Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2000-05-15
2001-07-24
Hofsass, Jeffery (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S545300, C340S567000, C250S342000, C250S353000
Reexamination Certificate
active
06265972
ABSTRACT:
BACKGROUND OF THE INVENTION
The present application relates to passive infrared motion detection sensors and in particular, relates to a sensor which has improved features with respect to false alarms caused by small pets.
Passive infrared detectors focus radiation from an area to be monitored in a particular manner such that movement of a human intruder through the monitored space is detected. A Fresnel focussing arrangement (lens or mirror) focuses infrared radiation emitted by a human or pet target onto a passive infrared detector. To improve the response characteristics of the sensor, the Fresnel lens has multiple lensets and each lenset includes a focussing element defining an infrared beam that collectively covers the protected area. These beams increase in size as an increasing function of proportional to the distance from the detector. This characteristic of the Fresnel lens makes it difficult to distinguish between small pets located in a region close to the detector from a human target located at a substantial distance from the detector. In the closer region to the detector, the beams are quite small, and as such, a small pet will produce a signal similar in level to a person a substantial distance away from the detector.
As can be appreciated, small pets and in particular, cats, have substantial vertical mobility and are not confined to an area adjacent the floor. Some systems have attempted to design a dead or reduced bottom zone in the region close to the sensor in order to reduce problems associated with false alarms, caused by pets. The substantial vertical mobility of cats defeats this type of system.
U.S. Pat. No. 4,849,635 discloses a single passive infrared detector sensor where substantial gaps are provided between the sensing beams or zones. These zones are spaced such that a small pet must enter a dead zone as they move across the space. In contrast, a human target is much larger and taller and will therefore, produce a signal regardless whether he is standing at a position which at floor lever, is in a dead zone. With this arrangement, a pet produces a pulse signal with a very low component when the pet is in the dead zone, whereas a human target, although producing a pulsed signal, the signal is much more constant and can be easily distinguished from a pet. Unfortunately, with this system, a cat located at a high point in close proximity to the sensor will bridge two active zones much in the manner of a human.
It has also been known to use two different types of sensors to help distinguish between a human intruder and a pet. In particular, a microwave sensor in combination with a passive infrared sensor has been used. This type of dual technology sensor greatly increases the cost of the system and as such, is not particularly desirable. It has also been proposed to use a two element passive infrared detector and analyze the signal from the two detectors to distinguish between an intruder and a pet. Typically one element receives low radiation and one detector receives high radiation. S small pet does not have the height to trigger both detectors. This system again experiences some difficulties and also has the additional cost of the two element detection.
A better approach for distinguishing between small pets and human intruders is needed.
SUMMARY OF THE INVENTION
A passive infrared motion sensor according to the present invention comprises a passive infrared detector, a Fresnel focussing arrangement in front of the detector for selective focussing of infrared radiation from an area to be monitored, and directing such radiation onto the detector and processing such circuitry for analyzing the signal from the detector and making a determination whether an intruder is present.
The Fresnel focussing arrangement is divided into at least two tiers comprising a first tier and a second tier. The first tier focusses radiation from a distant subdivision of the area being monitored and the second tier focusses radiation from a close subdivision of the area being monitored.
The second tier divides the close subdivision into narrow elongated vertically disposed sensing strips such that a pet in the close subdivision causes the detector to produce a signal less than 80% of the signal used to indicate the presence of an intruder in the close subdivision or the distant subdivision.
The Fresnel focussing arrangement can either be a mirror arrangement or a lens arrangement. Preferably, a Fresnel lens is used, comprising a number of stacked lensets of a Fresnel lens with the result being an elongation of the area which is capable of receiving radiation focussing the same on the detector. Basically, the lensets stacked one on top of the other, provides a series of vertical focal points in contrast to the prior practice of a single focal point. With this arrangement, what was previously a very small responsive area in close proximity to the sensor has now been elongated or shaped, whereby the radiation from a pet in close proximity is in proportion to the radiation received from a human intruder at a substantial distance from the sensor (i.e., it is substantially smaller in magnitude).
Basically the size of the active area has been elongated and in most cases, narrowe. With this arrangement, a small pet tends to traverse across this area while an intruder still has substantial vertical height, and as such, will trip the system. Thus, the system enlarges the sensing area and decreases the response caused by a small pet.
A passive infrared motion sensor according to an aspect of the invention, comprises a passive infrared detector, a Fresnel lens focussing arrangement in front of the detector for segmented focussing of infrared radiation from an area to be monitored onto the detector and processing circuitry for analyzing the detector, and making a determination whether an intruder is present.
The Fresnel focussing arrangement is divided into at least three horizontal tiers comprising an upper distant tier, an intermediate tier and a close tier, with each tier having a series of horizontally spaced focussing facets. Each focussing facet of the close and the intermediate tiers are segmented to vertically elongate and shape a detection region of the facet such that the passive infrared radiation received due to a small pet in the detection region is easily distinguished from passive infrared radiation received due to an intruder in the detection region.
REFERENCES:
patent: 3703718 (1972-11-01), Berman
patent: 4263585 (1981-04-01), Schaefer
patent: 4849635 (1989-07-01), Sugimoto
patent: 4990783 (1991-02-01), Muller et al.
patent: 5670943 (1997-09-01), Dipoala et al.
patent: 5923250 (1999-07-01), Pildner et al.
patent: 5936666 (1999-08-01), Davis
Digital Security Controls Ltd.
Hofsass Jeffery
Nguyen Phung T.
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