Principle and key points of anti-interference tech

Principle and key points of anti-interference technology of elevator video monitoring system

based on the research results of the generation principle of elevator video monitoring interference, the reasonable analysis of interference formation and anti-interference technology has achieved the unity of theory and practice, and provided some more practical anti-interference technical measures in the design and construction of elevator monitoring system. This paper only deals with the anti-interference technology of coaxial video transmission in elevator monitoring engineering, which is a reference for design and construction

in the closed-circuit monitoring project, the problem of elevator monitoring video interference has always been the most common, the most difficult to deal with, and one of the most concerned problems. This paper states that as long as the principle of interference generation is mastered, the problem of elevator anti-interference engineering will also be solved

I. master the types and characteristics of common coaxial cables

1 Consider transmission attenuation: when the floor is very high and far from the monitoring center, the transmission attenuation should be carefully considered. When choosing cables, we all know that thick cables are better than thin cables, but we should also understand that SYWV physical foaming cables are better than solid SYV cables, high braided cables are better than low braided cables, copper core cables are better than "copper clad steel" cables, and copper braids are better than aluminum magnesium alloy braids

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 the cable is that the higher the frequency, the greater the attenuation. High-frequency attenuation mainly affects the definition and resolution. Special attention should be paid to the observation method of summarizing the image quality. The characteristics and laws of cables in this regard are: thick cables are better than thin cables, and foaming is better than solid cables, but the "high-frequency attenuation of high-speed braiding and low-speed braiding" of the same model are the same

3. Consider cable life: flexible cable is better than ordinary cable, and thin cable is better than thick cable; Another problem that is most likely to be ignored is the adhesion between the layers of the cable, that is, whether relative sliding will occur when the layers of the cable are stressed in the opposite direction in the longitudinal direction. The cable length of the high-rise elevator can reach 100 meters, and the cable outer sheath is fixed on the traveling cable. This is a kind of "soft fixation", which does not allow the cable to deform (destroy the coaxiality). In this way, when the elevator moves repeatedly, the inner layer of the cable under the action of gravity, It will gradually "slide", slowly pull the braid or core wire, which shows that the signal is gradually weakened and the interference is getting greater and greater; At present, there is no technical standard for this cable. The simple inspection method is to take a meter of cable, peel off each layer at one end, one person holds both ends of the cable with his hand, and the other person pulls the inner layer of the cable with pliers: pull the core wire, insulating layer, braid in turn, experience the size of the adhesion, and make a reasonable estimate. Try not to choose those with poor adhesion and easy sliding. Many cables with this performance are not good, so they should be carefully selected

II. Introduction to interference generation principle

1 Power, lighting, fan, control, communication and other cables are usually arranged in the elevator shaft, and all kinds of cables will produce electromagnetic radiation. The same as the antenna receiving principle, the coaxial cable will also "receive" these interference materials. How to use the testing machine?, 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 outer conductor (Braid) of the cable. This interference induced voltage is just connected in series in the "long ground line" of the video signal transmission circuit to form interference

2. More importantly, these traveling cables are parallel to the video cables and tied together at a close distance. This forms a close to the "best and most effective" interference coupling relationship. In general engineering, the method of shielding interference through metal pipes or metal grooves can be used, but in the environment of elevator follow-up, this method is powerless. Therefore, it is very difficult to resist interference in the elevator environment, so we can only choose better design and construction methods

3. Understanding the basic principle of interference generation is very important for improving anti-interference design and construction

III. anti interference measures of common copper shaft cable transmission schemes

1 Common copper shaft cables: whether it is multi-layer high braided copper braided cables, "aluminum foil braided" double shielded cables, or "aluminum foil braided - aluminum foil braided" four shielded cables, they all belong to one shielding layer electrically. The interference induced voltage is directly connected in series in the video signal transmission circuit. Only the outer conductor resistance of multilayer high braided cable is small, and the interference induced voltage is relatively low. This has a certain effect against low-frequency power supply interference, motor spark interference, etc. (interference below tens of kHz). However, for high-frequency interference, due to the "skin effect", the high-frequency impedance is the same as that of low braided cable, and the anti-interference effect is basically the same; Therefore, we should clearly see that high-speed braided cables can only appropriately reduce the role of low-frequency interference, and there is nothing we can do to prevent strong interference and high-frequency interference

2. Anti interference measures for elevator wiring:

① location selection of video cable out of the elevator shaft: the ideal choice should be in the middle of the shaft, because at this time, the length of video cable traveling in the shaft is only a little more than half of the shaft depth, the shortest, and the interference naturally introduced is also the smallest; However, such outgoing requirements in the project can only be achieved according to the situation, which may not be allowed in the actual project

② in the past, without understanding the principle, most of the outlet positions went with other accompanying cables and came out from the top or bottom of the cable shaft. In this case, considering that only half of the cable is traveling, and the other half is only fixed extension connection and does not move, we call this part "fixed cable"; This provides a possibility: that half of the traveling cable can only be bundled with other traveling cables; The other half of the immobile cable can choose to route separately away from the accompanying cable. It is more effective to route the video cable vertically close to the well wall in the elevator shaft, and thread this part of the cable through metal pipes or metal grooves to shield the impact of interference on this part of the cable

③ when the video cable of the traveling part is bundled with other traveling cables, the designer should fully understand the structure and division of other traveling cables and can carry out the compression experiment; The tension connector cloth is installed between the moving beam and the upper beam. When strapping, the video cable should be far away from the cable with high current and high frequency, and close to the cable with low current and low frequency; Here, even if there is a choice of 1 cm, we may have to fight for it, because the size of the interference effect is at least inversely proportional to the square of the distance from the foam wheel of the stroller

④ the "ground" of video signals such as the metal shell of the camera, the shell of the NC head and the outer conductor of the coaxial cable should be insulated from the elevator car and guide rail, which should be paid special attention to when installing the camera

⑤ centralized DC power supply mode shall be preferred for camera power supply, followed by car lighting power, and power power power is not allowed

⑥ cables for power supply, control and other monitoring should be shielded to prevent interference signals from leaking outward

⑦ the video cable from the elevator shaft outlet to the control center should go through metal pipes or metal grooves to shield the influence of environmental interference on this part of the cable along the way, and pay attention to the shielding in this part of the shield and the elevator shaft, and make electrical connections

IV. apply anti-interference coaxial cable

1 The anti-interference coaxial cable is a kind of "double insulated and double shielded coaxial cable", and the core wire, insulation layer and shielding layer in it are still standard 75 ohm cables, without any difference. The difference is that in addition to the original shielding layer, the second insulation layer and the second shielding layer are added. 4. The motor is required to add a sheath when it is changed separately. From the above analysis of the interference generation principle, it is known that the induced voltage generated by the interference on the outer layer of the traditional coaxial cable is connected in series in the "long ground wire" of the video signal transmission circuit, thus forming the interference. However, after using anti-interference coaxial cable, the situation has changed qualitatively: the interference induced voltage can only be formed on the "second shielding layer", and the "second insulation layer" inside insulates it from the "long ground wire" of the video signal transmission circuit, excluding the interference from the video signal transmission circuit, so as to achieve the purpose of anti-interference

2. This kind of anti-interference cable has outstanding anti-interference performance for the interference below tens of kHz, such as super low-frequency power supply interference, motor spark interference, variable-frequency motor interference, control signal interference, etc. in the elevator environment

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