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Perimeter alarm system uses a technique such as microwave, active infrared, and electronic fence to form a “protective wall†that is visible or invisible at the boundary of protection. When there is a violation of the wall, the detector of the corresponding zone will issue an alarm. The signal is sent to the control center of the control center, and the alarm position is displayed at the same time as the sound and light alarm. It can also be used to simulate the perimeter electronic screen, even the linkage camera monitoring system, access control system, and strong electric lighting system.
In recent years, the perimeter prevention system has become a basic and indispensable security subsystem for security systems. It can be seen not only in high-end areas such as military factories, barracks, airports, ports, and government agencies, but also widely used in residential communities. , and maintain a fairly high rate of application growth in these areas. Cable theft is a common technique.
Several common cable anti-theft technologies
1, power carrier communication monitoring system anti-theft
The power carrier is a concept derived from the cross-discipline of power and communications. The power carrier communication method (shown in Fig. 1) utilizes signal transmission means in the wire and cable anti-theft system to load the detection carrier signal on the detection range cable, and the monitoring center alarm is connected to the carrier signal on the monitored cable. The cross-section of the cable leads to the interruption of signal transmission. If the alarm can't receive the signal, it will be automatically alarmed and serve as a warning. The power carrier communication method is currently the most widely used anti-theft measure because it transmits carrier signals on the monitored cable and does not require extra lines. Therefore, the power carrier communication method is easy to install, conceals detection, and has a wide monitoring range. A few kilometers or more) and so on. However, since the cable can only be stolen when the cable is stolen, it cannot be deterred to criminals, and video video forensics cannot be used. A single power carrier communication is facing challenges of integration and systematization.
2, capacitance detection and monitoring system anti-theft
The principle of the capacitance detection method is to use a pair of empty cables to short the cable terminals and the other end to connect to the multivibrator of the alarm. When the cable works normally, the oscillator of the alarm device will not generate vibration; when the cable outlet section is cross-sectioned, it is vacant. The cable becomes a simple capacitor and the oscillator outputs a signal that causes an alarm and the frequency of the oscillator output signal varies as the cable is switched off. Figure 2 shows the basic model of the capacitance detection method.
3, current detection monitoring system anti-theft
The system judges the integrity of the cable system by detecting on-line whether there is a transmission current in the cable loop. In other words, when the cable works normally, the detection system will not respond; when there is no current transmission in the cable, the detection system will start the alarm system. The current detection system cannot be used for voltage anti-theft systems with no current, such as subway tunnels or road tunnel blower power cables. In addition, the system can only alarm when all branch cables of the circuit system are cut off (because the total current of the cable loop is zero when all the cable branches are disconnected). Therefore, the current detection type monitoring system has the hidden danger of “not reportingâ€.
Cable anti-theft technology in different areas of application requirements analysis
1. Anti-theft alarm of underground cable for street light
With the rapid development of national urban and rural integration, the pace of rapid development of municipal construction has been accelerated. The street lamp is the main part of the city's night landscape lighting, and it plays an important role. The cable for the street lamp has now been converted from overhead to underground. The street lamp cable buried and laid mainly includes: direct laying, pipeline laying, cable trench laying and so on. No matter which kind of laying, there are hidden dangers of being stolen, leading to serious consequences such as economic loss and disorderly public order.
The ideal road buried cable anti-theft system, the most important requirement is in the off period, the alarm anti-theft system requires the end of the power without any form; turn on the light when the normal alarm alarm system needs and circuit status (cable length, street lamp model Etc.) No direct relationship. When possible, multiple lines are used to communicate with the control room through a communication device from a master control device.
2, highway power cable anti-theft alarm
Highways mainly use power cables, and one of the core issues in the development of highway cable anti-theft alarm systems is signal detection when cables are abnormal. For highway cable requirements, the main requirements of the anti-theft system are: availability, reliability (low false alarm rate), advanced (alarm discrimination), scalability (and high-speed traffic police system networking) and so on. At present, the main application of power carrier communication method, but there are still several issues to be solved:
â—‡ Power distribution system transformers have a blocking effect on power carrier communication information;
The three-phase power system in the circuit has a signal loss of 10-20dB. When the communication distance is relatively short, the general carrier signal can only be transmitted on a single-phase line.
The power line has its own pulse interference, the frequency of AC power in China is 50Hz, and the second peak is stored in a single cycle of the sine curve, then there will be a second pulse interference, that is, there is a 100Hz pulse interference in the power line, which is its inherent property;
â—‡ Different signal coupling methods correspond to different power carrier communication signal losses;
â—‡ Power lines can cause deformation of data signals.
To sum up the above points, it is worth pointing out that the power line communication method in the cable anti-theft system is worthy of research and development. This also shows that China's power carrier cable anti-theft system technology is not yet fully mature. The best cable anti-theft system in China is a certain Hangzhou produces anti-theft cable system companies.
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