The working principle of metal detector is very simple. A central transmitting coil and two equivalent receiving coils are installed in one exploration head. The oscillator emits a high frequency magnetic field through the central transmitting coil, which is connected with the two receiving coils, but the polarity is opposite. When the magnetic field is not disturbed by the outside world, the voltage output signals generated by the two coils cancel each other out. Once the metal impurities enter the magnetic field area, the output signals of the two output coils can not offset, and the metal detector can detect the presence of metal. The structure of the metal foil detector is slightly different. It is wound around a coil frame by many copper cables. There are many fixed pole magnets in the coil frame. Ferromagnetic metal will cut the magnetic field and generate current in the coil winding to detect metal impurities.