Eddy current testing is the use of electromagnetic induction principle, the sine wave current to stimulate the probe coil, when the probe close to the metal surface, the coil around the alternating magnetic field on the metal surface to produce inductive current. For flat metal, induction current flow in concentric circular coils, the shape of vortex, called eddy current. At the same time also produce the same frequency eddy current magnetic field, the direction and the opposite direction of the magnetic field coil.

The loss resistance of the eddy current channel and the reverse magnetic flux produced by the eddy current reflect the coil of the probe and change the current and phase of the coil, which changes the impedance of the coil. Therefore, the probe in the mobile metal surface, encountered defects or material, size change, the different effect on the eddy current magnetic field of the coil, the coil impedance change caused by eddy current testing instrument, measuring the change quantity can identify metal surface flaws or other physical properties change.

There are many factors that affect the eddy current field, such as the probe coil and the material to be measured the coupling degree of material, shape and size, conductivity, permeability, and defect. Therefore, the use of eddy current principle can solve the flaw and thickness of metal materials, sorting and other issues.