US3763424A
Expired / Lapsed
US
⏱️ 0.0 years remaining
Metal detector for identifying and discriminating between objects of different size, shape, orientation and ferrous content and including an auto nulling circuit
Assignee: Unisys Corp
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Filing: 1971-07-15
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Granted: 1973-10-02
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Expires: 1991-07-15
Abstract
Abstract A metal detector comprising a sensor coil positioned relative to an excited field coil for minimum mutual inductive coupling therebetween to provide a null output from the sensor coil in the absence of objects to be detected. The coils are rigidly secured in a lightweight epoxy casting to preclude deformation of the coils and relative displacement therebetween under varying environmental conditions to preclude drift of the null output. Low frequency excitation on the order of a few hundred up to a few thousand hertz is applied to the field coil to obtain a high degree of insensitivity to moisture conditions existing in the vicinity of the detector for further maintaining constancy of the null output. An auto-nulling circuit for automatically compensating for any long term drift of the detector output is also optionally included.
Inventors
- 👤 H Wilhelmsen
- 👤 C Bennett
- 👤 Lorenzo J De
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-10-23 13:14
CPC Classifications
G; G01; G01V; G01V3/00; G01V3/08; G01V3/10; G01V3/104; G01V3/105; G01V3/107
Claims Analysis
Total Claims: 4
Claim 1
Independent Claim
292 words
Metal detector apparatus comprising a coil assembly including a field coil and a sensor coil fixedly secured to one another in partially overlapping substantially minimum mutual inductive coupling relation so that the sensor coil provides a zero or small residual output signal, due to any unbalanced coupling between the coils, under an alternating current electrically energized condition of the field coil in the absence of extraneous metal objects within the detection region of the coil assembly; a source of alternating current electrical excitation coupled to the field coil; means coupled to the sensor coil for cancelling residual output signal of the sensor coil caused by either unbalanced coupling of the coils or the presence of extraneous metal objects moving at a rate less than a predetermined rate in the coil assembly detection region; said residual output signal cancelling means being tuned to a frequency of the electrical excitation applied to the field coil and including means for deriving from the sensor coil a reference signal at the tuned frequency having a phase identical to that of the sensor coil signal, means responsive to the reference signal for producing a nulling signal at the tuned frequency in phase opposition to the sensor coil output signal and variable in amplitude for cancelling the residual output signal of the sensor coil, means for combining the sensor coil output signal and the nulling signal to produce an error signal to the difference in amplitude therebetween and means responsive to the error signal to provide a signal for controlling the amplitude of the nulling signal having a time constant greater than the time required for an object moving at the predetermined rate to pass through the detection region whereby the residual sensor coil output signal is cancelled.
Claim 2
Dependent Claim
292 words
Metal detector apparatus comprising a coil assembly including a field coil and a sensor coil fixedly secured to one another in partially overlapping substantially minimum mutual inductive coupling relation so that the sensor coil provides a zero or small residual output signal, due to any unbalanced coupling between the coils, under an alternating current electrically energized condition of the field coil in the absence of extraneous metal objects within the detection region of the coil assembly; a source of alternating current electrical excitation coupled to the field coil; means coupled to the sensor coil for cancelling residual output signal of the sensor coil caused by either unbalanced coupling of the coils or the presence of extraneous metal objects moving at a rate less than a predetermined rate in the coil assembly detection region; said residual output signal cancelling means being tuned to a frequency of the electrical excitation applied to the field coil and including means for deriving from the sensor coil a reference signal at the tuned frequency having a phase identical to that of the sensor coil signal, means responsive to the reference signal for producing a nulling signal at the tuned frequency in phase opposition to the sensor coil output signal and variable in amplitude for cancelling the residual output signal of the sensor coil, means for combining the sensor coil output signal and the nulling signal to produce an error signal to the difference in amplitude therebetween and means responsive to the error signal to provide a signal for controlling the amplitude of the nulling signal having a time constant greater than the time required for an object moving at the predetermined rate to pass through the detection region whereby the residual sensor coil output signal is cancelled.
Claim 3
Dependent Claim
245 words
Metal detector apparatus for identifying and discriminating between objects of different size, shape, orientation and ferrous content, comprising a coil assembly including a field coil and a sensor coil fixedly secured relative to one another in partially overlapping substantially minimum mutual inductive coupling relation at a location constituting a detection zone so that the sensor coil provides a zero or small residual output signal under an alternating current electrically energized condition of the field coil in the absence of metal objects within the detection zone, a source of alternating current electrical excitation coupled to the field coil, electrically non-conductive conveyor means for carrying an object to be identified past the coil assembly at a predetermined speed so that the object is within the magnetic field produced by the field coil for at least several cycles of the electrical excitation applied thereto, and means coupled to the sensor coil tuned to a frequency of the electrical excitation applied to the field coil including means for cancelling residual signal produced at the output of the sensor coil and additional means for detecting the amplitude and phase of the sensor coil output signal as a function of time produced by an object carried past the coil assembly for presentation of respective amplitude versus time and phase versus time waveform signatures, in accordance with the position of the object relative to the coil assembly, to be compared with prerecorded amplitude and phase waveform signatures representative of a sample object.
Claim 4
Dependent Claim
245 words
Metal detector apparatus for identifying and discriminating between objects of different size, shape, orientation and ferrous content, comprising a coil assembly including a field coil and a sensor coil fixedly secured relative to one another in partially overlapping substantially minimum mutual inductive coupling relation at a location constituting a detection zone so that the sensor coil provides a zero or small residual output signal under an alternating current electrically energized condition of the field coil in the absence of metal objects within the detection zone, a source of alternating current electrical excitation coupled to the field coil, electrically non-conductive conveyor means for carrying an object to be identified past the coil assembly at a predetermined speed so that the object is within the magnetic field produced by the field coil for at least several cycles of the electrical excitation applied thereto, and means coupled to the sensor coil tuned to a frequency of the electrical excitation applied to the field coil including means for cancelling residual signal produced at the output of the sensor coil and additional means for detecting the amplitude and phase of the sensor coil output signal as a function of time produced by an object carried past the coil assembly for presentation of respective amplitude versus time and phase versus time waveform signatures, in accordance with the position of the object relative to the coil assembly, to be compared with prerecorded amplitude and phase waveform signatures representative of a sample object.
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USPTO Annuity & Maintenance Windows
| Annuity Stage | Window Opens | Payment Deadline | Surcharge Period Until | Estimated Fee | Status |
|---|---|---|---|---|---|
| 3.5 Years (Year 4) | 1976-10-01 | 1977-04-02 | 1977-10-02 | $2000.00 | Window Closed / Lapsed |
| 7.5 Years (Year 8) | 1980-10-01 | 1981-04-02 | 1981-10-02 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 1984-10-01 | 1985-04-02 | 1985-10-02 | $7700.00 | Window Closed / Lapsed |