Elevator monitoring video interference and solution

In the closed-circuit monitoring system project, elevator monitoring video interference has always been the most common, most difficult to solve, and one of the most concerned issues. Old-fashioned elevators use ordinary motors, the interference frequency is very low, and the anti-interference problem is still solved. Most of them use variable-frequency power motors. The high-order harmonics of interference are very rich, the spectrum is very wide, and the high-frequency interference is very serious. Especially the modern high-rise residential area, the office building is good, the floor is too high, the elevator video cable has taken measures from the "middle" into the elevator shaft, wearing metal pipes, metal wire troughs, elevator special cables and even "high-grade imports" Elevator cables have also been adopted, but the cable in the accompanying section is still very long and the interference has been a headache. The existing anti-jamming products were tested and although most of them have certain effects, the results are always unsatisfactory: some residual interference, especially residual high-frequency interference, makes the image unsatisfactory. Some produce "brightness blooming" distortion, and the floor displays character distortion, influence synchronization, and the like.

电梯监控视频干扰及解决方法——中国一卡通网

(1) Types and characteristics of coaxial cables commonly used in elevators.

a, consider transmission attenuation

When the floor is very high and the monitoring center is far away, the transmission attenuation problem should be carefully considered. When selecting the cable, we know the thick cable, but we should also understand that the SYWV buried 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 wrapped steel" cable. The net is superior to the aluminum-magnesium alloy mesh.

b, pay attention to high frequency attenuation

The brightness/contrast attenuation of the low-frequency components is easy to find and solve. The most important transmission characteristic of the cable is that the higher the frequency, the greater the attenuation, and the high-frequency attenuation affects the resolution and resolution. Pay special attention to the observation method of summarizing the image quality. The characteristics and regularity of the cable in this respect are: the thick cable is superior to the thin cable, and the foaming is better than the solid, but the same type of "high and low frequency high frequency attenuation".

c. Consider cable life

Soft cable life is better than ordinary cable, and thin cable is better than thick cable. There is also one of the most overlooked issues: the adhesion between the layers of the cable, that is, whether the relative sliding occurs when the cables are forced in opposite directions between the layers. The high-rise elevator cable has a length of 100m vertical wiring, and the outer sheath of the cable is fixed on the accompanying cable. This is a kind of “soft fixing”. When fixing, the cable is not allowed to be deformed (destruction of coaxiality), so that the elevator can be repeatedly moved. In the case of cable insults, the inner layer of the cable will gradually “slow down and break the braid or core wire under the force. The signal is gradually weakened and the interference is getting bigger and bigger. There is no technical standard for this cable. The simple inspection method is to take Lm cable, peeling off the layers at one end, one person holding the two ends of the cable by hand, and the other person pulling the inner layer of the cable with pliers: pulling the core wire, insulating layer, woven mesh in turn, experiencing the adhesive force, making a reasonable estimate ——The adhesive force is poor, and it is easy to slide. It is not used in the world. Many cables do not have this performance and should be carefully selected.

电梯监控视频干扰及解决方法——中国一卡通网

(2) Principle of elevator video interference generation.

a. Power, lighting, fans, control, communication, etc. in the elevator shaft, all kinds of cables will generate electromagnetic radiation. Like 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 ( The electromotive force), and the interference induced voltage is just connected in series in the "long ground" of the video signal transmission circuit to form interference.

b. The accompanying cables are parallel to the video cable and bundled together at close distances, which forms an "optimal and most effective" interference coupling relationship. c. In the general engineering, the shielding interference method of wearing metal pipes or metal grooves can be adopted, but in the environment where the elevators follow, this method cannot do anything. Therefore, the anti-interference in the elevator environment is very difficult, and only a good design and construction method can be selected. Understanding the basic principles of interference generation is important for improving anti-jamming design and construction.

(3) Anti-interference measures for common copper shaft cable transmission schemes.

Common copper shaft cable. Whether it is a multi-layer high-knit copper mesh cable, a double-shielded cable of "aluminum box-made mesh", or a four-shielded cable of "aluminum foil-made mesh-aluminum foil-made mesh", electrically belongs to a shielding layer, interference The induced voltages are all 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 below several tens of kilohertz). However, 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-interference effect is basically the same. It should be soberly seen that the high-frequency cable only has the effect of weakening the low-frequency interference properly, and it is powerless to prevent strong interference.

电梯监控视频干扰及解决方法——中国一卡通网

(4) Anti-interference measures for elevator wiring.

a. Select the location of the video cable out of the elevator shaft. The ideal choice should be in the middle of the well, because the video cable length in the well is only about half the depth of the well, and the shortest, naturally introduced interference is also minimal. However, such an outbound requirement in engineering can only be determined by the situation, and actual engineering may not be allowed.

b. In the past, where the principle was not understood, most of the outlet locations were taken with other accompanying cables, coming out of 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, does not move, and this part is called "immobilized cable." This provides the possibility that half of the accompanying motion cable can only be bundled with other accompanying cables; the other half of the fixed cable can be selected to be separated from the accompanying cable by a separate method of 'line the video line in the elevator shaftç’§ Vertically route the wire and put this part of the cable through the metal pipe or the metal groove to shield the interference on the cable. It is more effective.

c. When the video cable of the moving part is bundled with other accompanying cables, the designer should fully understand the structure and distribution of other accompanying cables. When strapping, the video cable should be kept away from the cable with high current and high frequency, close to the current with low frequency. Cable strapping. Here, even if there is a choice of 1cm, you may have to fight for it, because the magnitude of the interference is inversely proportional to the square of the distance.

d. The "ground" of the video signal of the camera metal casing, the outer casing of the BNC head, the outer conductor of the coaxial cable, etc., and the elevator car, the guide rail, etc., should be insured when installing the camera.

e. The camera power supply should preferably be concentrated in the DC power supply mode, followed by the selection of the car lighting power, and the power supply cannot be used.

f. Power supply, control and other monitoring cables, try to use shielded cables to prevent interference signals from leaking out.

g. From the outlet 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 between the shield and the elevator shaft. .

Common elevator interference problems can be solved by adding weighted anti-jamming devices and grounding. If the measures described above are still not solved, specific problems need to be specifically treated.

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