White's Electronics, Inc.

Pni Corporation

George C Payne

A metal detector uses a transmitting search coil inductively coupled to a receiving coil for detecting the presence of metal objects near the surface of the ground within the field of the coils. An oscillator generates a signal transmitted by the transmit coil, and the signals detected by the receive coil are coupled to the signal inputs of two synchronous demodulators. The output of the oscillator also is applied at different phases to the reference signal inputs of the two synchronous demodulators. The outputs of these demodulators then are passed through low pass and bandpass filters having a low cutoff frequency which is higher than the highest frequency components generated in the synchronous demodulators due to ground effects. The signals passing through the bandpass filters then are applied respectively to the signal input and reference signal input of a third synchronous demodulator, the output signal of which has an amplitude representative of the presence of metal objects and the polarity of which is an indication of the type of metal being detected. Undesired signals produced by ground effects are reduced by a considerable amount, and the output of the third synchronous demodulator is applied to a suitable indicator circuit.

Metal detector system with ground effect rejection

A metal detector for detecting and distinguishing between ferrous and non-ferrous objects in an area which may contain a ferrous, mineralized soil background is disclosed. The hysteresis effect is used to distinguish between ferrous and non-ferrous objects by generating groups of transmit current pulses, each group including a polarizing pulse of one polarity followed by a pair of read pulses of the opposite polarity and less magnitude. The transmit pulses are applied to a transmit coil to induce corresponding receive voltage pulses in a receive coil. A pair of receive pulses corresponding to the read transmit pulses are fed through a low pass filter and thereafter sampled to produce a sample output signal corresponding to the difference between the sampled voltage levels of the pair of receive pulses. Control means are provided to adjust the sampling times such that when the metal detector is placed over an area containing a ferrous, mineralized soil background and no metallic objects, the sample output signal will be a constant D.C. voltage. This eliminates the effect of mineralized soil, which otherwise tends to prevent the detection of metallic objects. Thereafter, as the metal detector is moved around and used to search for metallic objects, it will indicate the presence of a ferrous object by a first polarity sample output signal and will indicate the presence of a non-ferrous object by an opposite polarity sample output signal. Demodulator means, such as a balanced synchronous demodulator, is provided to generate detector outputs corresponding to the ferrous or non-ferrous sample output signals and to nullify the effect of relative motion between the metal detector and the objects being detected.

Ferrous/non-ferrous metal detector using sampling

A sampling type of metal detector is described in which an oscillator transmits an oscillator signal current through the transmit coil of the detector to produce an electromagnetic field in the region of a search head. A received signal is induced in the receive coil of the detector when a metal object is present within that portion of the electromagnetic field which is external to the search head. A sampling pulse generator produces sampling pulses, at a time related to the output signal of the oscillator, which are transmitted to a sampling circuit for sampling the received signal. In the preferred embodiment, the received signal is sampled at a time controlled by a phase shift adjustment means to produce an output sample voltage such that any indication due to mineral soils or other background conditions is excluded from the sample.

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Tempe, AZ, United States of America

George C Payne