of electromagnetic induction principle, which excites the probe coil with a sine wave current. When the probe approaches the metal surface, the alternating magnetic field around the coil generates induced current on the metal surface. For flat metal, the direction of induced current is a concentric circular coil, resembling a vortex, called an eddy current. At the same time, eddy currents also generate a magnetic field of the same frequency, which is opposite to the direction of the coil magnetic field. The loss resistance of the eddy current channel, as well as the anti magnetic flux generated by the eddy current, are reflected back to the probe coil, changing the current magnitude and phase of the coil, that is, changing the impedance of the coil. Therefore, when the probe moves on the metal surface and encounters defects or changes in material, size, etc., the reaction of the eddy current magnetic field on the coil is different, causing changes in the coil impedance. By measuring this change with an eddy current detection instrument, the presence of defects or other physical and material changes on the metal surface can be identified. There are many factors that affect the eddy current field, such as the degree of coupling between the probe coil and the tested material, the shape and size of the material, conductivity, permeability, and defects. Therefore, the principle of eddy current can be used to solve problems such as metal material inspection, thickness measurement, and sorting

技术参数：

Sensitivity of flaw detection: cracks and defects that extend from the surface of the workpiece to the inside of the metal with a depth of ≥ 0.05 mm and a length of ≥ 2 mm; For cracks inside the metal that have not yet extended to the surface of the workpiece, cracks with a depth of ≥ 0.20mm and a length of ≥ 1mm can be detected when the metal thickness between the upper edge of the crack and the surface of the workpiece is ≤ 0.2 mm.

Point impedance includes reactance and impedance. When displaying information, we use impedance R (Resistance) as the x-axis and reactance X (Resistance) as the y-axis to generate Cartesian coordinates. Based on the impedance changes of eddy current detection sensors, point information (Q) can be placed on the instrument and equipment for display based on signal analysis. Point Q is a two-dimensional vector material point with a certain amplitude and phase difference. Due to various reasons, the signal components R and X of eddy current data change, causing the position of point Q to also change. The trajectory of point Q is an impedance plane diagram. When the relative position between the inspection coil and the tested raw material changes, the relative density of eddy current caused by the inspection coil on the raw material changes. The relative density of eddy current decreases as the distance between the inspection coil and the raw material increases, which in turn causes the vector material point Q of the digital ultrasonic flaw detector to move on the display information plane. This type of situation is called the lifting effect. The basic principles of eddy current testing and magnetic particle testing can be applied to measure the thickness of non-metallic material coatings on metal surfaces.