Communications: electrical – Condition responsive indicating system – With particular system function
Reexamination Certificate
1999-03-22
2001-02-20
Swann, Glen (Department: 2736)
Communications: electrical
Condition responsive indicating system
With particular system function
C340S522000, C340S552000, C340S554000, C340S567000
Reexamination Certificate
active
06191688
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains generally to detecting attempts to bypass motion detectors, and more particularly to detecting, at power up of a motion detector, whether the motion detector has been masked.
2. Description of the Background Art
Motion detectors are widely used in alarm systems. State of the art motion detectors typically employ dual sensing technology, such as a microwave Doppler sensor combined with a passive infrared sensor (PIR), coupled with processing software. In most instances, the PIR sensor is the primary sensor and the microwave sensor is used as a secondary sensor to confirm a detection event from the PIR sensor. While the technology is reliable for detecting alarm conditions based on various sensed conditions, it is still possible to defeat a dual sensor motion detector by “masking” the PIR sensor. It is generally understood in the art that the term “masking” refers to placing a stationary object in front of a sensor, covering the sensor with a substance such as tape or paint, or the like. Even placement of a plate of glass or spraying clear varnish or hair spray over an infrared sensor window can be an effective mask. Most often, the PIR sensor is the target of masking since infrared signals are line of sight whereas microwave signals penetrate and bounce off of objects.
Understandably, mask detection is important if high levels of security are to be maintained at all times and various approaches to mask detection have thus been developed. The simplest is to monitor PIR activity and declare a mask condition if loss of activity occurs for a predetermined period of time, although this method is prone to false mask detects since an empty room will cause a mask condition to be indicated. Another approach is to detect a mask condition during the actual act of masking. In dual sensor detectors employing a microwave Doppler sensor, high level microwave signals are generated when a person or moving object comes into close proximity of the sensor. Therefore, items can be readily detected by a microwave Doppler sensor when they are moving into a position that will block the sensor. Unfortunately, however, once moved into position, a stationary object essentially becomes invisible to a microwave Doppler detector. Another approach is to use a near-infrared emitter/detector pair which looks for a reflected beam. A high reflected signal level would indicate a mask condition because of an object being placed in close proximity. However, this approach is costly and has a relatively high power consumption level.
Therefore, the most reliable approach to mask detection without incurring additional costs in price or power is to use the microwave Doppler sensor to detect close-up events; that is, movement to within approximately eighteen inches of the microwave Doppler sensor. Upon detection of the close-up event, a PIR detection window is opened. If PIR activity is detected during this window, then the mask detection routine ends. Otherwise, if no PIR activity occurs during that time period, a mask condition is declared.
A serious threat to security still exists, however, when using microwave-based mask detection, since this technology is dependent upon seeing the actual act of masking. Therefore, such technology cannot detect a mask if power is removed from the detector, such as, if a detector loses power while a sensor is masked, or the system is powered down during the daytime, or someone masks the sensor during a power outage. In any of those cases, since the masking has already occurred, the sensor will not give an indication that masking has taken place when it is powered up again. Therefore, a need exists for a system and method for detecting that a sensor has been masked without causing the sensor to declare a false masking condition when power loss occurs in an empty building. The present invention satisfies that need, as well as others, and overcomes the deficiencies found in conventional technology.
BRIEF SUMMARY OF THE INVENTION
The present invention pertains to determining if a motion detector is in a masked condition at the time power is applied to the detector. More particularly, the invention detects a situation where a person disconnects power to the detector by, for example, shutting down the power at the electrical panel, then masks the detector, and finally reapplies power.
By way of example, and not of limitation, to detect a mask condition in accordance with the present invention the detector is placed into a mask detection state when power is applied. Any infrared motion that is detected after the detector has warmed up and stabilized will terminate the mask detection state. However, if a predetermined amount of microwave sensor activity is detected within the field of view without detection of infrared activity, a mask condition is declared. This method of detecting a mask condition is based on the assumption that a large amount of microwave activity should be accompanied by at least a small amount of infrared activity if the infrared sensor has not been masked. The amount of microwave activity that required to trigger mask detection can be varied based on individual detector characteristics, but needs only be sufficiently large to avoid false mask detection resulting from microwave activity generated from radio transmitters, cellular telephones and other interfering sources.
An object of the invention is to detect attempts to bypass a motion detector.
Another object of the invention is to provide for reliable mask detection with virtually no additional component cost and virtually no additional power consumption as compared to using a near-infrared emitter/detector pair.
Another object of the invention is to determine if the infrared sensor in a motion detector has been masked.
Another object of the invention is to detect mask conditions in a motion detector after power up.
Another object of the invention is to detect masking of a motion detector occurring during a power outage.
Another object of the invention is to enable mask detection in a motion detector for a predetermined period after the motion detector is first powered on.
Another object of the invention is to detect masking of an infrared sensor in a motion detector using a microwave Doppler sensor as a trigger device.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.
REFERENCES:
patent: 4242669 (1980-12-01), Crick
patent: 5499016 (1996-03-01), Pantus
patent: 5581237 (1996-12-01), DiPoala
patent: 5796353 (1998-08-01), Whitehead
patent: 2 308 482 (1997-06-01), None
Honeywell International , Inc.
O'Banion John P.
Swann Glen
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