Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2000-10-13
2001-07-17
Wu, Daniel J. (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S565000, C340S506000, C340S521000, C250S353000, C250S341300, C250S342000, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06262661
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to passive infrared detectors and more particularly relates to an infrared sensor having an anti-masking device.
FIELD OF THE INVENTION
Known sabotage or masking methods to defeat passive infrared detectors include covering the detector with an object, such as, for example, a box, a hat or a screen, or spraying the entrance window with a spray that is opaque to infrared, such as, for example, glue or hairspray. Modern passive infrared detectors should be capable of automatically detecting such masking, preferably at the time of the masking or, at the latest when the detector or system is set. There are various strategies in this regard. In the case of detectors connected to a monitoring center, the detectors are always switched on and deliver signals to the center even while they are not set (e.g., in the standby mode). However, in standby mode, the center does not treat the received signals as alarm conditions but can use such signals for diagnostic purposes. Therefore, with the detector always switched on, the center can detect sabotage attempts without a time delay.
Anti-masking devices, such as those described, for example, in EP-A-0 186 226, in EP-A-0 449 177 and in EP-A-0 556 898, serve to detect attempts to sabotage the detector, such as, for example, by covering the entrance window with a foil or a cover or by spraying the entrance window with a spray that is opaque to infrared, such as, for example, hair lacquer. Phenomena or optical changes immediately in front of the detector, such as covering the detector, in most cases effect a reflection of the radiation emitted by the optical transmitter of the anti-masking device onto the optical receiver, which is manifested in a clearly defined change in the radiation received by the optical receiver.
Anti-masking devices generally include an optical transmitter and an optical receiver which are generally formed by an infrared LED and an infrared diode. Changes in the optical properties of the entrance window are then detected by measuring either the radiation and the radiation passing through the entrance window or reflected by it. In the case of an anti-masking device described in EP-A-0 772 171, an optical diffraction lattice structure is mounted on the outside of the entrance window and focuses the light emitted by the optical transmitter on the infrared detector. In the event of sabotage by spraying the entrance window, the focusing action of the optical refraction lattice structure is destroyed, with the result that the intensity of the light falling on the infrared detector is reduced.
To evaluate the signals of the anti-masking device, the signals of the optical receiver are generally compared with threshold or reference values. These values generally correspond to voltage values that have to be exceeded (or not reached) and maintained over a certain time interval.
The evaluation of the signals is usually carried out by one of two known methods. One method is the so-called proximity-latch (PL) method in which a masking alarm is triggered as soon as the predetermined criteria are reached. The alarm then remains active and can only be reset by an authorized individual by a specified procedure. The PL, method therefore responds rapidly and in a sharply defined manner. However, the method has a drawback in that the alarm can be set, in the case of short movements without any intent to mask, and the alarm cannot be reset automatically, but requires the intervention of an operator. The second method is the so-called real-time (RT) method, in which only sufficiently large and sufficiently stable changes trigger a masking alarm. This is automatically canceled when the signals return to the normal state. The RT method responds more slowly and tends to be less sharply defined, but has the advantage of automatic alarm cancellation.
Regardless of whether the signal is evaluated by the PL or the RT method, it is necessary to ensure that the threshold or reference values are chosen very much on the “safe” side so that the smallest changes in the environmental conditions do not trigger a false masking alarm. Such changes may be formed, for example, by insects, temperature variations, dust deposits or nicotine deposits, but occasionally also by mechanical vibrations or atmospheric pressure changes. This means that both in the case of both the PL method and in the case of the RT method, a compromise has to be sought between sensitivity and immunity to masking alarms. Such a compromise may have the result that, in certain cases, maskings are not discovered or, alternatively, a false masking alarm is inadvertently triggered.
SUMMARY OF THE INVENTION
The object of the invention is therefore to provide a passive infrared detector having a masking alarm device that has both an increased immunity to false alarms and a higher sensitivity.
This object is achieved, according to the invention, in that the signals of the anti-masking device are evaluated in two channels, wherein one channel, designated below as PL, channel, responds to temporally limited phenomena or changes and the other channel, designated below as RT channel, responds to temporally stable phenomena or changes, and in that a combined evaluation of the signals in both channels is carried out.
In the passive infrared detector according to the invention, the PL method and the RT method are mutually combined, the greatest advantage of this combination being that the threshold or reference values can be set lower in the individual channels or possibly omitted completely. The latter may be the case if the signals are evaluated using fuzzy logic or in a neural network.
A first embodiment of the passive infrared detector according to the invention is characterized in that, in each channel, the signal is investigated by comparison with at least one threshold or reference value or by means of fuzzy logic, and in that the combined evaluation is formed by a combination of the test results in the two channels.
A second embodiment of the passive infrared detector according to the invention is characterized in that, in each channel, various values are defined for pre-alarm stages in addition to the threshold or reference value corresponding to the respective alarm stage, and in that the signals are compared with the pre-alarm and alarm stages.
A third embodiment of the passive infrared detector according to the invention is characterized in that the combined evaluation of the signals of the two channels of the anti-masking device is combined with that evaluation of the signals of the infrared sensor that is carried out in a channel that is designated below as PIR channel, and in that intrusion or masking alarms are triggered on the basis of the signals in all three channels. This embodiment provides a further increase in the immunity to false alarms since it makes the detector largely immune to malfunctions due to insects. For example, if a fairly large insect is moving in the vicinity in front of the entrance window, that can result in an alarm signal being triggered in the PIR channel. Since, however, the insect would also trigger an alarm signal in the PL channel, the signal in the PIR channel can be disabled on the basis of the alarm signal in the PL channel. On the other hand, an alarm signal in the PIR channel without a simultaneous alarm signal in the PL channel would be a true intrusion alarm, and an alarm signal in the PIR channel with simultaneous alarm signals in the PL channel and in the RT channel would indicate a masking attempt.
REFERENCES:
patent: 4709153 (1987-11-01), Schofield
patent: 4746910 (1988-05-01), Pfister et al.
patent: 4752768 (1988-06-01), Steers et al.
patent: 4982094 (1991-01-01), Matsuda
patent: 5499016 (1996-03-01), Pantus
patent: 5942976 (1999-08-01), Weiser et al.
patent: 0289621 (1988-11-01), None
patent: 0499177 (1992-08-01), None
Hegnauer Stefan
Mahler Hansjurg
BakerBotts LLP
Nguyen Tai T.
Siemens Building Technologies, AG Cerberus Division
Wu Daniel J.
LandOfFree
Passive infrared detector does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Passive infrared detector, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Passive infrared detector will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2539553