How can we better solve the interference of plcdcs

2022-10-01
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How can we better solve the interference of S system

1: first of all, we need to understand how many kinds of on-site system interference will occur? Why interference? How to solve it

1) earth circulation interference

various automatic instruments, control systems and actuators are needed to realize monitoring and control in the process of industrial production. The signal transmission between them includes small signals ranging from weak to millivolt level and milliampere level; There are large signals of tens of volts, even thousands of volts, hundreds of amperes; There are both low-frequency DC signals and high-frequency pulse signals. After forming the system, it is often found that the transmission between instruments and equipment interferes with each other, resulting in system instability and even misoperation In addition to the performance reasons of each instrument and equipment itself, such as anti electromagnetic interference, another very important reason is that various instruments and equipment need to be grounded according to requirements and purposes, in which a and B are load ports. For example, the chassis needs to be grounded for safety; In order to make the circuit work normally, the system should have a common reference point; In order to suppress interference, add a shield, which also needs to be grounded However, due to the potential difference between the reference points between the instrument and the equipment (that is, the common location of each equipment is different), the problems of "ground circulation" and "ground loop" are formed. When the circulating current is large, there will be a high common mode noise voltage, which will be coupled to the signal line through distributed parameters, or directly connected to the level signal line, which will produce great series mode interference, and even damage the interface circuit and equipment earlier this year

2) natural interference

lightning is a major natural interference source, and the interference generated by lightning can be transmitted to places thousands of kilometers away. The time domain waveform of lightning interference is a large spike pulse superimposed on the background of a channeling random pulse. Cosmic noise is produced by ionizing radiation and changes constantly throughout the day. Solar noise changes dramatically with solar activity. Natural noise will mainly cause interference to communication, while lightning energy peak pulse can cause damage to many equipment, which should be avoided or reduced to reduce the degree of damage and loss

3) human interference

the root cause of electromagnetic interference is the change of voltage or current in the conductor, that is, a large dv/dtdi/or di/dt can make the conductor produce electromagnetic wave radiation. On the one hand, people can use this feature to realize specific functions, such as wireless communication, radar or other functions. On the other hand, when electronic equipment is working, accompanying electromagnetic radiation will be generated due to dv/dt or di/dt in the conductor. No matter what the subjective purpose, it has caused pollution to the electromagnetic environment objectively. In addition, factories and enterprises often have some large-scale equipment (motors, frequency converters, etc.) switched frequently in the production process, which will also cause some capacitive and inductive interference, and will also affect the normal display or collection of instruments and meters. Where there is a sudden change in voltage and current, there must be electromagnetic interference. Digital pulse circuit is a typical interference source. With the wide application of electronic technology, electromagnetic pollution will become more and more serious

2: how to solve the three kinds of interference in the system

first of all, the three elements of interference are interference source, sensitive source and coupling path. Without one of these three elements, electromagnetic compatibility problems will not exist. Therefore, we should start with these three elements. Find the most convenient solution. Generally, there is no way to solve interference sources and sensitive sources. It is usually from the coupling path, which is also the most commonly used method. Such as shielding and filtering. The circulation in the treatment area is the most common and troublesome. Now this is the topic of discussion

1) the first method:

all field devices are not grounded, so that all process loops have only one grounding point and cannot form a loop. This method seems simple. However, it is often difficult to achieve in practical applications, because some equipment requires grounding to ensure measurement accuracy or personal safety, and some equipment may form new grounding points due to long-term corrosion and wear or climate impact

2) the second method:

make the potential of the two grounding points the same, but because the grounding resistance is affected by many factors such as geological conditions and climate change, this method can not be completely achieved in practice

3) the third method:

use a signal isolator in each process link to disconnect the process loop without affecting the normal transmission of the process signal, so as to completely solve the problem of the ground loop

3: why does the signal isolator have good advantages

the signal isolation method used in each process loop can be achieved by using isolation cards such as DCS or PLC or isolated transmitters on site (which can be achieved by sub equipment) or signal isolators. In comparison, the use of signal isolators has the following advantages:

in most cases, the use of signal isolators + non isolated cards is cheaper than the use of isolated cards

the performance of signal isolators is better than that of isolated cards in terms of isolation ability and anti electromagnetic interference

the application of signal isolators is flexible, and it also has functions such as signal conversion, signal distribution and interface conversion, It is more convenient to use

signal isolators usually have single channel, double channel, and the channels are completely independent of each other, making the configuration and daily maintenance of the system more convenient

4: now there are so many brands of isolators in the market, and the prices are uneven. How to choose

the isolator is located between the two system channels, so the selection of the isolator must first determine the input and output functions, and make the isolator input and output mode (voltage type, current type, loop power supply type, etc.) adapt to the front and rear channel interface mode. In addition, there are many important parameters such as accuracy, power consumption, noise, insulation strength, bus communication function, etc. related to the performance of production 4 during equipment installation and commissioning, and in terms of cost performance. For example, noise is related to accuracy, power consumption and heat are related to reliability, which need to be carefully selected by users. In short, applicability, reliability and cost performance are the main principles for selecting isolators

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