How Eddy Current Works
When an oscillator is used to generate an AC signal in a coil, an alternating magnetic field is generated in the coil. When the coil is placed near an electrically conductive material, eddy currents are induced in the material. These eddy currents will generate a secondary magnetic field which will induce a secondary current back into the coil. This secondary current will cause a change in impedance of the coil. The eddy currents that flow in the material are affected by geometry, hardness, temperature and metallurgical make up of the material (especially near the surface). Therefore, the impedance change of the coil is affected by these same factors.
During eddy current testing, an oscillator driven coil is placed in a specific location on an acceptable part (master) near a test feature (machined detail, bushing, nut, etc.). An impedance reading is taken by the system and is used as a reference zero by the system. The same coil is then placed in the same relative position on a test part. If the test part is the same material, temperature and hardness, geometrical differences can be detected. Likewise if the test part is the same material, temperature and geometry, hardness differences can be detected. Because temperature, material, hardness and geometry are never exactly the same even on seemingly identical parts a window is set up to allow for slight variations in the parts. When a part with acceptable impedance reading is detected, the solid state relay is energized indicating the part has passed.
