Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1998-05-27
2001-09-11
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S339140, C250S252100, C340S567000
Reexamination Certificate
active
06288395
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to intruder detection systems and methods employing passive infrared (PIR) sensors.
BACKGROUND OF THE INVENTION
Intruder detection systems for home or commercial security applications often employ PIR sensors to detect the movement of heat-emitting objects within a detection area. A PIR sensor typically includes a pair of heat sensor elements. Each of the heat sensor elements comprises a pyroelectric material, or other radiation sensitive material, that generates electric charge in response to incident infrared radiation. The heat sensor elements generate oppositely poled signals. The PIR sensor includes a fresnel lens that defines the field of view of the sensor elements. The fresnel lens includes an array of sub-lenses that divides the detection area into a plurality of detection zones. The sub-lenses focus infrared radiation from each of the detection zones onto the heat sensor elements. Each of the heat sensor elements generates a signal representative of the incident infrared radiation. The PIR sensor sums the oppositely poled sensor element signals to produce a detection signal.
Signal processing circuitry associated with the PIR sensor receives the detection signal and performs appropriate amplification and filtering for presentation of the signal to a comparator circuit. The detection signal represents the difference between heat emitted by an intruder and background heat emission. If no intruder is present, the signals generated by both of the heat sensor elements will represent background heat emission and generally cancel out one another when summed. As a result, the detection signal will tend toward zero, or at least some base-line level, when no intruder is present.
As a heat-emitting object crosses from one zone to another, the amount of heat radiation received by the sensor elements will vary. In particular, boundaries between zones created by the sub-lenses will block portions of the infrared radiation emitted by the object. The sensor elements are spatially displaced relative to one another. Thus, as the object moves and interacts with zone boundaries, the sensor elements will receive radiation from a given zone in a temporally displaced manner. The summed detection signal therefore will rise and fall, taking on an alternating waveform. The frequency of the waveform is a function of the velocity of the object across the zones and the distance of the object from the sensor elements. The rise and fall of the detection signal provides an indication of movement within the detection area, whereas the amplitude of the signal gives an indication of its significance, i.e., whether the signal is indicative of a heat-emitting object that qualifies as an intruder.
To determine the significance of the detection signal, a comparator circuit is provided to compare the signal to a predetermined threshold. The threshold may take the form of a window having upper and lower thresholds. Upon a signal excursion outside of the window, i.e., above the upper threshold or below the lower threshold, a window comparator generates a signal indicative of the presence of an intruder. The detection system then initiates a response such as the activation of an alarm and/or the dispatch of security personnel.
The avoidance of false alarms is a concern in any detection system. Also important is the ability to detect the presence of an intruder under a variety of conditions. Due to a number of variations, however, the comparison of the signal to a predetermined threshold can result in false alarms or the failure to detect intruders. Such variations include changes in environmental conditions existing in the detection area, various features of an object entering the detection area, varying characteristics of the signal processing circuitry, or a combination of the above factors.
As an example, the detection signal can vary in amplitude as a function of intruder emission relative to an ambient temperature existing in the detection area. Specifically, as the ambient temperature varies, the background heat emission similarly varies. The result is a variation in the amplitude of the intruder detection signal. As the ambient temperature approaches human body temperature the difference in temperature between background and an intruder will decrease. Consequently, the amplitude of the detection signal decreases significantly. If the threshold window is set too wide, the signal processing circuitry may fail to resolve the presence of an intruder at ambient temperatures producing smaller signal amplitudes. If the threshold window is set too narrow, however, the vulnerability of the system to false alarms increases.
The detection signal also varies as a function of the frequency of the detection signal, and thus the velocity of an object within the detection area. As the velocity of an object increases, the amplitude of the detection signal tends to decrease. When the detection signal is compared to a threshold, variations in the signal amplitude due to the velocity of an object in the detection area can result in false alarms or detection failure.
Another variation in the amplitude of the detection signal that can lead to false alarms or detection failure is the introduction of a slow ac rise or fall. Specifically, over time, the detection signal can acquire a significant, but slowly changing, increase or decrease due to factors such as changes in ambient temperature or prolonged presence of an intruder within one or more zones. The increase or decrease tends to shift the magnitude of the signal. Although the output of PIR sensor typically will be accoupled to a signal processing circuit, the resulting rise or fall may have the local effect of a dc offset. With such a shift, a detection signal that otherwise would not be indicative of the presence of an intruder may extend outside of a given threshold window. As a result, the detection system may register a false alarm. With a shift causing the signal to fall inside of one of the window thresholds, the detection system may fail to detect the presence of an intruder.
SUMMARY OF THE INVENTION
In view of the foregoing problems associated with existing PIR-based detection systems, i.e., false alarms and detection failure due to variations in the amplitude of the detection signal, there is a need for an improved detection system.
The present invention is directed to a system and method for processing signals generated by a PIR sensor in an intruder detection system. A system or method in accordance with the present invention is capable of reducing the occurrence of false alarms and detection failures by compensating for variations in the amplitude of a detection signal generated by a PIR sensor.
For example, an adaptive threshold may be provided that varies according to ambient temperature and detection signal frequency. Comparison of the detection signal to the adaptive threshold allows compensation for temperature-and/or frequency-induced variations in detection signal amplitude. In this manner, the effects of such amplitude variations in producing false alarms or compromising detection effectiveness can be mitigated.
The adaptive threshold can be configured for standard detection area conditions. If desired, however, the adaptive threshold can be calibrated according to temperature and frequency variations existing in a particular detection area. In this manner, a detection system can, in effect, be tuned for the unique characteristics of a detection area.
The calibration can be comprehensive in scope, involving the measurement of detection signal amplitudes for a wide expanse of temperatures and frequencies. Alternatively, calibration may involve a “recalibration” in which a smaller number of measurements are taken, and adjustments are made to the adaptive threshold.
Further, relative measurement techniques and adaptive sampling may be used to compensate for the presence of a shift in the detection signal due to factors such as temperature change or prolonged intruder presence. Relative measurement te
Kuhnly Keith D.
Saldin Paul G.
Epps Georgia
Fish & Richardson P.C. P.A.
Hanig Richard
Interactive Technologies Inc.
LandOfFree
Passive infrared detection system and method with adaptive... 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 detection system and method with adaptive..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Passive infrared detection system and method with adaptive... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2439347