Application and selection of field isolation devic

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Application and selection of isolation devices in industrial field Abstract: This paper introduces the application of isolation devices in industrial field and the specific situation of its interface with computers and instruments

key words: isolation, interface, temperature transmitter

a variety of automatic instruments, computers and corresponding actuators are used in the monitoring and control of production process. There are both weak millivolt level small signals and tens of volts large signals transmitted to each other, There are even signals as high as thousands of volts and hundreds of amperes. In terms of frequency, there are DC low-frequency and high-frequency pulses. The mutual interference between equipment and instruments has become a problem that must be solved in the system commissioning. In addition to electromagnetic shielding, solving the "ground" of various equipment and instruments, that is, the potential difference at the signal reference point, is an important issue for on-site commissioning

I. two principles of isolation

for complete transmission of signals, it is necessary to solve the potential difference of reference points between signals of different equipment and instruments. Ideally, the signals of all equipment and instruments have a common reference point, that is, a common "ground". Further, the potential difference between the electrical reference points of all equipment and instruments is "zero". In the actual environment, this is almost impossible. In addition to the voltage drop caused by the wiring resistance between the "ground" of various equipment and instruments, there are many factors, such as the different interference of various equipment and instruments in different environments, and the decline of contact quality caused by wind and rain on the wire contact, resulting in the difference between the "ground". Take Figure 1 as an example

Figure 1 Schematic diagram of PLC and external instruments

in Figure 1, two field devices 1 and 2 send signals to PLC and PLC sends signals to two field devices 3 and 4. It is assumed that all transmitted signals are VDC. Ideally, the "ground" potentials of PLC and two field devices 1 and 2 are completely equal. There is no interference during transmission, and PLC receives correctly. However, as mentioned above, two field devices usually have a "ground" potential difference For example, the "ground" potential of 1equipment is the same as that of PLC, and the potential reference point of 2equipment is 0.1V higher than their "ground" potential. In this way, the signal from 1equipment to PLC is V, while the signal from 2\equipment to PLC is 0.1v-10.1v, and the error occurs. At the same time, the "ground" lines of 1#, 2\\\equipment are connected at the PLC, and 0.1V voltage is applied to the PLC ground wire strip, which may damage the PLC local circuit board line and display wrong data at the same time

problems caused by "ground" frequently occur during field commissioning. For example, a large building materials company used PLC of a foreign company and manual operators of a domestic manufacturer in the production line monitoring equipment. Each data acquisition board of the PLC is composed of eight channels, which share a 12 bit a/d. The converted digital signal is transmitted to the isolator composed of 12 optocouplers to realize isolation from the host, but there is no isolation between its eight channel inputs. When inputting signals, it is normal for each channel to be input to the acquisition board separately. If two or more external signals are input at the same time, the display will jump randomly, and the fault cannot be eliminated. Another example is that a certain Aerospace Department uses a K-type thermocouple as a sensor to test the temperature of each point of the engine. Similar to the above, when testing one point, everything is normal, but when connecting two or more points to the host, the temperature displayed is obviously wrong. In these two cases, the isolator works normally after the problem is solved

The reason why the

isolator can play this role is that it can completely isolate the input/output electrically. In other words, there is no common "ground" between input/output, and the external signal, whether V or V with common mode interference voltage, is v after isolation. That is, the newly established "ground" after isolation has nothing to do with the "ground" of external equipment and instruments. It is for this reason that the isolation between the instrument signals of multiple external equipment input to the PLC host is also realized, that is, there is no "ground" relationship between them

the isolation between the input signal and PLC is discussed above. Similarly, the output signal from PLC to external equipment also has a similar problem. The purpose of solving the problem can be achieved by using isolator

another frequently encountered situation is that a signal is required to be able to transmit signals to both display instruments and equipment such as frequency converters, which may cause mutual interference between the two equipment. At this time, it is required that the two outputs are also isolated. Isolated signal distributor is recommended. It can realize the isolation between input signal and external equipment, and realize the isolation between receiving signal equipment. Figure 2:

Figure 2 typical application of isolated signal distributor

to sum up, two principles should be followed to solve the "ground" problem. First: electrical isolation shall be conducted between external equipment and central processing system (such as PLC and DCS). Second: external equipment signals (whether external equipment sending signals to the central processing system or external equipment receiving signals) shall be electrically isolated from each other. Following these two principles, the system installation can completely overcome the interference caused by the difference between "ground"

most isolators need additional working power supply, generally DC 24V or AC 220V. When this power supply supplies power to the input and output parts, it must be seen that the performance of the tensile testing machine is very powerful. It must be electrically isolated from the input/output parts. The device with all mutual isolation between input/output/external working power supply is called three isolation or full isolation device. Theoretically, this power supply mode can be powered by one power supply regardless of the number of isolators, without introducing interference. Such products conform to the above two principles

II. Select the product according to the interface

the most common signals in the industrial field are ma and V. for physical quantities such as pressure, temperature and flow, they should also be processed into Ma and V signals for computer processing. The equipment that converts these physical quantities into Ma for instruments and requires V signals due to the data is called a transmitter. Figure 3 shows the application wiring diagram of the isolated temperature transmitter with PT100 as the sensor. The change of temperature is reflected by the change of resistance, so the resistance of field connecting lines will introduce errors. Generally speaking, transmitters such as ws9050 and ws2050 have long line compensation function, which can eliminate the error caused by lead resistance and have linearization function to ensure accurate conversion

Figure 3 two types of isolated temperature transmitters

two wire transmitters such as ws2050 share "ground" with their power supply and output and are not isolated. When multiple PT100 are transmitted to PLC through multiple ws2050, although the output shares a 24V power supply, as long as the output is connected to the analog input board with a common reference point, this connection conforms to the two principles pointed out above, and solves the disturbance caused by different "ground". It should be noted that when multiple ws2050 are connected to different PLCs, their respective 24V power supplies should be used

there is another way to isolate the transmitter. The sensor and transmitter are integrated and must be placed at the designated site. At this time, the isolator is generally placed in the cabinet of the central control room, and the isolator in the cabinet distributes power to the field transmitter. Figure 4 shows two product wiring diagrams for different interfaces. Which one to use depends on the site conditions

Figure 4 Schematic diagram of two PLC interfaces and isolated power distribution connection

mismatching of instrument interfaces may also occur during field commissioning. For example, the on-site transmitting equipment outputs Ma from a four wire transmitter, while the interface of the receiving terminal Ma is powered by a two-wire loop. If it is directly connected, it will cause power conflict. The solution is to use the isolator to receive and isolate the Ma from the field, and install a special circuit in the output part of the isolator to match the two-wire loop power supply mode of the interface. The isolator can also process voltage and current signals such as V and AAC from the site

the general ma current signal isolator needs an external working power supply. An isolator ws1562 without additional power supply is recommended here. As shown in Figure 6:

ws1562 is characterized by no external power supply, simple wiring, low power consumption and high reliability. Multi channel connection conforms to the above two principles

Figure 7 shows the relationship curve between output load and input voltage of ws1562 when the output is 20mA. Vin represents the input voltage, RL represents the output load resistance, and VO represents the voltage on the output load resistance

figure 8 shows the output load versus linearity curve of the two passive isolators. The performance depicted by the solid line is good. The output load resistance RL changes from 0 Ω -500 Ω to the downstream M & a tide of the industrial chain, and the linearity is within 0.2%

III. selection of main parameters

in addition to determining the function and paying attention to adapting to the front and rear interfaces, the isolator also has many parameters such as accuracy, power consumption, noise, insulation strength, bus communication function, etc., which need to be carefully selected by the user

The accuracy of potentiometer designed for displacement measurement is a very important parameter. There are many parameters related to it. Time drift and temperature drift indicate the stability of accuracy, and the smaller the value of these two parameters, the better

another parameter that affects accuracy is noise. Since the isolator generally uses dc/dc to generate isolation power supply to supply power to the internal circuit of the product, and the input signal must be modulated into a pulse first, and then demodulated to the output through the isolation band (optical couple or transformer). The isolator with CPU as the core also has pulse signals. The frequency of these working pulses is mostly in the range of 20kHz ~ 100kHz. It has steep edges and rich harmonics, so it is difficult to eliminate the signal pollution. If the noise is high, the error of the signal collected by the data collector will be large. So the smaller the noise peak and energy, the better

power consumption refers to the electric energy consumed by the isolator during operation, which involves the heat generated by the product. This parameter is closely related to the long-term reliability of the product

according to the analog circuit test, different power consumption produces different heat in the product shell, resulting in different temperature rise in the shell. When the product is in working state, the ambient temperature inside the product shell is 10 ℃ ~ 30 ℃ higher than that outside the shell. If the heat dissipation is not good, it may rise above 50 ℃. If the temperature in the shell is too high, the components that make up the product will work at high temperature for a long time, which will have an important impact on the product performance. Long time high temperature environment makes operational amplifier parameters degenerate, resistance value changes, capacitor leakage increases, etc., which will degrade product performance and even lead to product failure. Therefore, users should pay attention to the parameter of power consumption when selecting, especially when the consumption is large, the installation density is high, and the heat dissipation is poor

the design of isolation terminals is becoming increasingly miniaturized. The purpose of miniaturization is to occupy less space. Users should be allowed to install intensively, which will cause heat dissipation problems. In other words, it is necessary to reduce the power consumption and internal temperature of the product, which is one of the prerequisites to improve the reliability of the product

most isolators are installed by guide rail, and terminal wiring is used for wiring. This kind of isolator is usually referred to as isolation terminal, which is suitable for installation in cabinet, and its wiring is easy to disassemble and replace

IV. conclusion

due to the wide application of distributed systems, the system level is getting lower and lower, resulting in the problem of electrical interference in the remote transmission of electrical signals becoming prominent. The electrical isolation of electrical automation system is becoming more and more important in the electronic information age. The design and selection of electrical signal isolation devices is an important technical method to ensure the normal, stable and up to standard operation of the automation system. (end)

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