First, let us first analyze the principle of elevator interference generation.
1. Elevator wells are usually equipped with cables for power, lighting, fans, control, communication, etc. Various cables will generate electromagnetic radiation; if coaxial cable transmission is used, the same principle as antenna reception, coaxial cable will also "receive" these Interference, that is, the interference electromagnetic field generates an interference induced current on the cable. This interference induced current also generates an interference induced voltage (electromotive force) on the longitudinal resistance of the outer conductor (woven mesh) of the cable. This interference induced voltage is just connected in series to the video signal transmission circuit. In the "long ground line", interference is formed;
2. More importantly, these accompanying cables are parallel to the video cable and bundled together at close range. This creates an "optimal and most effective" interference coupling relationship. In general engineering, it is possible to use a shielded interference method that wears a metal pipe or a metal groove, but in an environment where the elevator is moving, this method is powerless. Therefore, the anti-interference in the elevator environment is very difficult, and only a good design and construction method can be selected;
3. Understand the basic principles of interference generation, and it is very important to improve anti-interference design and construction.
Second, the interference manifestations for elevators are as follows
1, the horizontal stripes scroll up and down
This phenomenon is manifested by the stripes rolling up or down, and the stripes are wider. It seems to be interference, but it is not the result of external electromagnetic waves. If the stripes are thrown away, the image is clear. This phenomenon accounts for more than 80% of the image interference. This phenomenon can be said to be a ground potential problem. It refers to the potential difference between the ground of the front-end equipment and the central control room. This problem cannot be tested by the multimeter. If the measured number is zero, it does not mean that it does not have a potential difference. The only way to do this is to disconnect its loop, which means that the ground at both ends is disconnected (preferably the front end). If the ground at both ends does not want to be broken, it is afraid of affecting lightning protection. That is to add a ground isolator to solve, this device is a passive device, as long as it is connected in series at any end of the coaxial cable, the installation is convenient and the effect is good, and the streaking phenomenon can be completely eliminated. The picture shows the Eupp signal isolators.
2, mesh interference
This phenomenon is very poor in image quality, basically no objects are visible, images are sometimes absent, and pictures are dead. This phenomenon is due to the shortness and shortness of the core and shield of the cable. Most of this happens on the connectors, and some are not properly constructed during wiring. Interference caused by welding problems or poor quality of joint parts is also common in engineering.
3. Interference of space electromagnetic waves
This kind of interference source is more complicated, mainly in the front-end equipment, the cable, the strong radiation source near the terminal or the high-power equipment. Mainly manifested as image distortion, jitter and so on. Therefore, before the construction, you should understand the surrounding environment and try to avoid the radiation source. In the case of already completed construction, and the interference is unavoidable, only anti-jamming equipment is added. This method is economical, fast, and the image is guaranteed.
Third, twisted pair transmission elevator monitoring application research
For the complex environment in the hoistway, the twisted pair transmission method can be used according to the following three recommended schemes:
A) Middle lane of the hoistway
1. Choose unshielded quality super five cable.
2. Open the hole in the wall in the middle of the hoistway.
3. The cable is bundled with the accompanying cable, and the cable bundle below the middle of the hoistway should be reserved.
4) The transmitter is selected according to the distance requirement, and the transmitter should be powered separately.
B elevator machine room outlet
1. Choose unshielded quality super five cable.
2. The cable is bundled with the accompanying cable. The cable bundle below the middle of the shaft should be reserved, and the upper cable of the shaft is tight.
3. The cable in the elevator machine room should not be used to take the strong bridge. It should be connected to the monitoring center separately from the bridge.
4. The transmitter is selected according to the distance requirement, and the transmitter should be powered separately.
C) Elevator room installation
1. Use the video cable that comes with the accompanying cable as the front media and install the transmitter in the elevator machine room.
2. The transmitter selects a passive or active transmitter depending on the length of the video line.
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 separated. We recommend that the remaining 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 achieve a common grounding device with a separate grounding device for all of the above groundings. It is generally required that the grounding resistance of the shared grounding device is not more than 1 Ω. The grounding wire can be connected to the PE wire of the building distribution room with a copper core wire (PE wire) with a cross-sectional area of not less than 16 square millimeters, and can be laid in the same way as the elevator power cable. The end of the grounding wire and the elevator The auxiliary equipotential plates in the equipment room are connected.
Fourth, pay attention to the following points when tying the network cable
1. Select a non-shielded network cable that is soft and flexible.
2. First fix the cable with the thick cable tie.
3. Bundle the cable with a thin strap and fix it in the thick tie.
4, the thickness of the thick tie is 30 cm, and each line of the cable is reserved for 3 cm.
Five, twisted pair wiring construction considerations
1. The twisted pair cable must meet the national twisted pair product standard:
1) The twisted pair used should be a pure copper core. Wire diameter is 0.5 mm
2), single core 100 meters resistance is 9.38 ohms, a box line 305 meters resistance value is about 28 ohms, not more than 30 ohms
3), the twist density should reach the five-category line standard
4) Flexibility and flame retardancy should meet the national inspection standards.
2. If used outdoors, please use outdoor waterproof twisted pair cable.
3. It is forbidden to mix and contact with strong voltage cables or equipment during project wiring, and it cannot be placed together with frequency conversion equipment and communication signal generator.
4. Each pair of four pairs of cables in the cable transmits one signal, so video can be transmitted at the same time. Audio, control signals, to prevent high-current power transmission within 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 control within 300 meters, the control signal is controlled within 1000.
5, active and some passive products built-in surge protection, need to be grounded, outdoor applications can be grounded with the camera, pay attention to the grounding resistance must be qualified (reference value 4 ohms)
6. The video line from the camera to the transmitter is as short as possible to ensure the quality of the signal is stable.
This article tags: elevator monitoring installation precautions