Sirens of alarm in engineering
A passive alarm is a type of detector that is used in a security project. But to its normal use, it is necessary to prevent omission, but also to reduce false positives, especially how to minimize the phenomenon of false positives is a large number of engineering designers in front of a subject. To do this, we must first understand the passive alarm some of the basic concepts and technical characteristics, so as to be based on these basic technical characteristics, from the installation, commissioning, use and other links, according to the basic technical characteristics of the detector, so In order to maximize the maximum effectiveness of the detector.
1, passive alarm infrared detection of the basic concepts
In the range of warnings, why can a passive alarm be able to generate an alarm signal when moving?
In nature, any object above the absolute temperature (- 273 degrees) will produce infrared spectra, different temperature objects, the release of infrared energy wavelength is not the same, so the infrared wavelength and temperature are related.
There are two key components in the passive alarm, one is the pyroelectric infrared sensor (PIR), which converts the infrared signal between wavelengths from 8 to 12um into electrical signals and can convert white light in nature The signal has a suppression effect, so the passive alarm in the warning area, when no human movement, the pyroelectric infrared sensor sensor is Sirens only the background temperature, when the human body into the alert area, through the Fresnel lens, pyroelectric Infrared sensor sensor is the difference between body temperature and background temperature signal, therefore, the alarm is the basic concept of infrared detection is to move the object and the background object temperature difference. Another device is the Fresnel lens, Fresnel lens in two forms, namely, refraction and reflection. Fresnel lens has two roles: one is the focus, that is, the thermal signal of the infrared signal refraction (reflection) on the PIR, the second role is to distinguish the area into a number of clear areas and dark areas, so that into the alert The moving object in the zone can change the pyroelectric infrared signal on the PIR in the form of temperature changes so that the PIR can produce a changed electrical signal.
Sirens Passive alarm development to today, the technology has been relatively mature, anti-small pets are passive alarm is an important function of each manufacturer against small pet interference is not the same way, but no more than two Kind of way: one is the physical way, that is, by Fresnel lens segmentation changes to reduce the probability of false positives caused by small pets, this approach is the surface, the effect is limited. The second way is to use the detection signal processing analysis, mainly for the detection of the signal data acquisition, and then analyze the signal cycle, amplitude, polarity. These factors specifically reflect the speed of moving objects, the size of pyroelectric infrared energy, and the displacement per unit time. Microprocessor in the detector will be the data collected for analysis and comparison, thus determining whether the moving object may be small or small animals.