Patent Application
20070176765
This invention relates to security systems, and in particular to a security device that uses a microwave detector for range determination to improve the performance of a PIR detector.
Security systems often employ the use of passive infrared (PIR) sensors for detecting motion in a region. A PIR sensor comprises a lens array that divides the protected region into sectors, a PIR detector that detects from each sector heat radiating from an object, and an amplifier/threshold detection circuit for determining if the detected heat is above a threshold producing an alarm condition. As an intruder passes through the protected region, the lens array collects and focuses the intruder's heat from each sector it passes through onto the PIR detector to produce a sine wave. The frequency of the sine wave corresponds to the speed of the intruder walking through the sectors, and the amplitude of the sine wave corresponds to the amount of heat collected by the lens array onto the detector. Additionally, because the lens array collects heat from finger-like cones that get larger as the distance from the sensor increases, the frequency and the amplitude of the sine wave are dependent on the distance of the intruder from the PIR sensor and the direction in which the intruder is traveling. If the intruder is close to the PIR sensor, the frequency and amplitude are much higher than if the intruder is on the far side of the region. The amplifier/threshold detection circuit must be designed to handle the wide range of frequencies and amplitudes produced by the extreme cases, i.e. slow walks at the far end of the region and fast walks at the close ends of the region. This causes the PIR sensor to be more susceptible to noise and false alarms.
A second problem with the PIR sensors occurs when the intruder walks directly at the PIR sensor (so-called “down the throat”) rather than across the field and through the sectors of the lens array. In this case, the PIR may not detect the intruder.
An additional problem with PIR sensors is that they are designed to detect motion over a large region but are typically used in a much smaller region. This oversizing leaves the PIR sensor more vulnerable to false alarms. Typically, the PIR sensor is designed with a frequency response that balances the fast catch characteristics of up close motion with the slow catch performance needed at maximum distance. To get crisp catch in both cases leaves the unit very false alarm prone.
To alleviate the false alarm problems, dual-technology sensors have been designed that supplement PIR detectors with other detectors such as microwave detectors. The microwave detector and the PIR detector must both detect the intruder before an alarm condition is set. An alternative design is that the microwave detector output causes the threshold of the PIR threshold detection circuit to be adjusted. Both of these designs do not obviate the problem of down the throat detection because the PIR sensor will not produce a detectable signal.
It is therefore an object of the present invention to provide a security device that uses a PIR sensor and a microwave sensor for increased performance in detecting an intruder within a region without increased false alarms.
It is a further object of the present invention to provide a security device that uses the microwave sensor to determine the distance of an object within the region to adapt the frequency response of the PIR sensor for a crisp catch without higher false alarm sensitivity.
It is a further object of the present invention to provide a security device that detects an intruder walking directly towards or away from the sensor, or “down the throat”.
It is a further object of the present invention to provide a security device that can detect motion in both a larger region and a smaller region without being prone to false alarms.
The present invention is a method and device for detecting an intruder in a region with increased performance and decreased false alarms. The security device has a microwave sensor and a PIR sensor operatively coupled to a processor. To increase the performance of the security device, the device determines distance information of an object in the region with the microwave sensor, processes the distance information to adapt the frequency response of the PIR sensor to provide a frequency adapted PIR signal, and determines if the object is an intruder by using the frequency adapted PIR signal.
The security device determines the distance information of an object in the region by transmitting a microwave pulse, receiving a microwave pulse reflected off of an object, determining the phase difference between the transmitted and received microwave pulses, and determining the distance of the object from the phase difference. The distance may also be determined in other ways such as measuring the time difference between the transmitted microwave pulse and the received microwave pulse.
The security device's processing circuitry processes the distance information to determine the desired frequency response of the PIR sensor and adapts the frequency response of the PIR sensor to correspond. This may be accomplished in the following manner. The processor inputs the distance information from the microwave sensor and selects the amplifier/filter parameters from stored filter parameters in memory, based on the distance information. If the filtering is performed digitally, the processing circuitry inputs the PIR signal from the PIR detector, stores the PIR signal, filters the PIR signal using the selected filter parameters, and generates the frequency adapted PIR signal. Digital filtering of the PIR signal is known in the art and is the preferred embodiment. One skilled in the art will recognize that the filtering may be performed by a parallel analog filter and analog switches.
The processing circuitry determines if the object is an intruder by using the frequency adapted PIR signal which is a more accurate representation of the object's motion and comprises less noise. The processing circuitry compares the frequency adapted PIR signal to a predetermined threshold, and if the frequency adapted PIR signal is above the predetermined threshold, the processing circuitry sets an intruder alert (such as by sending an alert signal to a centrally located control panel for further processing). An additional embodiment to further reduce false alarms and help with pet immunity is to change the predetermined threshold based on the distance information. The processing circuitry may perform this by storing a selection of predetermined thresholds and selecting which threshold is used based on the distance information received from the microwave sensor. For additional selections of stored thresholds, a pet immunity function may be enabled by an installer through selection of a jumper wire or programming means.
To alleviate the problem of down the throat intruder detection, the processing circuitry stores and updates the distance information of a detected object in the region and compares the distance information to a previously stored distance information to determine if the object is moving directly towards or away from the PIR sensor. If the processing circuitry determines this to be true, but the PIR sensor is not producing a detectable signal, the processing circuitry will set the intruder alert.
Lastly, to address the problem of using the PIR in a smaller room even though it is designed for a larger region, the processing circuitry determines if the distance information from the microwave sensor is greater than a predetermined distance, and if it is, then an intruder alert is not set even if the object is determined to be an intruder. The predetermined distance may be programmed during installation through wire jumpers or programming means. Additionally it may be necessary to provide exclusion areas within a large room where false alarms may be created by something in that area, such as a banner. In this case the processing circuitry determines if the distance information from the microwave sensor is within a predetermined zone, and if it is, then an intruder alert is not set. The predetermined zone may be programmed during installation through jumpers or programming means.
The preferred embodiments of the present invention will now be described with respect to the Figures.
Also shown in
It will be apparent to those skilled in the art that modifications to the specific embodiment described herein may be made while still being within the spirit and scope of the present invention. For example, the distance information may consist of more than three ranges, the ranges may be different sizes, or an actual distance information may be transmitted to the processor 140 from the microwave detector 100 rather than the three flags 110, 112, or 115. Also the distance information may be determined by measuring the time between the transmitted microwave pulse and the received microwave pulse. The size of the region 20 may be programmed differently than by the use of jumpers, and the information may be used by the processor to discriminate against distances out of range. Additionally, the digitizing may be performed internal to the processor, or there may be no digitizer and the filtering and the thresholding is performed using parallel analog circuits whose outputs are selected based on the distance information. Lastly, the processing flow may perform the same operations in a different order than described above.
