Wide pulse gated metal detector with improved noise rejection
First Claim
1. A metal detector circuit comprising:
- transmit coil means;
receive coil means;
search head means for enclosing and supporting said transmit coil means and said receive coil means in such a manner as to enable the detection of metal objects in an unobservable position external to the search head;
oscillator means coupled to said transmit coil means for transmitting an oscillator signal through said transmit coil and causing a received signal to be induced in said receive coil means during the transmission of said oscillator signal when a metal object is present thereby enabling the detection of said metal object, said received signal including a background signal component when said metal object is located in a background containing mineralized soil or undesired objects;
gating pulse generator means for producing gating pulses related in time to the oscillator signal, said gating pulses having a pulse width of at least about one third of one cycle of said oscillator signal;
a gate circuit coupled to the output of said pulse generator and having its input coupled to the output of said receive coil means, including gate means for gating a portion of said received signal through said gate means during the transmission of said oscillator signal when it is rendered conducting for a time period corresponding to the width of the gating pulse to produce an output signal corresponding to the gated portion of said received signal;
phase adjustment means coupled to the gate circuit for moving the gating pulses and received signal relative to each other to center said gating pulses at a centering position on the received signal waveform where the background signal component of said received signal is of minimum amplitude, and for causing substantially equal positive and negative polarity portions of the background signal component on opposite sides of said centering position to pass through said gate means during the gating pulse; and
noise reduction means for reducing noise, including random noise of high frequency and 60 hertz noise signals and harmonics thereof, in said output signal including an inverter amplifier having an inverting input connected to the output of said gate means and an averaging capacitor connected in a negative feedback path between the output of said inverter amplifier and said inverting input to cause said positive and negative portions to cancel and produce substantially no change in the output signal.
1 Assignment
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Accused Products
Abstract
A transmit-receive type buried metal detector is described employing wide gating pulses to transmit a portion of the received signal through a gate means to an averaging means for reducing any noise in the signal. The gating pulses are centered on the received signal waveform at a centering position where a background signal component of such received signal is of minimum amplitude. As a result, equal positive and negative polarity portions of the background signal component are transmitted through the gate means during each pulse and are averaged to zero in order to eliminate such background signal from the output voltage of the averaging means. The background signal component may be produced by mineralized soil or by undesired metal objects located near the metal object sought to be detected. As a result, the mineral soil signal component or the background signal component of the undesired metal objects is eliminated from the output voltage of the averaging capacitor. This enables detection of the desired metal object in mineral soil or enables such desired metal object to be discriminated from undesired objects. A wide gating pulse greater than about 1/3 cycle and preferably equal to approximately 1/2 cycle of the oscillator signal, is used for improved noise rejection including random noise of high frequency and harmonics of 60 hertz power line frequency which are close to harmonics of the oscillator signal.
81 Citations
18 Claims
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1. A metal detector circuit comprising:
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transmit coil means; receive coil means; search head means for enclosing and supporting said transmit coil means and said receive coil means in such a manner as to enable the detection of metal objects in an unobservable position external to the search head; oscillator means coupled to said transmit coil means for transmitting an oscillator signal through said transmit coil and causing a received signal to be induced in said receive coil means during the transmission of said oscillator signal when a metal object is present thereby enabling the detection of said metal object, said received signal including a background signal component when said metal object is located in a background containing mineralized soil or undesired objects; gating pulse generator means for producing gating pulses related in time to the oscillator signal, said gating pulses having a pulse width of at least about one third of one cycle of said oscillator signal; a gate circuit coupled to the output of said pulse generator and having its input coupled to the output of said receive coil means, including gate means for gating a portion of said received signal through said gate means during the transmission of said oscillator signal when it is rendered conducting for a time period corresponding to the width of the gating pulse to produce an output signal corresponding to the gated portion of said received signal; phase adjustment means coupled to the gate circuit for moving the gating pulses and received signal relative to each other to center said gating pulses at a centering position on the received signal waveform where the background signal component of said received signal is of minimum amplitude, and for causing substantially equal positive and negative polarity portions of the background signal component on opposite sides of said centering position to pass through said gate means during the gating pulse; and noise reduction means for reducing noise, including random noise of high frequency and 60 hertz noise signals and harmonics thereof, in said output signal including an inverter amplifier having an inverting input connected to the output of said gate means and an averaging capacitor connected in a negative feedback path between the output of said inverter amplifier and said inverting input to cause said positive and negative portions to cancel and produce substantially no change in the output signal. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. A metal detector circuit with improved noise reduction comprising:
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transmit coil means; receive coil means; search head means for enclosing and supporting said transmit and receive coil means to enable the detection of buried metal objects external to the search head; oscillator means coupled to the transmit coil means for transmitting an oscillator signal of audio frequency through said transmit coil and causing a received signal to be induced in said receive coil during the transmission of said oscillator signal when a buried metal object is present thereby enabling the detection of said metal object, said received signal including a background signal component when said metal object is located in a background containing mineral soil or undesired objects of the same type; gating pulse generator means for producing gating pulses having a pulse width approximately equal to one-half of one cycle of said oscillator signal at a time related to said oscillator signal; a gate circuit connected to the output of said pulse generator and having its input connected to the output of said receive coil, including gate means for gating a portion of the received signal through said gate means when it is rendered conducting for a time period corresponding to the width of the gating pulse to produce an output voltage corresponding to said gated portion of the received signal; phase shift means for adjusting the phase of the gating pulses and the received signal relative to each other to center said gating pulses at a centering position on the received signal where the background signal component of the received signal is of minimum amplitude, and for causing substantially equal positive and negative polarity portions of the background signal component on opposite sides of said centering position to pass through said gate means during the gating pulse; and noise reduction means for reducing noise, including random noise of high frequency and 60 hertz noise signals and harmonics thereof, in said output voltage and connected to the output of said gate means to cause said positive and negative portions to cancel, said noise reduction means including an inverter amplifier having an inverting input connected to the output of said gate means and an averaging capacitor connected in a negative feedback path between the output of said inverter amplifier and said inverting input so that any random frequency noise component of said gated portion of the received signal averages to zero in said averaging capacitor to produce substantially no change in the output voltage. - View Dependent Claims (11, 12, 13, 14, 15, 16, 17, 18)
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Specification