Capacitance change article removal alarm
First Claim
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1. An apparatus to detect removal of an object from a location, said apparatus comprising:
- a. a compressible detection pad comprising an upper conducting plate, a lower conducting plate, and an intermediate resilient dielectric layer which separates the upper and lower plates, with said pad being characterized in that when an object is placed on said pad, at least a portion of the dielectric layer is compressed, and a removal of such an object permits the dielectric layer to expand and change capacitance between the two plates;
b. pad detection and cable supervision monitoring means operably connected to said pad by means of a cable, said monitoring means comprising first circuit means to detect a change of capacitance between said upper and lower plates by reason of compression or expansion of said dielectric layer, or by change of capacitance due to presence of a person or other object, and second circuitry means to detect a fault in said cable means, said monitoring means being arranged to provide an alarm signal in response to change in capacitance between said upper and lower plates or because of fault in said cable means.
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Abstract
An object to be protected is placed on a compressible pad having upper and lower conductive plates, separated by a compressible dielectric. A sensing device is connected to the upper and lower plates to sense a first capacitance when the upper plate is compressed by presence of the object, and a change in capacitance when the object is removed so as to permit the dielectric layer to expand and change the capacitance. There is a time delay circuitry and other circuitry to alleviate the possibility of false alarms.
55 Citations
15 Claims
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1. An apparatus to detect removal of an object from a location, said apparatus comprising:
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a. a compressible detection pad comprising an upper conducting plate, a lower conducting plate, and an intermediate resilient dielectric layer which separates the upper and lower plates, with said pad being characterized in that when an object is placed on said pad, at least a portion of the dielectric layer is compressed, and a removal of such an object permits the dielectric layer to expand and change capacitance between the two plates; b. pad detection and cable supervision monitoring means operably connected to said pad by means of a cable, said monitoring means comprising first circuit means to detect a change of capacitance between said upper and lower plates by reason of compression or expansion of said dielectric layer, or by change of capacitance due to presence of a person or other object, and second circuitry means to detect a fault in said cable means, said monitoring means being arranged to provide an alarm signal in response to change in capacitance between said upper and lower plates or because of fault in said cable means. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. A method of detecting removal of an object from a location, comprising the steps of:
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a. providing a compressible detection pad comprising an upper conducting plate, a lower conducting plate, and an intermediate resilient dielectric layer which separates the upper and lower plates; b. placing the pad at the location; c. placing the object on the pad to compress the pad; and d. generating an alarm signal when the compression or expansion of the compressible detection pad causes the capacitive load between the upper and lower plates to change by a predetermined amount, the step of generating the alarm signal further comprising the steps of; i. measuring a pad capacitance signal corresponding to the capacitance between the two plates by generating a pulsed pad signal based on the voltages across the two plates, the duty cycle of the pulsed pad signal corresponding to the capacitance between the two plates, and so charging a capacitor that the voltage across the capacitor corresponds to the duty cycle of the pulsed pad signal, whereby the voltage across the capacitor is the pad capacitance signal, and ii. determining whether the pad capacitance signal changes by the predetermined amount. - View Dependent Claims (12, 13, 14, 15)
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Specification