Consideration of air–core coil size for magnetic metal plate identification

An eddy current method is a method for discriminating metals nondestructive and noncontact. It is very important to know the changes in the frequency profile of sensor coil inductance. If an air-core coil is placed to face an iron plate, the changes contain both the conductivity and permeability information. In order to find a suitable coil size, the same values of in inductance are designed for four coils which have different diameter to generate same magnetic flux at a constant current. The frequency profiles are measured with a conventional impedance analyzer, as a function of distance between the coil and metal plate. The measured values are as same as 200 μH for four coils until a few hundred Hz. When the coils are placed to face the metal, a decrease in lift off produces an increase in inductance at low frequency, and a decrease in inductance at high frequency. There is a cross frequency at which the measured value is as same as inductance of coil only. It is found that it seems to be defined by the coil diameter, not defined by the liftoff. Introducing normalized parameters, other useful considerations are also noted.