All objects whose temperature exceeds absolute 0℃ can produce heat radiation (infrared spectrum), while those whose temperature is lower than 1725℃ can produce heat radiation in the infrared region, so all objects in nature can radiate infrared heat outwards. The wavelengths of infrared energy released by objects with different temperatures are different, so the infrared wavelength is related to the temperature. However, the wavelength and distance of infrared radiation produced by any object due to its own physical and chemical properties and its own temperature are also different, which is usually divided into three bands.
- Near infrared: wavelength range 0.75 ~ 3μ m
- Mid-infrared: wavelength range 3 ~ 25μ m
- Far infrared: the wavelength range is 25 ~ 1000 μ m
The infrared light radiated by human body has a wavelength of 3 ~ 50 μ m, of which 8 ~ 14 μ m accounts for 46%, and the peak wavelength is 9.5 μ m.
There are two key elements in the passive infrared detector, one is pyroelectric infrared sensor (PIR), which can transform infrared signals with wavelength between 8-12um into electrical signals and inhibit white light signals in nature. Therefore, in the alert area of passive infrared detector, when no human body moves, the pyroelectric infrared sensor only senses the background temperature, and when human body enters the alert area, it passes through Fresnel lens. Pyroelectric infrared sensor senses the difference signal between human body temperature and background temperature. Therefore, the basic concept of infrared detection of infrared detector is to sense the temperature difference between moving object and background object. The other device is Fresnel lens, which has two forms, namely refractive and reflective. Fresnel lens has two functions: first, focusing function, that is, refracting (reflecting) the pyroelectric infrared signal on PIR; second, dividing the alert area into several bright areas and dark areas, so that the moving objects entering the alert area can generate varying pyroelectric infrared signals on PIR in the form of temperature changes, so that PIR can generate varying electrical signals.
At room temperature, everything has radiation. The higher the temperature, the stronger the infrared radiation. Man is warm-blooded animals, and infrared radiation is the most stable. We call it passive infrared, that is, the detector itself does not emit any energy, but only passively receives and detects infrared radiation from the environment. Several seconds after the detector is installed, it has adapted to the environment. When no one or animal enters the detection area, the infrared radiation on the spot is stable. Once the infrared radiation from human body comes in, the pyroelectric device will generate abrupt electrical signals after being focused by the optical system, thus giving an alarm. The cordon formed by passive infrared intrusion detectors can generally reach tens of meters.
Passive infrared detector is mainly composed of optical system, thermal sensor (or infrared sensor) and alarm controller. The core is that infrared detectors are missing, and the change of thermal radiation in a certain three-dimensional guard space can be detected through the cooperation of the science system. The detection wavelength range of infrared sensor is 8 ~ 14 μ m, and the infrared peak wavelength of human radiation is about 10μm, which is just within the range.
Passive Infrared Detector (PIR) can be roughly divided into single-beam type and multi-beam type according to its different structure, different warning range and different detection distance. Single-beam PIR adopts reflective focusing optical system, and uses curved mirror to converge infrared radiation from target on infrared sensor. In this way, the field of view of the detector is narrow, generally below 5, but the operating distance is far, up to 100 meters. Therefore, it is also called linear remote control passive red detector, which is suitable for protecting narrow corridors, passages and blocking doors, windows and walls. Multi-beam type adopts lens focusing optical system. At present, infrared plastic lens-Fresnel lens with multi-layer beam structure is mostly used. This kind of lens is made of special plastic at one time, and several small lenses are arranged on an arc surface. The warning range is in a plurality of single beam states in different directions, forming a three-dimensional fan-shaped heat sensing area and forming a three-dimensional warning. Fresnel lenses are divided into several rows from top to bottom, with more lenses on the top and less lenses on the bottom. Because the infrared radiation of human face, knees and arms is strong, just facing the lens above. There are fewer lenses on the lower side, one is because the infrared radiation on the lower part of human body is weak, and the other is to prevent the interference of infrared radiation from small animals on the ground. The warning view angle of multi-beam PIR is much larger than that of single-beam PIR, which can be greater than 90° horizontally and up to 90° vertically, but the action distance is relatively short. All lenses focus on pyroelectric devices arranged inside, so the sensitivity is high, and an alarm will be given if someone walks in the field of view of the lens.
The penetration of infrared light is poor, so there should be no tall objects in the guard area, otherwise, people in the shadow part will not be able to give an alarm, and do not face the heat source and strong light source, especially air conditioning and heating. Otherwise, the changing hot air flow will cause false alarm. In order to solve the problem of blocking objects, a ceiling passive infrared intrusion detector was invented. It is installed on the ceiling within 360 degrees downward for warning. As long as it is within the protection range, any intrusion from any direction will trigger an alarm, which is widely used in places with large space such as banking halls and public activity areas of shopping malls.
Passive alarm detectors are widely used because of their good detection performance, easy deployment and low price. Its disadvantage is that the false alarm rate is higher than that of active detection. Therefore, now we choose more double-method detector or multiple-method detector, and add microwave detection technology, which can effectively reduce the false alarm rate.
Precautions for Installation of Passive Infrared Detector
Because passive infrared detector is a kind of weak signal detection equipment, attention must be paid to some details in installation, such as height and sensitivity. To correctly install a passive infrared detector, we must master the following information: firstly, we must understand the performance characteristics of the detector; secondly, we must reasonably determine the installation position; finally, we must carefully debug it. It can't be said that the detector can alarm, so how to determine the installation position of a passive infrared detector?
- Determine the normal installation angle according to the manual: the installation height is not arbitrary, which will affect the sensitivity of the detector and the effect of preventing small pets. Imagine that a detector is installed at a height of 2M and a height of 2.5, so when a moving object moves from the ground, the frequency of cutting bright and dark areas is different.
- Inappropriate to face glass doors and windows: Passive infrared detectors face glass doors and windows, and there will be two problems: First, white light interference. Obviously PIR has a strong suppression function on white light, but it is not 100% suppression after all. Therefore, it can avoid facing glass doors and windows, and can avoid the interference of strong light. Second, avoid complicated environmental interference outside doors and windows, such as crowd flow and vehicles.
- It is not suitable to face cold and hot vents or cold and hot sources: the sensing effect of passive infrared detectors is closely related to the change of temperature. Cold and hot vents and cold and hot sources may cause false alarms of detectors. For some detectors with low performance, sometimes air convection through doors and windows may also cause false alarms.
- It is not suitable to face easily swinging objects: easily swinging objects will make the microwave detector work, so it can also cause false alarms. Therefore, pay attention to the illegal invasion route. The purpose of installing detectors is to prevent criminals from invading illegally. Before determining the installation location, the main entrances and exits of buildings must be considered. In fact, we prevented the entrance and exit and cut off the illegal invasion lines, thus achieving our goal.
- Reasonable selection: Passive infrared detectors have various models. The detector to be installed must consider the size of the guard space, the surrounding environment, the characteristics of the entrance and exit, and other actual conditions. Sometimes it is necessary to replace Fresnel lens to meet the requirements.
After installing the detector, debugging the detector is the last job to be done. There are two methods for debugging passive infrared detectors, one is step measurement, that is, the debuggers walk S-shaped lines in the warning area to perceive the equal width of the warning range, which can be understood from the figure.
Microwave sensitivity and infrared sensitivity should be adjusted separately by step measurement, too high or too low sensitivity will affect the preventive effect. Sometimes due to seasonal changes, sensitivity should be adjusted separately in winter and summer. The sensitivity of microwave must not be too high, because microwave only has penetrability, so we should pay attention to it when debugging.
The second method is instrument measurement. Some detectors have background noise voltage output interface, which is tested by voltage of multimeter. When the detector is on alert, the output voltage of static background noise indicates the interference degree of interference source, so as to judge whether this position is suitable for installing such detectors.
The above part is just the author's analysis of the basic generality of the use and installation methods of passive infrared detectors. Various brands have their own unique methods in infrared processing. Therefore, we must read the instructions carefully before using and installing. The most important thing is to gain a deeper understanding of the characteristics of the detectors through the accumulation of practical work experience, so as to better play the role of the passive infrared detectors in security engineering.