Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1982-03-15
1985-06-18
Gill, James J.
Communications: electrical
Condition responsive indicating system
Specific condition
G08B 1324
Patent
active
045243506
ABSTRACT:
In an electromagnetic passage surveillance system, first and second receiver means are employed in mutually-spaced locations across a guarded portal to detect perturbations in an electromagnetic field extending across the portal, resulting from presence within the field of a high-permeability, low-coercivity marker. From the outputs of such receivers a sum signal and a difference signal are produced, and the high-order harmonics of the sum signal are emphasized to produce a first analysis signal while the low-order harmonics of the difference signal are emphasized to produce a second analysis signal. The ratio of the first analysis signal to the second, and thus the ratio of high-order to low-order harmonics, is used in logic and processing circuitry to dynamically vary the detection threshold by which the presence of a marker is determined. To enhance sensitivity to various orientations of a marker, the phase characteristics of the electromagnetic field are periodically changed, and the processing circuitry is correspondingly switched. To improve noise rejection, signal reception is blanked out during the energization of the system's electromagnetic field-generation coils, and the processing of received signals is limited to the time segment during which perturbations resulting from a marker are most likely to be present.
REFERENCES:
patent: 2992325 (1961-07-01), Leaan
patent: 3758849 (1973-09-01), Susman et al.
patent: 4384281 (1983-05-01), Cooper
Gill James J.
Progressive Dynamics, Inc.
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