Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1996-06-27
1998-06-09
Mullen, Thomas
Communications: electrical
Condition responsive indicating system
Specific condition
340567, 340568, 340571, 331 65, G08B 1326
Patent
active
057641455
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a capacitive detector with an antenna for detecting small capacitive changes in an electric or electromagnetic field surrounding the antenna, and particularly to an alarm system which uses the capacitive detector to generate an alarm signal upon detection of capacitive changes in the generated electric or electromagnetic field.
2. Description of the Prior Art
The method of measuring changes in an electric or electromagnetic field in order to detect intruders is widely used in different kind of alarm systems. The object of such alarm systems is to protect an area from intruders or an object from being removed or stolen. These alarms typically give some kind of a first warning when a change in the electric or electromagnetic field occurs, i.e. they act to prevent the intruder from even getting close to the object or area being protected. However, these systems have many disadvantages that often trigger false alarms. Some of the disadvantages are: RF range. The problem with high frequencies is that they do not meet the requirements in the legislation concerning EMC (Electro Magnetic Compatibility), i.e. they interfere with other electronic equipment such as mobile telephones, TV and radio. detector or sensor is set to detect small changes. different conductive objects in the proximity to the antenna that distort the propagation of the detector field in space.
4) Drifting of various electronic elements and circuits due to changes in temperature which will affect the detecting accuracy.
WO 88/08595 discloses a capacitance proximity sensor which uses a conducting foil as a sensing electrode. The conducting foil is connected to an electronic circuitry. The electronic means comprises an inductive coil which together with the configuration of the conducting foil controls the frequency of an oscillator, typical 80 kHz. The presence of an intruder in the field changes the capacitance of conducting foil and alters the oscillator frequency. This frequency change is used to detect a capacitive change. This capacitance proximity sensor operates with a frequency of 80 kHz which results in one of the above mentioned problems, i.e. it will not meet the EMC requirements.
U.S. Pat. No. 4,987,402 discloses a alarm system which uses a proximity detector to detect an intrusion into an adjustable electromagnetic field set up around the object to be protected. This electromagnetic field is a RF field which results in the above mentioned problem.
EP 0 334 531 discloses a proximity detector made up of conductive paint and with a capacitor and switching means that form a an oscillator having a frequency of from 1 to 150 kHz. This detector measures the frequency change when an intruder enters the field. This detector, since using paint as an electrode, is directed toward surveillance of rooms or doorways and its application has therefore a very narrow range.
Thus, there is demand for a capacitive detector meeting the EMC requirements and which in a reliable way is capable of detecting small capacitive changes without being influenced by changes in temperature and humidity.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a capacitive detector meeting the EMC requirements and being capable of generating a balanced field surrounding the antenna and which is not affected by changes in humidity and temperature in order to detect small capacitive changes in the generated field surrounding the antenna.
The detector is realized with an antenna connected to an electronic circuitry which generates an electric field around the antenna. The electronic circuitry comprises, generating means for generating an electric or electromagnetic field around said antenna, adjustable means for building up a balanced electric field around the antenna, balancing means for maintaining the generated electric or electromagnetic field around said antenna balanced, filtering means which prevents the detector device from being affected by changes in temperat
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patent: 3760402 (1973-09-01), Magerle et al.
patent: 4124848 (1978-11-01), Clark et al.
patent: 4144529 (1979-03-01), Miller et al.
patent: 4325058 (1982-04-01), Wagner et al.
patent: 4621258 (1986-11-01), Campman
patent: 4987402 (1991-01-01), Nykerk
patent: 5406261 (1995-04-01), Glenn
Hansson Goran
Lindgren Bo
Soderberg Tommy
Mullen Thomas
Wu Daniel J.
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