ENCAPSULATED ELECTRICALLY RESONANT CIRCUIT AND INTERROGATING APPARATUS AND METHOD FOR FINDING SAME IN VARIOUS LOCATIONS

• US 3,836,842 A
• Filed: 01/22/1973
• Issued: 09/17/1974
• Est. Priority Date: 01/22/1973
• Status: Expired due to Term

• Current Assignee: Detlef Zimmermann, James Hugh Lougheed
• Original Assignee: Detlef Zimmermann, James Hugh Lougheed
• Inventors: Zimmermann, Detlef, Lougheed, James Hugh
• Primary Examiner(s): Corcoran, Robert J.
• Application Number:
  US05/325,518
• Time in Patent Office: 603 Days
• Field of Search: 324/34R,34O,41,3,6 340/258R,258C,258D,280,282 336/179,90,96
• US Class Current: 324/239
• CPC Class Codes: G01C 15/06 Surveyors' staffs; Movable markers
• CPC Class Codes: G01V 15/00 Tags attached to, or associated with, an object, in order to enable detection of the object record carriers for use with machines having a detectable tag or marker G06K19/00
• CPC Class Codes: G01V 3/15 specially adapted for use during transport, e.g. by a person, vehicle or boat

Abstract

Locations, typically outside locations, are marked by a passive marking device placed therewith. The marking device is responsive in a damped oscillatory manner when radiated by at least one pulse of magnetic energy, i.e., an induction field. An interrogating instrument is the source of the induction field. After an induction field has been generated, the interrogating instrument monitors the frequency spectrum within which the marking device is resonantly responsive. When a response is detected the interrogating instrument provides an indication of the response. The marking device is particularly useful for marking locations which are buried or are from time to time concealed. With a portable interrogating instrument, an operator, traversing the general area of a location, is thus able to find the location without the aid of a visible stake or other similar fixture.

Claims

1. A passive marking device for placement in a location so that the location is thereafter identifiable with the use of an appropriate interrogating instrument, the marking device comprising:

an elongated core in the form of a sintered body of ferrite magnetically soft material;

a coil of conductive material encircling a portion of the core;

a capacitor connected across the coil, the assembly of the core, the coil and the capacitor providing a resonant circuit;

an encapsulation, encapsulating the resonant circuit, the encapsulation bonded to the surface of the core and being of a magnetically transparent material at least as brittle as the sintered ferrite material, the core, mechanically reinforced by the encapsulation and co-acts therewith to fracture as would a unitary body in the event of mechanical damage, evidence of sUch fracture being detectable by visual inspection of the marking device.

2. An interrogating instrument for indicating the presence of a marking device which includes a coil in a passive resonant electrical circuit, comprising:

means for generating a continuous wave sweeping frequency on an interrupted basis, the sweeping frequency being within a predetermined frequency range within which the marking device is resonant;

means for generating a magnetic field corresponding to the continuous wave sweeping frequency to induce ringing in the marking device, the magnetic field generating means including a resonant circuit having parallel inductance and capacitive elements and a pair of diodes in a back-to-back parallel configuration, said pair of diodes being in series with the capacitive element in the resonant circuit and thereby electrically disconnecting the capacitive element when signals in the resonant circuit are of a voltage less than the forward drop across said pair of diodes;

damping means resistively loading the resonant circuit to reduce the Q thereof so that the magnetic field generated thereby tends to be constant with respect to signals from the sweeping frequency generator, throughout said frequency range;

receiving means, including an induction coil, for receiving a ringing signal from a nearby responsively ringing marking device;

means, responsive to the interruptions in the continuous wave sweeping frequency, for generating signals corresponding to the envelope shape of the signal from a typical ringing marking device, during said interruptions;

means, synchronized with the interruptions in said sweeping frequency, for selectively filtering the signals from the signal receiving means and passing only those signals within said frequency range, during said interruptions;

a variable gain amplifier for amplifying the signals from the filtering means, the amplifier controlled by signals from the means for generating signals to have a positive gain proportional to said signals only during the interruptions in the sweeping frequency, whereby signals at the output of the amplifier tend to have a constant signal-to-noise ratio, so that the probability of detecting a nearby marking device is maximized while the probability of a false indication due to background noise is minimized;

means for providing an indication when the output from the variable gain amplifier is detected to exceed a predetermined level, said indication being indicative of the nearby presence of said marking device.

3. A method or using an interrogating instrument characterized by having a pair of induction coils in parallel alignment one with the other one associated with circuitry for generating a magnetic field on an interrupted basis, the other associated with circuits for detecting a resonant response to the magnetic field from a buried marking device having a passive resonant circuit including a vertically oriented induction coil, with the instrument operating, the method comprising the steps of:

a. traversing said area along a first substantially straight line, with the longitudinal axes of the pair of coils in approximate parallel alignment with said coil in the marking device, to determine the detectable boundaries of the induction field, resulting from said resonant response, along the first line;

b. traversing said area along a second substantially straight line as in step (a), approximately perpendicular to the first line, from a point on the first line approximately equidistant between the boundaries thereon determined, to determine the boundaries of said resulting induction field along said second line, the location of the marking device being approximately determined to be about equidistant between the boundaries along the second line;

c. rotating said coils in the interrogating instrument so that their longitudinal axes are in approximately perpendicular relationship with respect to the longitudinal axis of said coil in the marking devIce and thereafter traversing the location, found in step (b), along a third substantially straight line in the direction of the longitudinal axes of said coils, in the interrogating instrument, until a first minimum induction field point is detected;

d. rotating the longitudinal axes of the coils in the interrogating instrument through 1/4 turn while maintaining said perpendicular relationship and thereafter traversing the location along a fourth substantially straight line perpendicular to the third line from said minimum point, that is along the longitudinal axes of the coils in the interrogating instrument, until a second minimum induction field point is detected, said second point falling at the center of the induction coil associated with the detection circuitry and being in direct alignment with the longitudinal axis of the coil in the marking device.

4. An interrogating instrument, for indicating the presence and location of a marking device which includes a coil in a passive resonant circuit, comprising:

a first induction coil for generating a magnetic field in response to a current flow in the induction coil, a capacitor connected in parallel with the first induction coil to provide a high Q tuned resonant circuit;

a pair of diodes connected in back-to-back parallel configuration in series with the capacitor, the pair of diodes electrically disconnecting the capacitor from the circuit at signal voltage levels below the forward voltage drop of the diodes;

oscillator means responsive to gating pulses for generating, in the first induction coil, bursts of continuous wave current of a frequency at which the marking device is resonantly responsive, means for generating the gating pulses each having a period greater than that of a plurality of cycles of the continuous wave current, a second induction coil for generating current corresponding to fluctuations in a magnetic field cutting through the second coil, detection means, responsive to the gating pulses to be inactive during the presence of a gating pulse and active in the absence of a gating pulse, for detecting a fluctuating current in the second induction coil corresponding to the resonant frequency of the marking device.

5. An interrogating instrument, for indicating the presence and location of a marking device which includes a coil in a passive resonant circuit, comprising:

a first induction coil for generating a magnetic field in response to a current flow in the induction coil, and including amplification means and means for controlling the gain of the amplification means in response to the gating pulses to synchronously vary the gain of the amplification means from relatively high to relatively low, generally in profile with the amplitude envelope of the resonant response in a typical marking device, during interruption in the magnetic field, whereby signals at the output of the amplification means tend to have a constant signal-to-noise ratio, so that the probability of detecting a nearby marking device is maximized while the probability of a false indication due to background noise is minimized. oscillator means responsive to gating pulses for generating, in the first induction coil, bursts of continuous wave current of a frequency at which the marking device is resonantly responsive, means for generating the gating pulses each having a period greater than that of a plurality of cycles of the continuous wave current, a second induction coil for generating current corresponding to fluctuation in a magnetic field cutting through the second coil, detection means, responsive to the gating pulses to the inactive during the presence of a gating pulse and active in the absence of a gating pulse, for detecting a fluctuating current in the second induction coil corresponding to the resonant frequency of the marking device.