When mention metal detectors, people will immediately think of the "treasure" boy story. In fact, there are not only mineral exploration detectors and landmine detectors, but also various detectors such as food inspection and fiber foreign bodies that are widely active in the industry.
Sensors have different shapes due to different ways, such as figure 1. The sensor is basically composed of a primary coil and a secondary coil, and is configured so that the magnetic fluxes generated by the primary coil just disappear when passing through the secondary coil. When approaching the metal body, the path of the magnetic flux changes slightly, the balance is broken, and a signal appears at the output.
Figure 1. Metal detectors
In practice, however, the amplitude of the output is so weak that it is almost indistinguishable from the leakage signal of the sensor without the metal. Therefore, a highly sensitive phase-locked amplifier is used, adjusting the phase of the reference signal so that the output of the PSD is zero in the absence of metal. When approaching the metal, small changes in phase are captured as output changes for the PSD.
Such metal detectors use a phase-locked amplifier to detect small changes in the sensor's transfer function. In addition, depending on the size and type of metal body, the sensor output amplitude and phase change (vector change) trajectory is also different. It is also possible to determine the type and quantity of foreign bodies based on this trajectory.