White's Electronics, Inc.

Donald K Shoemaker

Mark W Rohde

Peter T Smith

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

Plural frequency method and system for identifying metal objects in a

A housing for the magnetic induction coils of the search loop assembly of a metal detector is constructed of plastic resin reinforced with non-metallic electrically conductive graphite fibers, and is connected to ground potential of the main electronics package of the metal detector so that the housing acts as a static-draining shield for the search loop assembly. The mixture of graphite fibers and plastic conducts electricity well enough to prevent accumulation of static electricity charges and makes the housing stronger and more abrasion resistant than similar housings of non-reinforced plastic. The housing may be molded as two halves to include a central aperture, to allow the user of the metal detector to see the precise location on the ground at which to dig for detected targets. Upstanding walls concentric with the perimeter of the housing may be included in one half of the housing, to be used as spools for transmit, receive and balance coils wound directly onto these spools, simplifying the manufacture of a search loop assembly. Polyurethane foam is expanded in place in a cavity defined in the interior of the housing, further strengthening the search loop assembly. The density of the foam may be chosen to result in a search loop assembly having neutral buoyancy in fresh or sea water.

Static charge dissipating housing for metal detector search loop assembly

A portable transmit/receive induction balance type metal detector having an automatic ground exclusion balance (GEB) feature to facilitate the screening out of the effects of mineralized ground. The circuitry provides a static GEB in a single try and then, if selected by the user, continual automatic GEB on a dynamic basis as the system is used. The circuit includes a pair of quadrature phase detectors, one of whose outputs is approximately in phase with the ground component of the loop return signal. That output is modified, during an air balance of the detector, to remove the residual offsets in the circuit. This modified output in turn is summed with the other phase detector output to produce a phase detection signal that can be adjusted by automatic means to be precisely in quadrature with the ground component of the loop return signal. A controller is provided, with selectable user inputs, to initiate the static and dynamic GEB operations. An error amplifier in conjunction with two sample/hold elements and a variable duty cycle chopper operate in a feedback loop to appropriately adjust the phase detection signal. Audible beep indications are generated to signal the user when the steps in the automatic GEB are completed.

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Sweet Home, OR, United States of America

Donald K Shoemaker