In order to understand the nature of the instant invention, it is necessary for the reader to have a basic understanding about how metal detectors operate, since this invention is a device usable with a metal detector to refine the detecting capability of the detector, i.e. to render it more selective.
Early detectors were designated as "balanced coil" search units. They used two identical coils, each with a primary and a secondary winding. The primary windings are driven in series by AC current and thereby generating an alternating magnetic field. In a nonmetallic arena the induced voltages of the secondary windings will be equal and therefore no signal is produced. But when a metallic coin or other item is near one of the coils, it produces an induced magnetic field which interacts with the closer of the two secondary windings. The delta or net difference between the signals of the two coils can be amplified to create an audible indication of the presence of the coin or like item. Magnetic induction detectors of this nature are limited to shallow depths of about six inches or so.
The more modern detectors are of the "heterodyne" designation. While operating on the principle of induced magnetism with two coils, each of the coils have only one winding. Each coil is connected to its own oscillator. The two oscillators are adjusted to the same frequency in order for the two coils to produce an alternating magnetic field at the chosen frequency. When a metallic object is located by the detector, the coil which is closer to the object has its inductance changed which thereby changes the frequency of the oscillation of that circuit with respect to the other circuit. When the signals from the two circuits are mixed together, a beat is generated whose frequency can be amplified as may be necessary and reproduced on a meter and through earphones to enable the operator to both see and hear the found metallic item. While reference is made to FIG. 1 which illustrates these principles, the reader need not refer there now for an understanding of the discussion to follow.
Since persons doing metal detection want to discriminate between common nails and gold coins, between aluminum beverage cans and silver bracelets, means have been developed to help discriminate between the "desired targets" and "undesired targets". However oftentimes the very sand in which one is operating or the rocks nearby contain certain metallic minerals which can influence the detector. These are referred to in the trade as "mineralogy". But what happens when the gold coin is in the tin can. The detectors available today can not screen out one from the other. That is the function of the invention of this application.
It is an object therefore of this invention to provide an accessory for use with metal detectors to screen out unwanted targets when such are present with desired targets.
It is another object to provide a phase shifting device which allows the metal detector to eliminate or screen out the iron and not hear it, whereby on passage over a mix of nails and a gold ring only the ring is heard or sensed.
It is yet another object to provide a device for use with a metal detector to permit the detector to selectively provide information on the presence of a desired target which desired target is associated with undesired target(s).
A further object is to provide a device which influences the mode in which the detector processes the information concerning located targets.
A yet further object is to provide a means for the metal detector to broaden its phase response.
A still further object is to provide a tuneable phase shifting device tuneable over the phase angle range of from about -85 to about -55, capable of "blinding" the detector to mineralization and iron such that while detected, no audible or meter reading change will be presented to the user.
An additional object is to provide a device which nulls out the signal obtainable from a metal detector due to the presence of predefined undesirable targets when such are in the presence of desired targets.
Other objects of the invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises the product possessing the features, properties and the relation of components which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the appended claims.
For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings.