Metal detector with a resonating circuit being driven by a frequency higher than its natural resonance frequency

• US 3,896,371 A
• Filed: 12/17/1973
• Issued: 07/22/1975
• Est. Priority Date: 12/17/1973
• Status: Expired due to Term

• Current Assignee: Allen W. Hametta
• Original Assignee: Allen W. Hametta
• Inventors: Hametta, Allen W.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US05/425,520
• Time in Patent Office: 582 Days
• Field of Search: 324/3,5,41,40 340/258C 331/65
• US Class Current: 324/327
• CPC Class Codes: G01V 3/102 by measuring amplitude

Abstract

Concealed metal objects such as coins are detected by a metal detector including a novel discriminating circuit which discriminates between coins and unwanted metal objects such as ferrous and non-ferrous pull tabs from pop and beer cans. The discriminating circuit includes an L/C circuit which is forced to resonate at a frequency significantly higher than its natural resonance frequency. This L/C circuit is adapted to provide an output signal having different amplitudes depending on the type of metal object which is in the proximity of the L/C circuit. The L/C circuit is physically located in a sensing head or probe which the searcher moves over a given search area. When the probe is proximate a metal object, this object alters the resonance characteristics of the L/C circuit and the amplitude of the output signal from the L/C circuit. As the probe moves towards metal objects such as pennies, the amplitude of the output signal increases. As the probe moves towards other types of metal objects such as pull tabs, the amplitude of the output signal remains about the same or diminishes slightly. This discernible difference in amplitudes produced by the different types of metal objects is attainable because the L/C circuit is forced to resonate at a frequency which is different and, for most metal objects, is higher than its natural resonance frequency. Because coins have a different discernible effect on the resonance characteristics of the L/C circuit than unwanted metal objects such as pull tabs, coins can be distinguished from pull tabs. In the preferred embodiment of my detector the increase in amplitude of the output signal due to the proximity of the coin is used to activate a signal generator which provides audio and visual signals indicating the presence of a coin. The signal generator is not activated in the presence of the pull tabs. The metal detector is, however, adapted to operate in two modes. One mode distinguishes between pull tabs and coins as described above. The other mode does not distinguish between coins and pull tabs, but will distinguish between coins and bottle caps, metal foil and other ferrous junk. Depending on the prevalence of pull tabs in a given search area, one mode may be preferable to the other mode.

Claims

1. A metal detecting device having a discriminating mode in which said device discriminates between different types of metal objects such as, for example, coins like pennies and junk objects like pull tabs, bottle caps, foil, etc., comprising a probe containing an induction coil;

a non-bridge type resonating circuit including said coil and a capacitor;

an oscillator which during the discriminating mode provides a driving signal which has a predetermined frequency that is significantly higher than the natural resonance frequency of said resonating circuit;

means coupling the oscillator to the resonating circuit through isolation means which prevent feedback from the resonating circuit to the oscillator so that the frequency of the driving signal remains constant during the discriminating mode;

said resonating circuit being driven by the driving signal so that it provides three different types of output signals having characteristic amplitudes, namely, a. a first output signal having a predetermined amplitude when the probe is remote from a metal object, b. a second output signal having an amplitude greater then the amplitude of the first output signal when the probe is proximate a metal object having an effect on the electrical properties of the coil generally similar to that of an object such as a coin like a penny, c. a third output signal having an amplitude about the same as or less than the amplitude of the first output signal when the probe is proximate a metal object having an effect on the electrical properties of the coil generally similar to that of junk objects such as a pull tab, bottle cap, foil, etc., and means responsive to the second output signal for providing an indication that the probe is proximate a metal object such as a coin.

2. The device of claim 1 where the isolation means includes a transistor.

3. The device of claim 1 where the coil is flat with the individual turns of the coil being generally in the same plane.

4. The device of claim 1 where the indication means provides an audio indication.

5. the device of claim 1 being adapted to switch between the discriminating mode of operation and a non-discriminating mode of operation.