In the closed-circuit monitoring system project, the elevator monitoring video interference problem has been the most common and difficult to solve, and it is also one of the most concerned issues. Old-fashioned elevators use ordinary motors. The interference frequency is very low, and the anti-jamming problem is still a good solution. Nowadays, most of the inverters use frequency-converting power motors. The high-order harmonics of the interference are very rich, and the spectrum is very wide. High-frequency interference is very serious. In particular, modern high-rise residential buildings, office buildings, and floors are too high. Although the video cables of elevators have taken measures from the “middle†into elevator shafts, they are wearing metal pipes, metal trunking, special cables for elevators, and even “high-grade importsâ€. "Elevator cable" has also been adopted, but the accompanying cable is still very long, and interference has always been a headache. The existing anti-jamming products were all tested. Although most of them had certain effects, the results were always unsatisfactory: Some had residual interference, especially residual high-frequency interference, which made the image unsatisfactory. Some produce “brightness blossoming†distortions, character distortion on the floor display, and effects on synchronization.
(1) Types and characteristics of common coaxial cables for elevators.
a. Consider the transmission attenuation When the floor is very high and the distance from the monitoring center is far away, the transmission attenuation problem should be carefully considered. When selecting the cable, we all know the thick cable, but we should also understand that the SYWV embedded foam cable is superior to the solid SYV cable, the high cable is better than the low cable, and the copper cable is better than the copper-clad steel cable. The net is superior to aluminum-magnesium alloy netting.
b. Pay attention to the attenuation of brightness/contrast of high-frequency attenuation low-frequency components. It 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 must be paid to summing up the image quality. Observe the 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-end high-frequency attenuationâ€.
c. Considering the cable life The soft cable life is better than the ordinary cable. The thin cable is better than the thick cable. There is also a problem that is most easily overlooked: the adhesion between the layers of the cable, ie whether relative slipping occurs when the layers of the cable are forced in opposite longitudinal directions. The high-rise elevator cable is up to 100m vertical, and the outer jacket of the cable is fixed on the accompanying cable. This is a kind of “soft fixingâ€. When the cable is fixed, the cable is not allowed to deform (break coaxiality). In this way, consider the repeated movement of the elevator. The internal cable layer of the cable will gradually “slid down and pull off the braid or core wire under force. The signal is gradually weakened and the interference is getting bigger and bigger. At present, there is no such cable technical standard. The simple inspection method is to take lm. In the cable, one layer is peeled off at one end, one person holds the cable at both ends by hand, and the other pulls the inner layer of the cable with pliers: Pull the core wire, insulating layer, and woven mesh in order to experience the size of the adhesion and make a reasonable estimate— - Adhesion is poor, easy to slide in the world, many cables do not have this performance, should be carefully selected.
(2) The principle of elevator video interference generation.
a. Power, lighting, fan, control, communication, etc. in the elevator shaft, all kinds of cables will produce electromagnetic radiation. Like the antenna reception principle, the coaxial cable will also “receive†these interferences, that is, the interference electromagnetic field will produce interference induced currents on the cable, and the interference induced current will also generate interference induced voltages on the longitudinal resistance of the cable outer conductor (woven mesh). (Electromotive force), and the interference induced voltage is just connected in series in the "long ground" of the video signal transmission circuit, forming interference.
b. The accompanying cables are all in parallel with the video cable and are bundled together in close quarters. This results in a near "best and most effective" interference coupling relationship. c. In the general project, the method of shielding and interfering with the metal pipe or the metal channel 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. It is important to understand the basic principles of interference and improve anti-jamming design and construction.
(3) Common anti-jamming measures for copper shaft cable transmission scheme.
Commonly used copper shaft cable. No matter whether it is a multi-layered high-copper braided cable, or an “aluminum box-wired†double-shielded cable, or a “four-shielded aluminum foil-braided-aluminum-foil braided cableâ€, it all belongs to a shield layer. Inductive 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 kilohertz) against low-frequency power disturbances and motor spark disturbances. 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. Should be clearly aware that: high cable only moderately weaken the role of low-frequency interference, anti-strong interference or powerless.
(4) Anti-jamming measures for elevator wiring.
a. The location of the video cable going out of the elevator shaft. The ideal choice should be in the middle of the well because at this time the length of the accompanying video cable in the well is only about half the depth of the well, and the shortest is the natural introduction of interference. However, this kind of qualifying requirements in engineering can only be achieved by looking at the situation. Actual projects may not be allowed.
b. In the past, without knowing the principle, most of the outlets were taken 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 cables are accompanied by the movement, the other half is only a fixed extension connection and does not move. This part is called “immovable cableâ€. This provides the possibility that half of the traveling cables can only be routed together with other traveling cables; the other half of the fixed cable can be routed away from the accompanying cables. 'The video line is stuck in the shaft in the elevator shaft.ç’§ Vertically route the cable and put the cable through the metal pipe or the metal slot to shield the influence of the interference on this part of the cable.
c. When accompanied by the video cable and other accompanying cables, the designers should fully understand the structure and distribution of other accompanying cables. When tying, the video cable should be far away from the large current and high frequency cables, and the frequency near the current should be low. Cable strapping. Here, even if there is a choice of 1cm, you may have to fight for it, because the interference effect is inversely proportional to the square of the distance.
d. The "ground" of the video signal of the metal shell of the camera, the casing of the BNC head, and the outer conductor of the coaxial cable must be insulated from the elevator car and guide rails. This requires special attention when installing the camera.
e. The power supply of the camera should be preferably centralized DC power supply mode, followed by the selection of car lighting power, not power.
f. Monitoring and control cables for power supply, control, etc. Use shielded cables as much as possible to prevent external signals from leaking.
g. The video cable from the elevator shaft exit to the control center should take the metal pipe or take the metal slot to shield the impact of the environmental interference on this part of the cable, and pay attention to this part of the shield and the shielding in the elevator shaft, should make the electrical connection .
The common elevator interference problem can be solved by adding a weighted anti-jamming device and grounding. If the above described measures cannot be solved, specific problems need to be specifically addressed.
(1) Types and characteristics of common coaxial cables for elevators.
a. Consider the transmission attenuation When the floor is very high and the distance from the monitoring center is far away, the transmission attenuation problem should be carefully considered. When selecting the cable, we all know the thick cable, but we should also understand that the SYWV embedded foam cable is superior to the solid SYV cable, the high cable is better than the low cable, and the copper cable is better than the copper-clad steel cable. The net is superior to aluminum-magnesium alloy netting.
b. Pay attention to the attenuation of brightness/contrast of high-frequency attenuation low-frequency components. It 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 must be paid to summing up the image quality. Observe the 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-end high-frequency attenuationâ€.
c. Considering the cable life The soft cable life is better than the ordinary cable. The thin cable is better than the thick cable. There is also a problem that is most easily overlooked: the adhesion between the layers of the cable, ie whether relative slipping occurs when the layers of the cable are forced in opposite longitudinal directions. The high-rise elevator cable is up to 100m vertical, and the outer jacket of the cable is fixed on the accompanying cable. This is a kind of “soft fixingâ€. When the cable is fixed, the cable is not allowed to deform (break coaxiality). In this way, consider the repeated movement of the elevator. The internal cable layer of the cable will gradually “slid down and pull off the braid or core wire under force. The signal is gradually weakened and the interference is getting bigger and bigger. At present, there is no such cable technical standard. The simple inspection method is to take lm. In the cable, one layer is peeled off at one end, one person holds the cable at both ends by hand, and the other pulls the inner layer of the cable with pliers: Pull the core wire, insulating layer, and woven mesh in order to experience the size of the adhesion and make a reasonable estimate— - Adhesion is poor, easy to slide in the world, many cables do not have this performance, should be carefully selected.
(2) The principle of elevator video interference generation.
a. Power, lighting, fan, control, communication, etc. in the elevator shaft, all kinds of cables will produce electromagnetic radiation. Like the antenna reception principle, the coaxial cable will also “receive†these interferences, that is, the interference electromagnetic field will produce interference induced currents on the cable, and the interference induced current will also generate interference induced voltages on the longitudinal resistance of the cable outer conductor (woven mesh). (Electromotive force), and the interference induced voltage is just connected in series in the "long ground" of the video signal transmission circuit, forming interference.
b. The accompanying cables are all in parallel with the video cable and are bundled together in close quarters. This results in a near "best and most effective" interference coupling relationship. c. In the general project, the method of shielding and interfering with the metal pipe or the metal channel 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. It is important to understand the basic principles of interference and improve anti-jamming design and construction.
(3) Common anti-jamming measures for copper shaft cable transmission scheme.
Commonly used copper shaft cable. No matter whether it is a multi-layered high-copper braided cable, or an “aluminum box-wired†double-shielded cable, or a “four-shielded aluminum foil-braided-aluminum-foil braided cableâ€, it all belongs to a shield layer. Inductive 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 kilohertz) against low-frequency power disturbances and motor spark disturbances. 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. Should be clearly aware that: high cable only moderately weaken the role of low-frequency interference, anti-strong interference or powerless.
(4) Anti-jamming measures for elevator wiring.
a. The location of the video cable going out of the elevator shaft. The ideal choice should be in the middle of the well because at this time the length of the accompanying video cable in the well is only about half the depth of the well, and the shortest is the natural introduction of interference. However, this kind of qualifying requirements in engineering can only be achieved by looking at the situation. Actual projects may not be allowed.
b. In the past, without knowing the principle, most of the outlets were taken 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 cables are accompanied by the movement, the other half is only a fixed extension connection and does not move. This part is called “immovable cableâ€. This provides the possibility that half of the traveling cables can only be routed together with other traveling cables; the other half of the fixed cable can be routed away from the accompanying cables. 'The video line is stuck in the shaft in the elevator shaft.ç’§ Vertically route the cable and put the cable through the metal pipe or the metal slot to shield the influence of the interference on this part of the cable.
c. When accompanied by the video cable and other accompanying cables, the designers should fully understand the structure and distribution of other accompanying cables. When tying, the video cable should be far away from the large current and high frequency cables, and the frequency near the current should be low. Cable strapping. Here, even if there is a choice of 1cm, you may have to fight for it, because the interference effect is inversely proportional to the square of the distance.
d. The "ground" of the video signal of the metal shell of the camera, the casing of the BNC head, and the outer conductor of the coaxial cable must be insulated from the elevator car and guide rails. This requires special attention when installing the camera.
e. The power supply of the camera should be preferably centralized DC power supply mode, followed by the selection of car lighting power, not power.
f. Monitoring and control cables for power supply, control, etc. Use shielded cables as much as possible to prevent external signals from leaking.
g. The video cable from the elevator shaft exit to the control center should take the metal pipe or take the metal slot to shield the impact of the environmental interference on this part of the cable, and pay attention to this part of the shield and the shielding in the elevator shaft, should make the electrical connection .
The common elevator interference problem can be solved by adding a weighted anti-jamming device and grounding. If the above described measures cannot be solved, specific problems need to be specifically addressed.
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