Metal detector is used to detect metal foreign matters mixed in food, medicine, cosmetics, textile and other production processes Its principle is a central transmitting coil and two equivalent receiving coils, which are installed in a detector head. The oscillator emits a high-frequency magnetic field through the middle 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 them cancel each other. Once the metal impurities enter the magnetic field area, this balance will be destroyed, and the induced voltage of the two receiving coils cannot be offset. The non offset induced voltage is amplified by the control system, and the metal detector can detect the presence of metal and generate an alarm signal (metal impurities are detected). The system can use the alarm signal to drive the automatic elimination device, so as to exclude metal impurities from the production line.