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Metal detector and classifier with automatic compensation for soil magnetic minerals and sensor misalignment

  • US 4,628,265 A
  • Filed: 04/22/1983
  • Issued: 12/09/1986
  • Est. Priority Date: 04/22/1983
  • Status: Expired due to Fees
First Claim
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1. In apparatus for locating a metal object:

  • a transmitting coil, means for applying a square wave excitation voltage to the transmitting coil to transmit an alternating magnetic field, a receiving coil located within the transmitted field for producing a signal having components of different phases corresponding to metal objects and other disturbances in the field, first and second synchronous demodulators for demodulating the received signal at the phase of the transmitted field and in quadrature with the phase of the transmitted field, means for providing a square wave signal in quadrature with the transmitted field, an integrator for providing a signal of generally triangular waveform in response to the square wave signal in quadrature with the transmitted field, means for clipping the triangular signal to provide a sinusoidal signal at the fundamental frequency of the magnetic field, means for applying the sinusoidal signal to the first demodulator, a differentiator responsive to the sinusoidal signal for providing a signal which is shifted in phase by 90 degrees with respect to the sinusoidal signal, means for applying the phase shifted signal to the second demodulator, means responsive to one of the demodulated signals for indicating the presence of a metal object in the field, a bandpass filter responsive to the received signal for passing a signal corresponding to the third harmonic of the received signal, a third synchronous demodulator for demodulating the third harmonic signal at the phase of the transmitted field, means for applying a reference phase signal to the third demodulator at the phase of the transmitted field, means for integrating the demodulated third harmonic signal, means responsive to the integrated signal for proving a correction signal in phase with the transmitted field, and means for combining the correction signal with the received signal to null out a component of said signal of the phase of the transmitted field.

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