Oscillator type metal detector with switch controlled fixed biasing

• US 4,323,847 A
• Filed: 06/11/1979
• Issued: 04/06/1982
• Est. Priority Date: 06/11/1979
• Status: Expired due to Term

• Current Assignee: Discovery Electronics Incorporated
• Original Assignee: Triple DEE Electronics Incorporated
• Inventors: Karbowski, James P.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US06/047,225
• Time in Patent Office: 1,030 Days
• Field of Search: 324/327, 324/236, 324/237, 331/65, 331/109, 331/117, 331/136, 331/172
• US Class Current: 324/327
• CPC Class Codes: G01V 3/102 by measuring amplitude
• CPC Class Codes: G01V 3/15 specially adapted for use during transport, e.g. by a person, vehicle or boat

Abstract

An apparatus is described for detecting the presence of metal objects which may be subterranean. The apparatus comprises an amplifier equipped with a feedback network for forcing the amplifier into oscillation. The amplifier is critically biased such that the minimum gain necessary to maintain oscillation at the frequency of resonance is achieved. The presence of metal near the search coil which forms the tuned circuit of the amplifier loads the output circuit of the amplifier to a degree which suppresses oscillation. An indicator is provided to indicate when the oscillation has decreased in amplitude, whereby a metal object may be located.

Claims

1. A metal detecting device comprising:

(a) a transistor amplifier including a tuned resonant circuit in the collector circuit of said transistor amplifier;

said tuned resonant circuit being adapted to be located near a metal object;

(b) a feedback circuit for providing a current from said resonant circuit to said transistor amplifier input for forcing said amplifier into an oscillating condition;

(c) means for supplying a fixed biasing current to said transistor amplifier in response to a voltage occuring at said amplifier output upon momentary closure of a switch means connected to said amplifier output for maintaining the minimum gain necessary to cause said amplifier to oscillate at substantially the resonant frequency of said tuned resonant circuit in the absence of a metal object in proximity of said resonant circuit, said means providing a new fixed biasing current upon subsequent momentary actuation and release of said switch for reestablishing a minimum gain for producing oscillations in said amplifier compensating for a change in the effective Q of said resonant circuit due to the presence of said metal object; and

(d) means for indicating when the amplitude of oscillations produced by said transistor amplifier decreases whereby the presence of said object is detected.

2. A metal detector according to claim 1, wherein said means for indicating a decrease in amplitude of said oscillations includes a means for producing an audible sound.

3. A metal detector for locating a metal object comprising:

(a) a transistor amplifier having a collector circuit, emitter circuit, and base circuit;

(b) a tuned resonant circuit in series with said collector circuit;

at least a portion of said tuned resonant circuit being adapted to be located in the vicinity of the metal object;

(c) feedback means for supplying current from said resonant circuit to the base of said transistor, said current having a phase which forces said transistor into oscillation;

(d) means for biasing said transistor at a fixed level to maintain a minimum gain necessary for producing oscillations of said transistor in response to a momentary closure of a switch, said switch connected to provide a voltage from said amplifier to said means for biasing, said biasing level remaining until said switch is reclosed whereby a new fixed bias level is established and maintained on said transistor amplifier reestablishing the minimum gain for maintaining oscillations when said metal object is in the vicinity of said tuned circuit; and

(e) means for indicating when the amplitude of the oscillations produced by said transistor has decreased whereby the presence of said object is detected.

4. The apparatus of claim 3, wherein said means for biasing said transistor comprises:

(a) a differential amplifier having a first input connected to said emitter circuit and a second input connected to a reference voltage;

(b) a second differential amplifier having a first input connected to said reference voltage and a second input, a switch having first end connected to the output of said first amplifier and a second end connected to the remaining input of said second differential amplifier;

whereby the closure of said switch will provide a bias voltage for said transistor to provide the minimum gain necessary to maintain oscillation; and

(c) means for retaining said bias voltage on said transistor when said switch is opened.

5. An apparatus for detecting the proximity of metal objects comprising:

(a) an amplifier having a resonant tuned circuit in the output circuit of said amplifier, said circuit being adapted to be located in the vicinity of the object to be detected, whereby electromagnetic energy from said circuit is free to interact with said object;

(b) means for introducing feedback to said amplifier input from said tuned circuit whereby an oscillatory condition is produced in said amplifier;

(c) means for biasing said amplifier to achieve a gain sufficient to maintain oscillation of said amplifier in the absence of a metal object in proximity with said tuned circuit, said means for biasing including

feedback means providing a path from the output of said amplifier to the input of said amplifier for applying a biasing voltage to said amplifier for establishing a forward gain for said amplifier sufficient to support an oscillatory condition;

a switch for interrupting said path;

means for storing and applying said biasing voltage to said amplifier input when said switch is interrupting said path;

whereby closure of said switch establishes a new bias voltage for said amplifier for restoring the amplitude of oscillations produced in the presence of said metal object; and

(d) means connected to said amplifier for indicating when said amplifier oscillations have decreased in amplitude in response to the presence of said metal object near said resonant tuned circuit.

6. An apparatus for detecting the presence of a buried metal object comprising:

(a) a transistor oscillator having a resonant tuned circuit positionable near a buried object wherein said tuned circuit is magnetically coupled with said object altering the Q of said resonant circuit;

(b) a feedback network connected to said oscillator, said feedback network providing a voltage proportional to the amplitude of oscillations produced by said oscillator;

(c) means connected to said feedback network for biasing said oscillator to provide a sufficient forward gain for maintaining said oscillations, including a switch for momentarily applying a voltage from said feedback network to said means for biasing whereby said oscillator gain is momentarily set;

(d) means for storing and maintaining a fixed bias voltage generated in response to said switch closure on said oscillator after said switch is opened; and

(e) means connected to said feedback network for monitoring the magnitude of oscillations produced by said oscillator whereby the change in Q of said tuned circuit in response to the proximity of said metal object is detected as a change in amplitude of said oscillations, the amplitude of said oscillations being restored in response to a momentary closure of said switch whereby a new fixed bias voltage is established.

7. An apparatus for detecting the presence of a buried metal object comprising:

(a) a transistor oscillator circuit comprising a transistor having a collector, emitter, and base, a tuned resonant circuit having a first end connected to said collector, and a second end, said circuit being positionable in proximity with a buried metal object;

(b) a first feedback circuit magnetically coupled to said resonant circuit, said feedback circuit providing a signal to said base member for initiating oscillation of said transistor;

(c) an emitter circuit having first and second ends, said first end being connected to said emitter;

(d) means for applying a voltage between said tuned resonant circuit second end and said emitter circuit second end;

(e) a second feedback circuit connected to receive a voltage from said emitter indicative of the magnitude of oscillations produced by said transistor, including a switch, to supply a momentary bias voltage to said base in response to the closure of said switch, said bias voltage having a magnitude sufficient to establish the minimum gain for maintaining said transistor in an oscillating state;

(f) voltage storing means for storing and holding said bias voltage on said base in response to the opening of said switch whereby oscillation of said transistor is maintained at a fixed amplitude until said resonant circuit is brought into proximity with said metal object whereby the magnitude of said oscillations are decreased altering said emitter voltage; and

(g) means for indicating changes in said emitter voltage whereby the proximity of said metal object is detected.