Elevator video surveillance anti-jamming technical measures - Database & Sql Blog Articles

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In the monitoring project, the elevator monitoring is a common engineering problem due to the complicated application environment and video interference. If the monitoring image is not well solved, the monitoring image will produce horizontal stripes, twill, netting, spikes, etc. Severe interference makes the project quality less than expected and even the system cannot be delivered.

First, master the common coaxial cable types and characteristics

1. Consider transmission attenuation: When the floor is very high and far from the monitoring center, the transmission attenuation problem should be carefully considered. When selecting a cable, the thick cable is better than the thin cable. The SYWV physical foam cable is better than the solid SYV cable. The high braided cable is better than the low braided cable. The copper core cable is better than the "copper clad steel" cable. The copper mesh is better than the aluminum and magnesium. Alloy mesh.

2. Pay attention to high-frequency attenuation: the brightness/contrast attenuation of low-frequency components is easy to find and solve. The most important transmission characteristic of cable is that the higher the frequency, the greater the attenuation. The high-frequency attenuation mainly affects the resolution and resolution. Pay special attention to summarizing the image quality. Method of observation. The characteristics and rules of this cable are: thick cable is better than thin cable, foam is better than solid, but the same model of "high and low frequency high frequency attenuation."

3. Consider cable life: soft cable is better than ordinary cable, thin cable is better than thick cable; there is also a problem that is easily overlooked: the adhesion between the layers of the cable, that is, when the cable is vertically reversed When force is used, will there be relative sliding, the high-rise elevator cable can be up to 100 meters in vertical wiring, and the cable outer sheath is fixed on the accompanying cable, which is a kind of "soft fixed", which does not allow the cable to be deformed when fixed (destruction of coaxiality) In this way, in the repeated movement of the elevator, the inner layer of the cable will gradually "slide" under the action of gravity, and slowly pull off the woven mesh or core wire, which shows that the signal is gradually weakened and the interference is getting larger and larger; Without this cable technical standard, the simple inspection method is to take a one-meter cable, peel off the layers at one end, one person holds the two ends of the cable with one hand, and the other uses the pliers to pull the inner layer of the cable: the core wire, the insulation layer, Weaving the net, experience the size of the adhesion, make a reasonable estimate, the adhesion is poor, and the slippery is not used as much as possible. This performance is not good for many cables and should be carefully chosen.

Second, the principle of interference generation

1. Cables for power, lighting, fans, control, communication, etc. are usually arranged in the elevator shaft, and various cables generate electromagnetic radiation. As with the antenna receiving principle, the coaxial cable also "receives" these interferences, that is, the interference electromagnetic field generates an interference induced current on the cable, and the interference induced current also generates an interference induced voltage on the longitudinal resistance of the outer conductor (woven mesh) of the cable. (electromotive force), this interference induced voltage is just connected in series in the "long ground" of the video signal transmission circuit to form interference.

2. More importantly, these accompanying cables are parallel to the video cable and bundled together at close distances, which forms a close to “best and most effective” interference coupling relationship. In general engineering, metal pipe or metal can be used. The shielding of the metal trough interferes with the method, 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.

3. Anti-jamming measures for common copper shaft cable transmission schemes

1. Common copper shaft cable: whether it is multi-layer high-woven copper mesh cable, "aluminum foil-woven mesh" double shielded cable, or "aluminum foil-woven net-aluminum foil-woven net" four-layer shielded cable, electrical Belongs to a shielding layer. The interference induced voltages are directly connected in series in the video signal transmission loop. Only the outer conductor of the multi-layer high-wound cable has a small resistance, and the induced induced voltage is relatively low. This has a certain effect against low-frequency power supply interference, motor spark interference, etc. (interference of tens of KHZ or less). However, for high-frequency interference, due to the "skin effect", the high-frequency impedance is the same as that of the low-wound cable, and the anti-interference effect is basically the same. Therefore, the high-frequency cable only has the effect of properly reducing the low-frequency interference, and the anti-strong interference and high-frequency cable are still powerless. .

2. Anti-jamming measures for elevator wiring:

1) Location of the video cable out of the elevator shaft: The ideal choice is in the middle of the well, because at this time the length of the video cable accompanying the well is only about half of the depth of the well, and the shortest, naturally introduced interference is also minimal.

2) Most outlet locations are taken with other accompanying cables and exit from the top or bottom of the cable well. In this case, considering that only half of the cable is moving, the other half is only a fixed extension connection, no movement, this part is called "no moving cable", which provides a possibility: that half of the accompanying motion cable can only Bundle the cable with other accompanying cables, and the other half of the fixed cable can choose to separate the cable from the accompanying cable. In the elevator shaft, the video cable should be placed close to the vertical cross-section of the well wall, and the cable should be worn through the metal pipe or Metal trough to shield the interference on this part of the cable.

3) When the video cable of the accompanying moving part is bundled with other accompanying cables, the structure and distribution of other accompanying cables should be fully understood. When bundled, the video cable should be kept away from the cable with high current/high frequency, and the cable with low current and low frequency should be bundled. Even a small distance, because the magnitude of the interference is inversely proportional to the square of the distance.

4) The “ground” of the video signal such as the metal casing of the camera, the outer casing of the BNC head, and the outer conductor of the coaxial cable should be insulated from the elevator car, the guide rail, etc., which should be paid special attention when installing the camera.

5) The power supply for the camera should be selected from the centralized DC power supply mode, followed by the lighting of the car, and the power supply cannot be used.

6) Cables for monitoring such as power supply and control, try to use shielded cables.

7) From the exit of the elevator shaft to the control center, the video cable should be taken away from the metal pipe or the metal trough to shield the impact of the environmental interference along the road, and pay attention to the shielding of the part and the shield in the elevator shaft. .

Fourth, the application of anti-interference coaxial cable

The anti-interference coaxial cable is a "double insulated double shielded coaxial cable", in which the core wire, insulation layer and shielding layer are still standard 75 ohm cables, no difference. The difference is that, in addition to the original shielding layer, a second insulating layer and a second shielding layer are added, and a sheath is added on the outside. Therefore, the interference induced voltage can only be formed on the second shielding layer, and is insulated from the "long ground" of the video signal transmission circuit by the second insulating layer inside, and the interference is excluded from the video signal transmission circuit. To achieve the purpose of anti-interference. The characteristics of this anti-jamming cable are excellent for the interference of ultra-low frequency power supply in the elevator environment, motor spark interference, frequency conversion motor interference, control signal interference and other tens of KHZ interference.