Anti-theft alarm system principle and characteristics

Many foreign manufacturers also admitted that foreigners had relied heavily on technological superiority to cope with the cost advantage of Chinese national products. However, in recent years, the gap has narrowed and the advantages have gradually diminished. It can be seen that Chinese manufacturers have gradually caught up with foreign manufacturers in technology, and some manufacturers are more innovative. The ability to introduce unique products has greatly improved the level of Chinese security products. This phenomenon mainly comes from the attention of many manufacturers to brand awareness and product quality, and has increased investment and R&D efforts.

Active infrared detector principle and characteristics

The human body has a constant body temperature, generally around 37 degrees, and emits infrared light with a specific wavelength of about 10 μm. A passive infrared detector works by detecting infrared radiation emitted by a human body at about 10 μm. The 10 μm or so infrared emitted by the human body is enhanced by the Fresnel filter and collected on the infrared sensor. Infrared sensors usually use pyroelectric elements, which lose their charge balance when they receive a change in human infrared radiation temperature, and release charge outward. The subsequent circuit can generate an alarm signal after detection and processing.

1. Passive infrared intrusion detectors are aimed at detecting human radiation, so pyroelectric elements must be very sensitive to infrared radiation having a wavelength of about 10 μm.

2. In order to be sensitive only to the infrared radiation of the human body, a special Fresnel filter is usually covered on its radiation surface, so that the interference of the environment is obviously controlled.

3. Its sensor contains two pyroelectric elements connected in series or in parallel with each other. In addition, the two polarization directions produced are exactly the opposite. The ambient background radiation has almost the same effect on the two pyroelectric elements, causing them to cancel each other's charge generation effect. Thus, the detector has no signal output.

4. Once the person invades the detection area, the infrared radiation of the human body is partially focused by the mirror, and is received by the pyroelectric element. However, the heat received by the two pyroelectric elements is different, and the pyroelectricity can not be cancelled by the signal processing.

Passive infrared depth detector advantages and disadvantages

Advantages: Does not emit any type of radiation itself, the device power consumption is very small, better concealment, and low prices.

Disadvantages: easy to be interfered by various heat sources and sunlight sources; passive infrared penetrating power is poor, human body's infrared radiation is easily shielded, is not easily received by the detector, is susceptible to interference by radio frequency radiation, and when the ambient temperature and human body temperature are close, the detection and The sensitivity has decreased significantly, sometimes causing short-term failure.

How to properly install and use a passive infrared detector

Passive infrared detectors are a type of detector that is most commonly used in security engineering. However, for its normal use, it is necessary to prevent omissions, but also to reduce false positives, mainly to minimize false positives. To achieve this, we must first understand some of the basic concepts and technical characteristics of passive infrared detectors, so that in accordance with these basic technical characteristics, from the installation, debugging, use and other aspects, in accordance with the basic technical characteristics of the detector, In order to maximize the detector's maximum efficiency.

First of all, we must understand the following information, which will be of great help to the correct use of passive infrared detectors.

According to the instructions to determine the installation height

The installation height of the detector is not random, and it will directly affect the sensitivity of the detector and the effect of preventing animals. Generally, the installation height of the wall-mounted infrared detector is 2.0-2.2 meters.

Should not face glass doors and windows

There are two problems facing the glass doors and windows: First, white light interference, although the passive infrared detector (PIR) has a strong suppression of white light, but after all, is not 100% inhibition, do not face the glass doors and windows, you can avoid glare The second is to avoid complex environmental disturbances outside the doors and windows, such as direct sunlight, crowds, and mobile vehicles.

Not suitable for hot and cold vents or cold and heat sources

The induction effect of the passive infrared detector is closely related to the temperature change. Both the cold and hot vents and the cold and hot sources may cause false alarms for the detector. For some low-performance detectors, air convection through windows and doors sometimes occurs. Caused false positives.

It is not right to swing large objects

A large swing of a large object can momentarily cause sudden changes in the air flow in the detection area. It can also cause false alarms; for example, an outdoor detector should avoid large trees and tall shrubs.

Reasonable position

There should be no screens, furniture, large bonsai or other partitions within the scope of detection; it is best not to install two wireless infrared detectors in the same space to avoid the phenomenon of interference due to simultaneous triggering; infrared detectors should be The indoor walking line is at a certain angle. The detector is the least sensitive to radial movements, and the detector is most sensitive to tangential movement (ie, the direction perpendicular to the radius). Selecting the proper installation location on site is an extremely important step to avoid false alarms for infrared detectors and to obtain the best detection sensitivity.

Reasonable selection

Passive infrared detectors have a variety of models, from indoor to outdoor, from wired to wireless, from single infrared to three infrared, from wall-mounted to ceiling-mounted, so the detectors to be installed must consider the size of the prevention space , the surrounding environment, the characteristics of the entrance and exit and other actual conditions.

After installing the detector, the commissioning of the detector is the last thing to do. The commissioning of passive infrared detectors is usually step-by-step. That is, the commissioning personnel walks the S-type line in the alert zone to sense the length and width of the alert range to test whether the entire alarm system meets the requirements. The instructions can be consulted to properly adjust the detector's sensitivity. Excessively high or low sensitivity will affect the preventive effect.

Detector's false alarm factors and solutions

The ideal intrusion detector merely responds to the presence of personnel and does not respond to the activities of animals such as dogs, cats, and mice, and does not respond to changes in the indoor environment such as changes in temperature, humidity, and wind, rain, and vibration. It is not easy to do this, most devices not only respond to people's existence, but also respond to the influence of some unrelated factors.

Alarms issued when there are no intrusions are called false alarms. False positives may be caused by component failures or some external influences. The adverse consequences it produces are disastrous. The lightest consequences are annoying because of the increase of many unnecessary troubles, which greatly reduces the number of alarms. Reliability. The worst consequence is that it causes the security personnel to rush to the scene unnecessarily, so that their own safety and the safety of people around them will be jeopardized. Therefore, false alarm is the fatal weakness of the alarm.

Let us analyze the main reason that infrared detection alarms may cause false alarms. At present, there are several reasons for false alarms in the alarm system. The poor performance of the wireless detectors in terms of anti-jamming performance is likely to cause false alarms due to the same frequency interference. Infrared detectors cause false alarms due to inaccurate judgment in intrusion behavior. Infrared detectors are vulnerable False positives due to environmental factors such as temperature, light, etc.; host and detectors use wireless coding to set the code to have duplicates causing host and detector re-numbers to cause false positives; some alarms also have poor quality, such as components. Damage and poor production processes cause false positives; there are also related to the selected equipment, installation methods, angles, locations, and also under the influence of the environment such as air movement, pet actions, etc., as well as human factors. Improper operation by users, accidental triggering of alarms, mistakes, and mistaken access to the already secured visiting areas can result in false alarms. The reasons for false positives are many and complex. Therefore, the most important thing to reduce the false alarm of burglar alarm is to consider it from many aspects, such as selecting the detector from the aspects of technology and performance, including the selection of the sensor detector, the shape design of the Fresnel lens, and micro processing. Program, multi-technology, automatic code skipping coding technology, production process, use methods, temperature compensation, sensitivity detection distance adjustment and so on. These comprehensive factors determine the detector's performance and false alarm rate.

A new generation of infrared detectors designed to meet the above conditions adopt some unique technologies to solve such problems. First of all, in the focus of the infrared lens, two photothermal sensors connected by different circuits are applied. The special lens divides the coverage area into multiple protected areas with a trapezoidal sensitivity and guarantees signal strength in the protected area. The unique lens also acts as an infrared filter, and the specially treated black lens on the surface allows visible and shortwave infrared (which exists in most white light sources) to be injected and then absorbed by its black base. The black infrared lens reflects only the infrared rays whose wavelength matches the body's movement and reflects it to the photothermal sensor. With a white filter, the detector can prevent white light interference. If a person enters or leaves one or more areas covered by the detector, the detector should detect the change in infrared radiation energy. The A/D converter digitizes the signal sent from the sensor and analyzes it with the processor. Then it will send an alarm signal.

This process involves all the criteria in the probe, such as signal amplitude, time, format, energy, and spectrum, plus statistical information actually extracted from the site. These criteria must be combined with the judgement to detect their rationality. The detector will not issue an alarm until the results meet the criteria for forced entry. Digitizing the signal of the photo-thermal sensor eliminates signal transients and electromagnetic interference.

At present, Shuangjian detectors occupy a certain share in the market, and microwaves are insensitive to changes in temperature and light. Through the combination of two detection units, the smart Shuangjian passive infrared detectors of the SuiLi detector have greatly reduced errors. Possibility.

The sensitivity and reliability of the alarm detectors influence each other. Reasonably selecting the detection sensitivity of the alarm detector and adopting different measures against external interference can improve the performance of the alarm detector. The use of different anti-jamming measures determines the performance of the alarm detector in different environments. Understand the performance and characteristics of various alarm detectors. According to different use environments, rational allocation of different alarm detectors is the key link of an anti-theft alarm system. Various detectors have different working principles. Each has its own advantages and disadvantages. To make the detector work effectively in any situation, it should be carefully selected and carefully installed.

The Difference and Application of Active Infrared and Passive Infrared Detectors

The active infrared intrusion detector is composed of a transmitter and a receiver. The transmitter is composed of a power source, a light source, and an optical system. The receiver is composed of an optical system, a photoelectric sensor, an amplifier, and a signal processor. The active infrared detector is an infrared beam shield type alarm. The infrared light emitting diode in the transmitter emits a modulated infrared beam (the wavelength of this beam is between 0.8 and 0.95 microns) under the excitation of the power source. After the action of the optical system becomes parallel light emission. This beam is received by the receiver, the infrared photoelectric sensor in the receiver converts the optical signal into a signal, passes through the circuit and passes it to the alarm controller. The infrared rays emitted by the transmitter reach the receiver through the defense zone, forming a warning line. Under normal circumstances, the receiver receives a stable light signal. When someone invades the cordon, the infrared beam is blocked and the infrared signal received by the receiver changes. This change is extracted, amplified and appropriately processed. Alarm signal from the controller. At present, such detectors have two beams, three beams, and multiple beam infrared fences. Generally used in the perimeter to prevent more, the biggest advantage is to prevent the distance, can reach more than ten times the detection range of passive infrared.

Passive infrared detectors are mainly based on changes in the external infrared energy to determine whether someone is moving. The infrared energy of the human body is different from the environment. When people pass through the detection area, the position of the different infrared energy collected by the detector changes, and then an alarm is issued through analysis. But the external environment is: not only the human body will emit infrared energy, many objects will emit infrared energy under certain conditions, and this energy is particularly prominent in the visible light, so the anti-white light interference of any passive infrared detector becomes an important index. In the case of stable indoor light and relatively constant infrared energy, this detection method performs very well. However, the outdoor situation is different. For a long time, passive infrared (IR) infrared detection has only a handful of manufacturers in the outdoors. As a result of a small step in indoor and outdoor and three steps in technology, SAFESTNET has launched a series of highly stable wired and wireless outdoor detectors in the early 1990s with a powerful R&D team. Civil product detector series, such as window guard radar, super bodyguards, security guards, laser fighter and other products.

Active infrared detector equipment selection

1. According to the minimum, maximum temperature and duration of the protection scene, active infrared intrusion detectors are selected to suit the operating temperature; if the ambient temperature is too low, dedicated heaters can be used to ensure the normal operation of the detectors.

2. Active infrared intrusion detectors are severely affected by fog. Devices used for automatic gain should be selected for outdoor use (such devices will automatically adjust the sensitivity when the climate changes); in addition, the distance of the selected device to the actual warning distance is set 20 More than % of margin to reduce systemic false alarms caused by climate change.

3. When using outdoors, it is necessary to use dual-beam or 3-beam active infrared intrusion detectors to reduce false alarms caused by birds and fallen leaves.

4. The wavelength of infrared light emitting diodes used in active infrared intrusion detectors is near 0.85 μm and 0.95 μm, respectively. The former has red exposure, and its concealment is not as good as the latter.

5. It is not advisable to use active infrared intrusion detectors outdoors in areas with heavy fog and in areas where the environment is dirty and windy.

6. When using active infrared intrusion detectors in open areas or on walls and roofs, equipment with lightning protection should be selected.

7. When there is a folded wall and the distance is close, reflective devices may be used to reduce the number of detectors used.

8. The maximum beam distance for outdoor use of active infrared intrusion detectors should be more than six times the detection range specified by the manufacturer.