Elevator monitoring anti-jamming technology principles and points

In the closed-circuit monitoring project, the monitoring of video interference by elevators has always been the most common and difficult to deal with, and it is also one of the most concerned issues. This article will clarify from the principle: as long as we master the principle of interference, elevator anti-jamming engineering problems will be solved. This article only deals with the anti-jamming technology of coaxial video transmission in elevator monitoring engineering, for design and construction reference.

First, master the types and characteristics of commonly used coaxial cable 1. Consider the transmission attenuation: When the floor is very high, far away from the monitoring center, you should carefully consider the issue of transmission attenuation. When selecting a cable, it is known that the thick cable is better than the thin cable, but it should also be understood that the SYWV physical foam cable is superior to the solid SYV cable, the high-knit cable is superior to the low-knit cable, the copper core cable is better than the copper-clad steel cable, copper Webbing is better than aluminum-magnesium alloy wire mesh;

2. Focus on high-frequency attenuation: The attenuation of brightness/contrast of low-frequency components is easy to find and solve. The most important transmission characteristic of cables is that the higher the frequency, the greater the attenuation. The high-frequency attenuation affects the definition and resolution. Special attention should be paid to summarizing the image quality. The observation method. The characteristics and laws of the cable in this respect are: thicker cable is better than thin cable, and foaming is better than solid, but the same type of "high and low frequency attenuation is the same" (after analysis);

3. Consider cable life: soft cables are better than ordinary cables, thin cables are better than thick cables, and there is one problem that is most easily overlooked: the adhesion between the layers of the cable, ie when the cables are vertically opposite When the force is applied, whether relative slippage occurs or not, high-level elevator cables are up to 100 meters vertically and the outer jacket of the cable is fixed on the accompanying cable. This is a kind of “soft fixing”, and the cable is not allowed to deform when fixed (breakage of coaxiality) ) In this way, the inner layer of the cable during the repeated movement of the elevator will gradually "slide down" under the action of gravity, and slowly pull off the braided wire or core wire, which shows that the signal gradually weakens and the interference becomes greater and greater; Without this cable technology standard, a simple check method is to take a one meter cable, peel off the layers at one end, hold the cable at one end with one hand, and pull the inner layer of the cable with a pair of pliers: pull the core wire in turn, the insulation layer, Weave mesh, experience the size of the adhesive force, make a reasonable estimate - poor adhesion, easy to slide as much as possible. This performance, many cables do not have, should be carefully chosen.

Second, understand the principle of interference generation Brief introduction 1. Elevator well power, lighting, fan, control, communications, etc., all kinds of cables will produce electromagnetic radiation. Like the antenna reception principle, the coaxial cable will also “receive” these interferences, ie, the interference electromagnetic field will produce interference induced currents on the cable. This interference induced current will also cause interference induced voltage on the longitudinal resistance of the cable outer conductor (braided network). (Electromotive force), this interference induced voltage just in series in the video signal transmission loop "long ground" in the formation of interference;

2. More importantly, these accompanying cables are all in parallel with the video cable and bundled together at close range. This results in a near "best and most effective" interference coupling relationship. In general engineering, the method of shielding and interfering with metal pipes or metal channels can be used. However, in the elevator follow-up environment, this method cannot do anything. Therefore, the anti-jamming in the elevator environment is very difficult and only good design and construction methods can be selected.

3. Understand the basic principles of interference, and it is very important to improve anti-jamming design and construction;

III. Anti-jamming measures for common copper shaft cable transmission transmission schemes 1. Commonly used copper shaft cables: whether it is a multi-layered high-copper braided network cable, a double-shielded cable of “aluminum foil-braided wire mesh”, or “aluminum foil — braided wire mesh — The four-shielded cable made of aluminum foil-woven nets is electrically a shield. Interference induced voltages are directly connected in series in the video signal transmission loop. However, the resistance of the outer conductor of the multi-layer high-knit cable is small, and the interference induced voltage is relatively low. This has a certain effect (interference below tens of Khz) against low-frequency power interference, motor spark disturbances, and the like. But for high-frequency interference, due to the “skin effect”, the high-frequency impedance is the same as that of the low-profile cable, and the anti-jamming effect is basically the same; therefore, it should be clear to see that the cable of Gaobian only has the effect of weakening the low-frequency interference properly, and the anti-strong interference is still Helpless

2. Anti-jamming measures for elevator wiring:

1 The position of the video cable to get out of the elevator shaft: The ideal choice should be in the middle of the well because at this time the length of the video cable accompanying the well is only about half the depth of the well, and the shortest is the natural introduction of the least interference; but this is the engineering Outgoing requirements can only be achieved by looking at the situation. Actual projects may not be allowed;

2 In the past, without knowing the principle, most of the outlets were walking along with other accompanying cables and they exited from the top or bottom of the cable well. In this case, considering that only half of the cable is accompanied by the movement, the other half is only a fixed extension connection and does not move. We call this part “immovable cable”; this provides a possibility: that half of the cable accompanying the movement Only cable bundles can be bundled together with other traveling cables; the other half of the fixed cable can be routed away from the accompanying cable. In the elevator shaft, the video cable is placed in close contact with the vertical line of the well and the cable is passed through the metal pipe. Or take the metal slot to shield the influence of interference on this part of the cable, more effective;

3 When accompanied by video cables and other accompanying cables, designers should fully understand the structure and distribution of other accompanying cables. When tying, video cables should be kept away from large current and high frequency cables, and near low current and low frequency cables. Bundling; here, even if there is a choice of 1 cm, we may have to fight for it, because the impact of interference is at least inversely proportional to the square of the distance;

4 The camera's metal case, the casing of the BNC head, and the outer conductor of the coaxial cable must be insulated from the floor of the video signal, such as the elevator car and the guide rail. This requires special attention when installing the camera.

5 The camera power supply should be preferably centralized DC power supply mode, followed by the choice of car lighting power, can not use power.

6 The monitoring and control cables for power supply, control, etc. should use shielded cables as much as possible to prevent external signal leakage;

7 The video cable from the elevator shaft exit to the control center should be taken away from the metal pipe or metal slot to shield the impact of environmental interference on this part of the cable, and pay attention to this part of the shielding and shielding in the elevator shaft, should make electrical connections;

Fourth, the application of anti-jamming coaxial cable 1. Anti-jamming coaxial cable is a "double insulated double shielded coaxial cable", inside the "core - insulation - shield" is still the standard 75 ohms Cable, 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. From the above analysis of interference generation principle, it has been known that the induced voltage generated by the interference on the outer layer of the conventional coaxial cable is connected in series with the “long ground line” of the video signal transmission loop, thereby forming interference. However, after the use of anti-interference coaxial cable, the situation has undergone a qualitative change: the interference induced voltage can only be formed on the “second shield layer”, and the “second insulation layer” inside it is used to “long” it with the video signal transmission circuit. The long ground wire is insulated and isolated from the video signal transmission circuit to achieve anti-interference.

2. The characteristics of this anti-jamming cable, for the ultra-low-frequency power supply interference in the elevator environment, motor EDM interference, frequency conversion motor interference, control signal interference and other disturbances below tens of kilohertz, anti-jamming performance is very prominent.

3. In the long project of transmission line design, after adopting "double insulated and double-shielded coaxial cable", some anti-jamming measures in traditional engineering can also be greatly simplified, and the total project cost can be effectively reduced.

Solve electromagnetic interference, including elevator interference:

The "defense" method, such as wearing iron pipes and walking wire troughs, is 100% effective. However, it cannot be used in the case of elevator accompanying cables;

The methods of “avoiding”, such as radio frequency, optical cable, microwave, and digital conversion, all avoid the 0-6M band and transmit in other bands, but in the elevator environment, the methods are more effective and the cost is not too high. It should be radio frequency. These transmission methods must be equipped with a modem device or a reverse conversion device. It is advantageous to interfere with 0-6M. Needless to say, all of these transmission methods will avoid the need for protection. However, each type of transmission method will bring about new unfavorable problems and need to be understood. clear. Everything is "one in two."

The "resistance" method is to still maintain the video source signal (baseband) transmission method and use direct interference-resistant anti-jamming measures. At present, the more practical and effective method is to improve the "video/interference ratio" method, that is, the front-end to improve the video The signal amplitude also relatively increases the "video/interference ratio" of the entire transmission process. After the terminal recovers the standard amplitude of the video signal, it still maintains a sufficiently high "video/interference ratio." The choice of this kind of equipment, one is the lifting factor of the video signal, that is, the interference suppression multiple size; Second, this interference suppression multiple (suppression ability), whether there is a difference between low frequency and high frequency, high frequency suppression ability than low frequency Thirdly, as a video transmission device, does it also have the function of compensating cable attenuation and frequency distortion and restoring the characteristics of the video source signal? At present, the development of anti-jamming technology for such video source signal (baseband) transmission mode is also Soon, most common electromagnetic interferences, including elevator disturbances, can be solved immediately.

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