Currently, in the field of security systems, motion detectors are generally provided to detect intruders. Motion detectors often incorporate microwave (MW) technology and/or passive infrared (PIR) technology. Dual technology motion detectors incorporate multiple sensing technologies in order to provide efficient intruder detection with minimized risk of false alarms. Many dual detectors incorporate PIR technology and MW technology.
PIR technology has long been used in motion detectors. The PIR sensor detects the difference between the infrared energy emitted from an intruder and that emitted from the ambient environment. Many PIR detectors utilize Fresnel lenses or custom shaped mirrors to focus infrared energy on a pyrodetector. The output signal from the pyrodetector is then processed via analog hardware and/or digital signal processing. Lenses and mirrors are designed to provide various detection zones emanating radially from the sensor. As a target moves across the PIR detection zones, the sensing elements within the pyrodetector are alternately exposed to the target IF energy, resulting in an alternating voltage output from the PIR sensor. The amplitude and frequency of this voltage vary with a number of factors including target size, speed, and direction relative to the PIR zones, difference between ambient and target temperature, width and spacing between the detection zones, and frequency response of the pyrodetector.
Upon receiving the signals, the detector may perform processing by comparing the received signal to one or more voltage thresholds. These threshold crossings produce positive and negative pulses that can be counted and timed, with certain combinations of pulse height, duration, and frequency being considered PIR alarms.
MW technology often operates on the principle of phase shift or Doppler effect. Unlike PIR, MW technology is an active technology. The MW detector transmits MW energy, which reflects off objects and returns to the MW detector. Moving objects result in a received signal that is frequency shifted from the original transmitted signal. The detector receives this signal, and generates an alternating voltage difference frequency signal which is then processed via hardware or digital signal processing. Because only the AC (alternating current) component of the signal is processed, only moving objects are detected. The frequency of the returned signal is dependent upon the target movement direction and speed. The amplitude is a function of transmitted signal strength, target size, distance, and reflectivity. Highly reflective materials such as metal will return significantly greater energy to the detector than less reflective objects such as people or animals. Processing may include comparison of the MW signal to one or more thresholds with certain combinations of quantity, duration, or frequency of threshold crossings considered MW alarms.
Typically, dual technology detectors incorporate an AND function, whereby both individual sensor technologies must be in the alarm state simultaneously to produce an alarm at the detector output. PIR and MW technologies have many differing and distinct potential false alarm sources, as well as some common sources such as small animals. Minimization of false alarms from either detector technology is likely to greatly reduce the incidence of false alarms for the dual detector system.
MW motion detectors are susceptible to false alarms caused by small animals and insects within the detection range of the MW detector. Furthermore, many MW detectors also suffer from decreased sensitivity in the area beneath the sensor because very little MW energy is transmitted to, or can be received from that direction due to the shape of the MW transmitter and/or receiver patterns.
Some currently existing systems, such as that of U.S. Pat. No. 5,578,988 provide sensitivity adjustments for MW detection systems. However, such systems do not consider a distance of the target object from the dual detector in order to reliably minimize the probability of false alarms. A solution is needed that minimizes MW false alarm effects and improves MW detector performance by considering target location in order to enhance accuracy.