In locating current-carrying objects, such as buried pipes in the presence of extraneous magnetic fields by detecting the magnetic field produced by the current, first and second spaced primary sensors measure the magnetic field at first and second locations, and a compensating sensor measures the extraneous field at a location midway between the first and second locations. The signal from the compensating sensor is combined with the signals from both of the primary sensors so as to compensate for the extraneous magnetic field at the primary sensors. The invention compensates for ambient magnetic fields associated with propagating electromagnetic energy that induces current in an object, and compensates for extraneous magnetic fields produced by current flow in an adjacent object.

Apparatus and method employing extraneous field compensation for locating current-carrying objects

A metal detector circuit includes a transmit coil (12) and a receive coil (56) arranged in a balanced induction configuration in an electromagnetic field. The receive signal from the receive coil (56) is input to electronic switches (146, 148) which receive quadrature reference inputs from a phase shift circuit (112). The phase demodulated outputs of the switches (146, 148) are passed through amplifiers and input to an analog-to-digital converter (324) to produce digital signal samples which are transmitted through a bus (326). The bus (326) is connected to random access memory (414, 416) and a read only memory (418) which includes a stored signal processing program. A microprocessor (374) is connected to the bus (376) for receiving the digital signal samples and the stored program from memory (418). The microprocessor (374) executes the stored signal processing program to produce a digital output signal which is transmitted through the bus (326) to a digital-to-analog converter (360). The converter (360) produces an analog output signal which is passed to an output driver circuit which produces a target indication signal at a speaker (284). The digital signal processing provided by the microprocessor (374) includes concurrent ground cancellation and discrimination without the need for operator selection of these functions. The digital output produced by the microprocessor (374), upon detection of an object in the electromagnetic field, can be displayed in digital readouts (474-482).

Metal detector having digital signal processing

A system for detecting metal targets located in a background environment such as the ground evaluates metal targets by processing components of signals at two or more different frequencies. A component of the received signal induced by the ground can be eliminated by subtracting signal components measured at two different frequencies. The received signal can be processed to account for changes in the background response with frequency. The type of an unknown target can be identified by processing background excluded components to produce data characterizing the target type.

Plural frequency method and system for identifying metal objects in a background environment

A metal detector utilizes digital signal processing and a microprocessor to process buffers of information which is received at a periodic rate. Both high and low gain phase quadrature and in-phase signals are provided via a multiplexer to an analog-to-digital converter from a first receive antenna. A second receive antenna provides phase quadrature and in-phase signals also through the multiplexer to the analog-to-digital converter. The received signals are averaged, decimated and low pass filtered to eliminate noise and reduce the quantity of data which must be processed. A threshold (triggering) processing operation is performed to determine whether a valid target signal is present in the data. If not, no further processing is performed. The in-phase and quadrature components are processed using Fourier transforms to select a frequency band which includes the energy for the target signal. The energy in this frequency band is utilized to determine the identification of the target. The depth of the target is determined by comparing the quadrature phase components received from the first and second receive antennas. The size of the target is determined by reference to a look-up table based on the depth factor and the signal amplitude determined for the target object A display screen has a plurality of horizontal depth symbols, each of which has a plurality of size indicators and upon determining the depth and size of a target, one depth symbol is activated together with one of the size indicators to concurrently display this information to an operator.

Metal detector for identifying target electrical characteristics, depth and size

A metal detector apparatus and a metal detection method for detecting and discriminatory interrogating a metal target buried in the ground. The detector includes a transmitter for providing a first varying magnetic field including at least two frequency components and a receiver for providing a resultant voltage dependent upon a second magnetic field resulting from the combined effects of the ground and metal targets buried in the ground, upon the first alternating magnetic field. The detector samples at least three voltage measurements of the resultant voltage over three different time periods to provide three sampled values which are then processed by a microprocessor to provide three average ground balanced signals from which at least two ratios are calculated and compared against a pre-defined response. The method of detecting and discriminating between metal targets includes providing a first varying magnetic field having at least two frequency components and receiving a resultant voltage dependent upon a second magnetic field resulting from the combined effects of the ground and metal targets buried in the ground, upon the first alternating magnetic field. The resultant voltage is sampled and processed over three different time periods to provide three average ground balanced signals from which at least two ratios are calculated and compared against a predefined response. This comparison is used to distinguish between ferrous and non-ferrous metals.

Metal detector for detecting and discriminating between ferrous and non-ferrous targets in ground

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.

George C. Payne

Papers

Metal detector system with ground effect rejection