GB1534039A - Metal detector - Google Patents

Abstract

1534039 Metal detector CANDLE INTERNATIONAL RESEARCH & DEVELOPMENT Ltd 13 Feb 1976 05845/76 Heading G1N A metal detector includes a search head having transmit and receive coils 1, 3 respectively, the former energized directly by a 2À5 kHz oscillator 2. The receive coil output signal is passed to a gate circuit 5. The oscillator output signal is also fed to a shaper 6 producing from it an alternating squarewave which is differentiated 7 and the resulting pulses fed to a delay circuit 8. This produces an output pulse of adjustable duration in response to each pulse from the differentiator. A further differentiator 9 is arranged to produce a pulse at the trailing edge of each pulse from the delay circuit. Thereby a gate pulse shaping circuit 10 controls the gate 5 so as to sample the signal from the receive coil with a phase delay determined by the adjustment of the variable delay circuit 8. In use, the search head is moved alternately between a free air position and a position adjacent to the ground and the delay 8 is adjusted so the ammeter 13 reading does not change for such movement. This represents the condition where the sampling point c corresponds with the crossover point of the two waveforms A, B produced from the receive coil at the respective positions. The voltage from adjustable D.C. source 12 is then set to null the gate 5 output in comparator 11. Thus the effect of the conductivity of the ground to be searched is nullified. Equally the circuit may be adjusted to nullify the effect of cigarette packet foil, which is said to require a phase delay of 180 degrees.