Q: Generally speaking, there is a rule when choosing and installing intruder detectors, namely "passive indoors, active outdoors". Is this the universal matching principle?
A: The so-called passive and active are distinguished by the detection means of the detector. A passive infrared detector is a detector that uses a pyroelectric sensing element (PIR element) as the main detection means. The alarm is triggered by a temperature change in the detection range. Usually, a three-dimensional area is sensed, and the general ambient temperature in the room is constant. When there is an intruder, the detector will sense the difference between the human body temperature and the background temperature and alarm, while the outdoors due to the unstable ambient temperature, if you use an ordinary passive detector will cause a great false positive rate. The range of detection of passive infrared detectors is 120 degrees left and right, and 75 degrees up and down. Its detection range is wider than that of active infrared detectors. It is usually installed in a room or hall.
Active infrared detectors generally refer to photoelectric detectors. These detectors always consist of a photo-emission and a photo-detection end. The emission end emits an infrared beam that is invisible to the human eye, and the receiving end receives the infrared beam. Energy, it is always used in pairs: one launch, one receive. When someone traverses the surveillance zone, an invisible infrared beam is blocked and an alarm is triggered. The ambient temperature has little effect on it. As mentioned earlier, the detection range of a passive infrared detector is a three-dimensional space. Relatively speaking, the detection range of an active infrared detector can be described as a surface. It is mainly installed on windows, fences, and important entrances and exits. Its detection range is Only between the emitter and the receiver is an alarm triggered when someone or object passes through it. Due to the problems of detection efficiency, range, and غير مجاز مي باشدt in the room, active infrared detectors are rarely used.
Q: There are still dead angles when the active infrared detector is used for perimeter protection. How to solve this problem?
A: Due to the problem of the active infrared detector itself or the geographical environment of the detection area, there may be some small areas or small dead angles that cannot be completely sealed by the radiation detector. This brings certain hidden troubles to the security system and reduces the entire area. System reliability. Like the barrel effect, the entire security rating is determined by the weakest link in the system. Therefore, in order to solve these weak links, it is very necessary to select some outdoor curtain detectors to seal the corners.
Q: Since the PIR detector is a kind of weak signal detection device, some details of the installation may directly affect the detection effect of the detector. Then, what are the requirements on the angle and height when installing it? What are the places that are not suitable for installation?
A: To a large extent, the sensitivity, detection range, and installation height of the detector are directly related to each other. The optimal installation height of a typical passive infrared detector is 2.1 to 2.5 meters. If it is a pet-proof detector, then it is Mounting height requirements will be higher. If the installation is too high, the blind spot will be large. If it is too low, it may not be able to detect the far position. Therefore, the actual application also needs to be measured. For example, a curtain detector that belongs to the principle of passive infrared, if it needs to be installed on a window, the installation height at this time is 25CM away from the window sill. If it is too high, people can drill it from under the probe if the distance is too low. May be crossed from above.
The principle of passive infrared is to monitor the temperature difference. When the outside temperature is about 37 degrees, it basically fails, so do not install near the heat source. In addition, it is necessary to pay attention to avoid fluidity areas such as vents, air conditioners and radiators that can change the ambient temperature, and also to block the shielding of the middle shield (even if it is transparent, such as glass). Also, avoid moving objects such as fans, drying clothes, and curtains. Places where animal activity is frequent should also be avoided. If it is difficult to avoid the actual needs, then special attention should be paid to using a pet-proof detector.
Q: What are the rules for the installation of active infrared detectors?
A: The installation of the active infrared detector has some common sense installation specifications according to the specific conditions of the human body. For example, if a human invades, the invasion distance will be greater than 20cm regardless of whether it is horizontal or lateral. Therefore, the installation height should generally not be lower than the minimum. 20cm; In the case of an arc-shaped corner, the distance from the tangent to the arc shall not be greater than 20cm; when mounted on the fence, the distance between the edge of the fence and the center of the detector shall not be greater than 20cm. In addition, active infrared detectors require that the mounting bracket is stable and firm, and there should not be shaking. A slight deviation may cause the detector to fail. At the same time, there should not be sheltered branches and weeds in the detection range, so as to avoid causing too many false alarms. . There is also a need to consider the linearization of its detection, which is difficult to shape for some complicated and varied shapes.
Q: The detection distance of the active infrared detector determines its detection area. The nominal distance and the actual working distance of the general manufacturer do not match. How to determine the optimal installation distance?
A: Due to the defect of infrared technology, there is a certain error between the measured distance and the nominal distance when the active infrared detector is used for perimeter protection. This is also reasonable, but if the error is too large, it is the quality of the product itself. Manufacturers mark the working distance in the best weather conditions, but it is actually difficult to achieve such conditions, even if it does not last for a long time. Therefore, in a stable environment, the detector's detection distance can also reach the manufacturer's nominal distance, and sometimes the detection distance will be much higher than the nominal distance. However, under actual circumstances, the outdoor environment is an unstable environment. In particular, the energy loss of infrared light during rainy and foggy days is very large, and the nominal distance is often not reached at this time. Therefore, in practice, it is often installed in accordance with 50% to 70% of the nominal distance of the manufacturer. Therefore, the actual measurement is the most critical when selecting the anti-theft product.
Q: The commonly used detectors are wired. If it is installed indoors, there are many indoor circuits such as TV, stereo, and lighting. How can these lines not affect the normal operation of the detector? In addition, there is a certain distance between the detector and the line connected to the host. How can these lines be protected from being cut off?
A: Ordinary detectors generally use four-wire system, in which two lines are power (usually DC12V), and the other two lines are signals (usually NO/NC switch signals). In accordance with the engineering regulations, strong and weak currents should be routed separately, and the signal line of the detector is not easily affected by environmental disturbances. Therefore, TV, audio and lighting circuits will not be affected. Also, when installing, do not put together with strong electricity and electric appliances that can generate strong magnetic fields. Lines cannot go along with strong electric lines.
Whether it is indoor or outdoor, the wired alarm detector has a certain distance from the host circuit, so the protection of the detector circuit is also very important. At present, the detectors generally have a shear protection function. For the protection of the connection line, on the one hand, according to the grade of the project, it can be protected by means of steel pipes, PVC pipes, bridges or buried burial; on the other hand, the simple and easy way to prevent the line from being cut is the access line resistance. Each regular manufacturer's host has this feature. In addition, the qualified alarm host's zone detection circuit will respond to the cut of the line. Once the line is cut, the host will consider this detector as abnormal.
However, according to the current state of security development, it is not enough to have only the shear protection function. Because with the open management of the security industry, internal theft or technical crimes increase year by year, only the detectors with self-protection function can better protect others.
Q: The active infrared working devices are mainly the transmitting and receiving lenses. What problems do they have during adjustment?
A: If these two lenses are misaligned, the direct result is to deviate from the center area, resulting in poor reception.
The adjustment of the active infrared detector should generally be based on the voltage output of the light receiving end. Through the fine adjustment of the deflection angle and vertical angle of the lens, the output voltage of the light receiving end can be observed. The higher the voltage, the more accurate the detector is, and the false alarm rate. The lower it is. Generally, the radiation detector will have a lowest output voltage index. The current shots are relatively simple to install and debug. As long as they are correctly installed in accordance with the installation method provided by the manufacturer, there is no problem under normal circumstances.
Q: Sensitivity is the most direct factor that affects the false alarm rate or missed rate of the detector. How to adjust the sensitivity so that the false alarm rate and false negative rate achieve the desired effect?
A: In the conventional detection technology, sensitivity is a contradiction between false positives and false negatives. To reduce false positives and lower sensitivity, it is bound to bring about the problem of missing reports; instead, it is increased to reduce false negatives. Sensitivity, in turn, raises the problem of false positives. It is difficult to accurately grasp this "degree" in practical applications.
The specific setting of sensitivity depends on the actual environment. In general, if the application environment is rather harsh, for example, there are often birds or blade floats blocking the beam, it should be set in a low-sensitivity range. The active infrared detector should choose the appropriate response time: if it is too short, it will cause unnecessary disturbances, such as flying over small birds, passing through small animals, etc.; too long will result in missed reports. The shortest shading time is usually determined at a speed of 10 meters per second. Sometimes due to seasonal changes, sensitivity is adjusted separately in winter and summer.
Q: Currently there are two-beam and four-beam infrared detectors.
A: The four-beam detector means more stability and lower false alarm rate, and the ability to adapt to the environment is stronger. It also means that the increase in غير مجاز مي باشدt is generally used in long-distance environments (nominal distance exceeds 100 meters). The more the beam is, the better the detection effect is. We need to look at where it is used. If the place is relatively small, we will use less beams, so we can reduce its false negative rate. If we use a larger area, we must use more. Beams of light, so that it can reduce its false positive rate.
The main purpose of multi-beam detectors is to prevent false alarms, because the principle is to cover the entire beam to alarm, from this point of view, as long as it is more than two beams, it is detected in the body of this range of objects, and the beam How much does not matter, only with the coverage area of the beam. Of course, some products are currently considered to be infrared fences, which is due to the combination of multi-beam detectors.
Q: Shuangjian detector utilizes the dual technology of infrared and microwave, which reduces the false alarm rate and improves the accuracy. However, microwave technology itself can penetrate walls and other obstacles (and its shielding of metal). Does the stability and reliability of Shuangjian detectors suffer greatly?
A: The microwave detection range of the Shuangjian detector is generally wider than that of the passive infrared detector. The Shuangjian detectors of general manufacturers are equipped with microwave range adjustment knobs. During the debugging process, the detectors are observed by walking. The indicator light, through the microwave range adjustment knob and the angle of the mounting bracket, make the microwave range as close as possible to the passive infrared detection range. Microwave sensitivity must not be too large, because microwaves are only penetrating, and care should be taken during commissioning.
Will microwave technology affect the stability and reliability of the detector itself? It is understood that the answer is of course negative. Some manufacturers have also done a lot of useful work in this regard: such as increasing the frequency (the K-band frequency is 2-5 times that of other commonly used microwave frequencies), reducing leakage; increasing the degree of microwave molding, using a dedicated waveguide The shaped antenna makes the microwave and infrared highly complex, further reducing the microwave energy leaked outside the protected area.
This article comes from China Security Network: http://news.c-ps.net/2011/8/72788.html
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