Magnetic article surveillance system, method and coded marker
First Claim
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1. An article surveillance system, comprising:
- means for generating and transmitting phase locked reference signals in a detection zone;
a plurality of coded markers, each marker having means for generating harmonic signals of a predetermined phase shift responsive to the phase locked reference signals;
means for receiving the phase shifted harmonic signals generated by coded markers in the detection zone;
means for determining the relative phase shift between the reference signals and the harmonic marker signals; and
,means for generating a control signal responsive to identification of a valid code by the determining means.
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Abstract
A surveillance system is based upon generation and detection of phase shifted harmonic signals responsive to transmission of a reference signal at a fundamental frequency in a detection zone. Phase shifted harmonic signals may be generated by markers comprising a core of soft magnetic material and an electrically conductive material at least partly surrounding the core. Phase shifted harmonics are not accidentally generated by biassed ferrous alloys, the cause of most system failures.
36 Citations
17 Claims
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1. An article surveillance system, comprising:
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means for generating and transmitting phase locked reference signals in a detection zone; a plurality of coded markers, each marker having means for generating harmonic signals of a predetermined phase shift responsive to the phase locked reference signals; means for receiving the phase shifted harmonic signals generated by coded markers in the detection zone; means for determining the relative phase shift between the reference signals and the harmonic marker signals; and
,means for generating a control signal responsive to identification of a valid code by the determining means. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A method for conducting surveillance of articles in a detection zone, comprising the steps of:
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providing each article with a coded marker having means for generating harmonic signals of a predetermined phase shift responsive and relative to reference signals; transmitting phase locked reference signals at a fundamental frequency into the detection zone; receiving phase shifted harmonic signals generated by each marker in the detection zone responsive to the phase locked reference signals; and
,measuring the phase shift between the reference signals and the harmonic marker signals, the degree of the phase shift being related to positive identification of a marker in the detection zone. - View Dependent Claims (10, 11, 12, 13)
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14. A magnetic marker for use in article surveillance systems wherein coded markers are carried by monitored articles, the marker comprising:
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a core of soft magnetic material; and
,an electrically conductive material at least partly surrounding the core, whereby the marker will generate a clearly identifiable harmonic signal of predetermined phase shift responsive and relative to a phase locked reference signal of a fundamental frequency, the degree of phase shift enabling reliable detection and identification of each coded marker. - View Dependent Claims (15, 16, 17)
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Specification