Transmission and interference of elevator video monitoring system

Traditional elevator monitoring is to mix the video line with the accompanying cable, and then connect the output from the elevator room to the monitoring center. The system architecture is simple. However, the elevator shaft is a relatively complex environment. The video surveillance images of many projects are interfered to varying degrees in the elevator shaft environment. This has always been one of the most common, most difficult to deal with, and most concerned issues. Let's discuss the solution to this problem.

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

1). The elevator shaft is usually equipped with power, lighting, fan, control, communication and other cables. Various cables will generate electromagnetic radiation; if coaxial cable is used for transmission, the same as the antenna reception principle, the coaxial cable will also "receive" these interferences, that is, the interference electromagnetic field generates interference induced current on the cable, and this interference induced current will also generate interference induced voltage (electromotive force) on the longitudinal resistance of the cable outer conductor (braided mesh). This interference induced voltage is just connected in series in the "long ground wire" of the video signal transmission loop, forming interference;

2). More importantly, these accompanying cables are parallel to the video cable and are bundled together at a close distance. This forms an interference coupling relationship close to the "best and most effective". In general projects, the shielding interference method of wearing metal pipes or metal grooves can be used, but in the environment where the elevator moves, this method is powerless. Therefore, it is very difficult to resist interference in the elevator environment, and only a better design and construction method can be selected;

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

2. There are several forms of interference for elevators:

1). Horizontal stripes rolling up and down

This phenomenon is manifested as stripes rolling up or down continuously, and the stripes are relatively wide. It looks like interference, but it is not caused by external electromagnetic waves. If the stripes are left aside, the image is 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 means that there is a potential difference between the ground of the front-end equipment and the ground of the central control room. This problem cannot be tested by a multimeter. If the measured number is zero, it does not mean that there is no potential difference. The only way is to disconnect its loop, that is, disconnect one of the grounds at both ends (it is best to choose the front end). If you don't want to disconnect the ground at both ends, you are afraid of affecting lightning protection. Then add a ground isolator to solve it. 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 good effect. It can completely eliminate this stripe phenomenon. The picture shows the Utop signal isolator.

2) Mesh interference

This phenomenon has very poor image quality. Basically, no objects can be seen. The image is sometimes there and sometimes not, and the screen freezes. This phenomenon is due to the breakage and shortness of the cable core and shield. Most of this situation occurs on the connector, and some are not well constructed during wiring. Interference caused by welding problems or poor quality of connectors also accounts for a large number of projects.

3) Interference of electromagnetic waves in space

This interference source is relatively complex, mainly because there are strong radiation sources or high-power devices running near the front-end equipment, cables, and terminals. It is mainly manifested as image distortion, jitter, etc. Therefore, before construction, you should have some understanding of the surrounding environment and try to avoid radiation sources. In the completed project, if interference is unavoidable, only anti-interference equipment can be 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 can be carried out according to the following three recommended solutions:

A) Routing in the middle of the shaft

1) Select unshielded high-quality Category 5e cables.

2) Open a hole on the wall in the middle of the shaft for cable outlet.

3) The cables are bundled with the accompanying cables, and the 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) Elevator room outlet

1) Select unshielded high-quality Category 5e cables.

2) The cables are bundled with the accompanying cables, and the cable bundles below the middle of the shaft should be reserved, and the cables in the upper part of the shaft should be tied tightly.

3) The cables in the elevator room should not be connected to the strong power bridge, and should be separated from the bridge as soon as possible and connected to the monitoring center.

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

C) Elevator room installation

1) Use the video cable of the accompanying cable as the front-end medium and install the transmitter in the elevator room.

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

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

In addition, the transmitter and the elevator grounding system should be separated. We recommend that the remaining wire pairs in the network cable be grounded in the monitoring center, and the receiver grounding is to ensure that the equipment is protected from overvoltage. It is often difficult to use independent grounding devices for all the above grounding. Usually a shared grounding device is used. It is generally required that the grounding resistance of the shared grounding device is not greater than 1Ω. The grounding wire can be connected to the PE wire in the building distribution room with a copper core wire (PE wire) with a cross-sectional area of ​​not less than 16 square millimeters at the head end, and can be laid on the same route as the elevator power cable. The end of the grounding wire is connected to the auxiliary equipotential board in the elevator machine room.

4. Pay attention to the following points when bundling the network cable

1) Select a qualified and relatively soft non-shielded network cable.

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

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

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

V. Precautions for twisted pair wiring construction

1. Twisted pair cables must comply with national twisted pair cable product standards:

1) The twisted pair cables used must be pure copper cores. The wire diameter is 0.5 mm

2) The resistance of a single core per 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 must meet the Category 5 cable standard

4) Flexibility and flame retardancy must meet national inspection standards.

2. If used outdoors, please use outdoor waterproof twisted pair cables.

3. When wiring the project, it is prohibited to mix with high-voltage cables or equipment, and it cannot be placed together with frequency conversion equipment and communication signal generators.

4. Each of the four pairs of wires in the cable transmits one signal, so video, audio, and control signals can be transmitted at the same time to prevent high current power from being transmitted in the cable. If you need to use a pair of low-voltage power transmission in the network cable, the DC12V should be controlled within 150 meters, the AC24V should be controlled within 300 meters, and the control signal should be controlled within 1000.

5. Active and some passive products have built-in surge protection and need to be grounded. For outdoor applications, they can be grounded together with the camera. Note that the grounding 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 the stability of the signal quality.