The principle and application of curtain detector

First, the principle of passive infrared detector

Passive pyroelectric infrared detector referred to as passive infrared detector, in the field of electronic anti-theft, human body detector, passive infrared detector (referred to as PIR, Passive infra-red detectors) is widely used, because of its low price, stable technical performance and used in various fields.

Theoretically, as long as the temperature of all objects in nature is above absolute zero (-273.15 °C), its surface will constantly radiate infrared rays. The wavelength of infrared energy released by objects at different temperatures is not the same, so the infrared wavelength is related to the temperature. According to the difference in thermal radiation of various parts of the human body (including in vivo), the peak value is between 8-12 μm.

Passive infrared probes work by detecting about 10UM infrared rays emitted by the human body. About 10UM of infrared rays emitted by the human body are enhanced by Finier filters and concentrated on the infrared sensing source. The infrared induction source usually uses pyroelectric elements, which will lose the charge balance when the temperature of the infrared radiation of the human body changes, release the charge outward, and the subsequent circuit can generate an alarm signal after detection and processing.

Second, the principle and development of curtain detector

The PIR detector has two key components: a pyroelectric sensor and a Fresnel optical lens set on the front of the detector. Fresnel optical lens groups usually consist of many sets of lenses, ranging from a few lenses to dozens of lenses. Fresnel filters have different focal lengths (sensing distances) according to different performance requirements, resulting in different monitoring fields, and the more fields of view (the greater the number of lenses in the corresponding lens group), the tighter the control. Most detectors can change different lenses according to different detector requirements, including: wide-angle lenses, long-distance lenses, insect-proof lenses, curtain lenses, etc., which are actually combinations of Fresnel optical lenses with different numbers and focal lengths. Among them, the curtain detector has more characteristics in the detection mode, so it is widely used, and Figure 1 is the detection area diagram of the curtain detector.

From the top view of the curtain detector in Figure 1, it can be seen that the detection area (shadow area) is a narrow band, so that the detector is only triggered when crossing the detection area.

The traditional detector is transformed into a curtain detector by replacing the curtain lens, but in terms of performance, such detectors are only detected in the curtain area, and other aspects, such as: installation methods, shape design, detection angle, detection accuracy, etc. are subject to different degrees of restrictions. Therefore, some security manufacturers design special curtain detectors.

Third, the principle and development of directional curtain detector

The characteristics of curtain detectors determine that they are more suitable for protecting intrusion channels such as doors and windows, such as installing a curtain detector on the side wall of indoor windows, which can protect the entire wall with windows, which is equivalent to installing an "electronic curtain" on the wall with windows, so as to leave more space for indoor people. Although more space is given to indoor people, indoor people inadvertently enter the detection area of the curtain probe, which also triggers an alarm. On this basis, Israel EL Company first proposed the "motion vector judgment" technology, which applied the movement direction recognition technology to the curtain detector, so that the detector can identify different directions of movement of the object. Take the curtain detector of EL Company in Israel as an example: when a person moves from the unprotected area of the detector to the protected area, when passing through the detection area of the curtain probe, the alarm is immediately triggered; When a person moves from the protected area of the detector to the unprotected area, when passing through the detection area of the curtain probe, the detector delay is triggered, and during the delay, the detection area of the curtain probe entering from the outside will not trigger an alarm. This leaves more range of activity for indoor protected objects, and there is no need to worry about triggering the detector alarm from the inside, and the indoor protected objects can also pass through the detection area from the inside to the outside for a short time and then come back, without triggering the curtain detector alarm (Figure 2 is a schematic diagram of the working principle of the steering curtain).

Curtain detectors are classified from the aspect of pyroelectric induction elements: dual pyroelectric induction unit, four-element pyroelectric induction unit. The so-called dual (quaternary) that is, each detector has two (four) pyroelectric inductive sensor units, foreign detector manufacturers usually integrate two (or four) induction units on one electrical element, so it is difficult to distinguish from the appearance of the detector's pyroelectric electrical components is several element inductors. Some domestic manufacturers have also developed directional curtain detectors, such as double-curtain directional detectors and four-curtain directional detectors. From a technical point of view, the four-element directional curtain detector is more advanced than the dual-element directional curtain detector, and has better direction judgment accuracy and reliability. Let's describe the working principle of the dual directional curtain detector and the quaternary directional curtain detector.

It can be seen from Figure 3: whether binary or quaternary, the curtain detector is determined by judging the order of the sensing unit of the triggering detector, and at the same time combining the logic of the microprocessor to determine whether to trigger delay or trigger alarm output. Four-element directional curtain detector, respectively, two sensing units first sense internal infrared changes and two sensing units first sense external infrared changes, which plays a double identification of human body in/out movement, and also reduces false alarms caused by local temperature and airflow changes. At the same time, the four-element directional curtain detector is more convenient for the positioning of the detector and the position adjustment of the detection area during installation. The dual directional curtain detector has only two sensing units, which work on the same principle as the quaternary directional curtain detector, but is slightly inferior to the quaternary directional curtain detector in terms of performance.

4. Application conditions

Directional curtain detector in the protection bedroom and living room, to prevent illegal intrusion when there are people in the home is very suitable and more characteristics, to achieve the overall comprehensive protection program, should be a variety of detectors, a combination of various detection methods.

Although the directional curtain has many advantages, it also has some weaknesses, such as the delay of the directional curtain detector, which not only gives the protected object more time to travel from the inside to the detection area to the outside activity, but also illegal intrusion in this delay is also ignored by the detector, so it is usually recommended that the delay time is not too long. Although the directional curtain detector has the function of identifying the direction of movement, if the detector is installed on the bed, sofa, table or place where people often continue to move, due to the interference of factors such as temperature and airflow, false alarms may be caused, so the above position is not suitable for installing the directional curtain detector. In addition, the directional curtain detector itself belongs to the category of passive infrared detector, so the passive infrared detector is usually not suitable for installation, and the directional curtain detector is also not suitable for installation, such as: the lower part of the air conditioner, the upper part of the heating and hot fan, the place where the air flow is large, the place close to the curtain, the rotating fan in the detection area and other places that are easy to cause false alarms.