METHOD AND APPARATUS FOR SELECTIVELY DETECTING METAL

摘要

PURPOSE:To detect a specified metal with a lessened intensity of an AC bias magnetic field, by exiting a metal to be detected in an AC magnetic field with a required frequency to detect a harmonic distortion component intrinsic to metal based on a harmonic component integrally multiplying the AC magnetic field. CONSTITUTION:Magnetic field generating coils 3 and 4 outputs a sine wave of about 3-5KHz with an AC power source 7 to excite metal attached to an article passing through a detection zone. The metal is mounted to a price tag sheet and generates a harmonic component proper to the metal. This component is detected with detection coils 5 and 6 to be converted into an AC signal and the harmonic component alone is inputted into a synchronous amplifier 9 with a high-pass filter 8 to amplify an Norder harmonic alone according to a frequency output N fold as much as a fundamental wave from a PLL circuit 10. The N-order harmonic signal is converted into a DC level signal with a low-pass filter 11 and compared with a reference value by a DC comparator 12 to detect a specific metal.