Transmission and interference of elevator video monitoring system

Traditional elevator monitoring is to mix the video cable in the accompanying cable, and connect it to the monitoring center from the output of the elevator machine room. The system structure is simple. However, the elevator shaft is a relatively complex environment. The video surveillance images of many projects are disturbed to varying degrees in the elevator shaft environment. This has always been one of the most common, most difficult to deal with, and one of the most concerned issues. Here we discuss the solution to the problem.

1. First, let's analyze the principle of elevator interference

1). Cables for power, lighting, fans, control, communication, etc. are usually arranged in the elevator shaft, and various cables will generate electromagnetic radiation; if coaxial cable transmission is used, the same principle as the antenna reception, the coaxial cable will also "receive" These interferences, that is, the interference electromagnetic field generates interference induction current on the cable, and this interference induction current will also generate interference induction voltage (electromotive force) on the longitudinal resistance of the cable outer conductor (braided net). This interference induction voltage is just in series in the video signal transmission. Interference is formed in the "long ground wire" of the loop;

2). More importantly, these accompanying cables are parallel to the video cable and are bundled together at close range. This forms a close to "best and most effective" interference coupling relationship. In general engineering, the shielding interference method through metal pipes or metal troughs can be used, but in the environment where the elevator is following, this method is powerless. Therefore, the anti-interference in the elevator environment is very difficult, and only a better design and construction method can be selected;

3). Understanding the basic principles of interference generation is very important for improving anti-interference design and construction.

2. There are the following forms of interference to the elevator

1), horizontal stripes scroll up and down

This phenomenon is manifested by the stripes rolling up and down continuously, and the stripes are relatively wide. It seems to be interference, but it is not caused by external electromagnetic waves. If the stripes are set aside, the image will be clear. This phenomenon basically accounts for more than 80% of the image interference. This phenomenon can be said to be a problem of ground potential, which refers to the potential difference between the ground of the front-end equipment and the central control room. This problem is that the multimeter cannot test If the measured number is zero, it does not mean that there is no potential difference. The only way is to disconnect its circuit, that is to say, disconnect one of the two ends of the ground (preferably choose the front end). If the ground at both ends does not want to be broken, it is afraid that the lightning protection will be affected. To solve this problem, add a ground isolator. This device is a passive device. As long as it is connected in series at any end of the coaxial cable, it is easy to install and has a good effect, which can completely eliminate this streak phenomenon. As shown in the figure is the signal signal isolator.

2), mesh interference

The image quality of this phenomenon is very poor. Basically, no objects can be seen. Sometimes the image is absent, and the screen freezes. This phenomenon is due to the broken and short core and shield of the cable. Most of this situation occurs on the connector, and some of them are not well constructed during wiring. The interference caused by welding problems or poor quality of joint parts is also predominant in the project.

3) Interference of space electromagnetic waves

This kind of interference source is relatively complicated, mainly because there are strong radiation sources or high-power devices in front-end equipment, cables, and terminals. The main performance is image distortion and jitter. Therefore, understand the surrounding environment before construction and avoid radiation sources as much as possible. In a project that has already been constructed, and interference is unavoidable, only anti-interference equipment is added. This method is relatively economical, fast, and the image is guaranteed.

3. Research on the application of twisted pair transmission elevator monitoring

For the complex environment in the shaft, the use of twisted pair transmission mode can be carried out according to the following three recommended schemes:

A) Route in the middle of the shaft

1). Choose unshielded high-quality Cat5e cable.

2) Open a hole in the wall in the middle of the shaft.

3) Cables are bundled with the accompanying cables. Cable bundles below the middle of the shaft should be reserved.

4) The transmitter is selected according to the distance requirements, and the transmitter should be powered separately.

B) Outlet of elevator machine room

1). Choose unshielded high-quality Cat5e cable.

2) Cables are bundled with the accompanying cables. Cable bundles below the middle of the shaft should be reserved, and the cables above the shaft should be tightly tied.

3) The cables in the elevator machine room should not be used for strong electric bridges, and should be connected to the monitoring center separately from the bridges as early as possible.

4) The transmitter is selected according to the distance requirements, and the transmitter should be powered separately.

C) Installation of elevator machine room

1) Install the transmitter in the elevator machine room by using the video cable provided with the accompanying cable as the front-end medium.

2) The transmitter selects passive or active transmitter according to the length of the video cable.

3) The receiver is installed in the monitoring center, and the passive or active receiver is selected according to the distance of the twisted pair.

In addition, the transmitter and elevator grounding system should be carried out separately. We recommend that the remaining wire pairs in the network cable be grounded in the monitoring center. The receiver is grounded to protect the device from overvoltage. It is often difficult to use separate grounding devices for the above grounds, and common grounding devices are usually used. It is generally required that the grounding resistance of the common grounding device is not greater than 1Ω. The grounding wire can use a copper core wire (PE wire) with a cross-sectional area of ​​not less than 16 square millimeters to connect with the PE wire in the building power distribution room, and can be laid in the same way as the elevator power cable. The auxiliary equipotential plates in the equipment room are connected.

Four, pay attention to the following points when the network cable is bundled

1). Choose qualified and soft unshielded network cable.

2) First, fix the accompanying cable with a thick cable tie.

3) Bind the network cable with a thin cable tie and fix it in the thick cable tie.

4), the distance between the thick cable tie is 30 cm, and each section of the network cable is reserved for 3 cm.

5. Precautions for twisted pair wiring construction

1. The twisted pair must meet the national twisted pair product standard:

1). The twisted pair used is pure copper core. The wire diameter is 0.5 mm

2) The resistance of a single core 100 meters is 9.38 ohms, and the resistance of a box of 305 meters is about 28 ohms, not more than 30 ohms

3). The twisting density should reach the standard of Category 5 wire

4) The flexibility and flame retardancy must meet national inspection standards.

2. For outdoor use, please use outdoor waterproof twisted pair.

3. It is forbidden to mix with high-voltage cables or equipment during project wiring, and cannot be placed together with frequency conversion equipment and communication signal generators.

4. Each pair of four pairs of cables in the cable transmits one signal, so video can be transmitted simultaneously. Audio, control signals, prevent high-current power transmission in the cable. If you need to use a pair of low-voltage power supply DC12V in the network cable to control within 150 meters, AC24V within 300 meters, and control signals within 1000.

5. The active and some passive products have built-in surge protection and need to be grounded. Outdoor applications can be grounded together with the camera. Note that the ground electrode resistance must be qualified (reference value 4 ohms)

6. The video cable from the camera to the transmitter should be as short as possible to ensure stable signal quality.

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