Hollysys PLC - the isolation technology of the input/output loop of the PLC control system

1. Circuit isolation

The isolation of analog circuits is more complex, mainly depends on the accuracy requirements of the transmission channel, the higher the accuracy requirements, the higher the cost of the channel; However, when the performance requirements rise to the main contradiction, the isolation components should be selected based on performance, and the cost should be put in the second place, on the contrary, the isolation components should be selected from the cost perspective. The isolation of analog circuits is mainly based on transformers, transformers, DC voltage isolators or linear isolation amplifiers.

For occasions with DC components and common mode noise interference, measures must be taken in the measurement of analog signals to completely isolate the input and output, insulate each other, and eliminate the coupling of noise. Isolation can prevent the interference of the analog system, especially the ground interference of the power system can effectively prevent the ground interference of the power system from entering the logic system, resulting in the operation of the logic system. In the precision measurement system, it can also effectively prevent the pulse fluctuation interference of the digital system from entering the analog system, because the signal of the analog circuit preamplification part is very weak, and the small interference fluctuation signal will drown out the useful signal.

For high-voltage, high-current signals, transformer isolation is adopted, and the principle of noise suppression is similar to that of isolation transformer. The application of the transformer isolation method is shown in Figure 1(a). However, the isolation system of microvoltage and microcurrent analog signals is relatively complex, and it is necessary to consider the factors of accuracy, frequency band width, and its price. In general, for a small amount of common-mode noise, a difference amplifier or instrumentation amplifier can achieve good results, but for occasions with large common-mode noise and high measurement accuracy requirements, a high-precision linear isolation amplifier should be selected. The application of a linear isolation amplifier is shown in Figure 1(b).

2. Physical isolation

Physical isolation refers to isolating the interference source from the circuit and the part that is susceptible to interference by using a physical medium, so that they do not have electrical contact. The following physical isolation measures are commonly used in PLC control systems:

Correct selection of connecting cables and wiring. The low-frequency circuit should use twisted pair as much as possible, and the high-frequency circuit should use double coaxial shielded cable as much as possible, and try to replace the long cable with optical cable, so that all signal lines are well insulated and do not leak. This effectively prevents interference due to contact. According to the different types of signals, they are classified according to their ability to resist noise interference, and different kinds of signal cables are isolated and laid according to their categories. If similar types of signal cables must be routed in the same cable trough, they must be separated by metal partitions, and the signal source and the system are electrically isolated by isolation technology, which will greatly reduce the harm caused by common mode interference to the system. The input terminal of the signal is completely isolated from the system circuit part with the isolation amplifier (some systems use isolation transformers or relays to isolate, and the switching amount can be isolated by photoelectric devices or relays), so that the interference signal generated by the different ground potentials cannot form a loop, and the harm of interference can be effectively suppressed. The anti-interference ability of analog signals to high-frequency pulse signals is very poor, and should be transmitted by shielded twisted pair or shielded cables, and these signal lines must be routed separately through the conduit or in the cable trough, and cannot be routed in the same cable pipe (or slot) with other signals. Low-level switching signals (some state dry contact signals) and data communication lines (RS232, EIA485, etc.) have stronger anti-interference ability to low-frequency pulse signals than analog signals, and should also be transmitted over shielded twisted pair (at least with twisted pair). This kind of signal should also be routed separately, and should not be routed together with the power line and the large load signal line. The input and output of high-level (or high-current) switching quantities and other relay input and output signals have stronger anti-interference ability than the above two, but these signals will interfere with other signals, which can be connected with twisted pair cables, or can be laid separately with cable conduits or cable troughs. AC 220V, 380V power line and circuit breaker with large on-off capacity, switch signal line, etc., the selection of these cables is not mainly based on anti-interference ability, but determined by the current load and withstand voltage level, and its laying should be strictly separated from the signal transmission line. In order to prevent the introduction of common-mode high-frequency interference signals on the power supply line, an isolation transformer can be set on the power supply line for interference isolation. In order to achieve a good interference suppression effect, the shielding layer of the transformer should be well grounded, and the secondary coil of the transformer should be twisted pair.

3. Galvanic isolation

Potential isolation is divided into mechanical, electromagnetic, photoelectric and floating isolation methods, the essence of which is to artificially cause electrical isolation to prevent electromagnetic interference caused by circuit coupling. Mechanical isolation uses relays to achieve its coil to receive the signal, and the mechanical contacts to send the signal. When the mechanical contacts are broken, the impedance is large and the capacitance is small, which prevents electromagnetic interference caused by circuitic coupling. The disadvantage of mechanical isolation is that the coil has a low operating frequency, which is not suitable for use in occasions with a high working frequency, and there are bouncing interference and contact resistance when the contact is turned on and off.

Relays are commonly used digital output isolation components, and using relays as isolation elements is simple and practical, and the price is low. Figure 2 is a schematic diagram of relay output isolation. In this circuit, the low-voltage DC and high-voltage AC are isolated by a relay, so that interference from the high-voltage AC side cannot enter the low-voltage DC side.

4. Electromagnetic isolation

Electromagnetic isolation is the use of transformers to transmit electrical signals, which prevents electromagnetic interference caused by circuit coupling, and is more convenient to use in AC occasions. Due to the large distributed capacitance between the windings of the transformer, it should be used in conjunction with shielding and grounding. The number of turns of the primary and secondary windings of the pulse transformer is very small, and they are wound on both sides of the ferrite core, and the distributed capacitance is only a few picos, which can be used as an isolation device for pulse signals. For analog input signals, the actual sampling waveform is also a pulse waveform due to the short sampling period of each point, which can also achieve isolation. The pulse transformer isolation method is adopted, and the dynamic and static norm interference should still be suppressed by the filtering link in the line. Pulse transformer isolation can be used in signal circuits of several megahertz.

The main problem of pulse transformer isolation is that there is a common ground wire between the control system and the control object, even if the coaxial cable is used as the transmission medium, the interference signal on the site will be introduced into the PLC control system, which will affect the reliable and stable operation of the whole system. Because the pulse transformer does not transmit the DC component when transmitting the input and output pulse signals, it has been widely used in the PLC control system. Generally speaking, the signal transmission frequency of the pulse transformer is between 1kHz and 1MHz, and the transmission frequency of the new high-frequency pulse transformer can reach 10MHz. Figure 3(a) is a schematic diagram of the structure of a pulse transformer, and Figure 3(b) shows an example of an application for a pulse transformer.

5. Analog/digital conversion isolation circuit

The isolation between analog circuits and digital circuits mainly adopts analog/digital conversion devices; For circuits with high requirements, in addition to the analog/digital conversion device, analog isolation components and digital isolation components should be added at both ends of the model/digital conversion device.

In the PLC control system, the analog/digital conversion is often carried out on the spot, and the analog-to-digital converter is used to convert the analog signal that is susceptible to interference into a digital signal for transmission, and then the photoelectric isolation is used at the receiving end to enhance its anti-interference ability in the signal transmission process. The location of the analog-to-digital converter is a very specific problem in practical applications. For the application in the industrial production site, the first is to consider keeping the analog/digital converter away from the production site and placing it in the main control room; The second is to place the analog/digital converter at the production site, away from the main control room, both of which have their own advantages and disadvantages.

The analog/digital converter is placed in the main control room, so that the digital information generated by the analog/digital converter is conveniently transmitted to the processor of the PLC control system, and the control information of the PLC is also very convenient to be transmitted to the analog/digital converter, thus conducive to the management of the converter. However, due to the fact that the analog/digital converter is far away from the production site, the analog transmission line is too long, the influence of distribution parameters and interference increases, and it is easy to cause analog signal attenuation, which directly affects the working accuracy and speed of the converter. Although the above problems can be solved by placing the converter at the production site, the digital information transmission line is too long, and it is not easy to manage the converter.

In the application of analog-to-digital (A/D) converters or digital-to-analog (D/A) converters, if certain measures are not taken, the high-frequency oscillation signals in the digital circuit will bring certain interference to the analog circuit and affect the accuracy of the measurement. In order to suppress the high-frequency interference caused by the digital circuit to the analog circuit, the analog ground and the digital ground must be routed separately, and this wiring method cannot completely exclude the high-frequency interference from the digital circuit. In order to eliminate high-frequency interference from digital circuits, it is necessary to isolate the digital circuit from the analog circuit, and the common isolation method is to add a photocoupler between the A/D converter and the digital circuit to isolate the digital circuit from the analog circuit, as shown in Figure 4(a). However, this kind of circuit can not fundamentally solve the interference problem in the analog circuit, and there are still certain defects, because the common-mode interference and differential-mode interference in the signal circuit have not been effectively suppressed, and the requirements can not be met for the occasion of high-precision measurement. For measurements with severe interference, the circuit shown in Figure 4(b) can be used. In this circuit, the signal receiving part is also isolated from the analog processing part, because a linear isolation amplifier is added between the pre-processing stage and the A/D converter to separate the signal ground from the analog ground, and a photocoupler is used to isolate the analog ground from the digital ground between the A/D converter and the digital circuit. In this way, the high-frequency interference signal of the digital system is not only prevented from entering the analog part, but also the common-mode interference and differential-mode interference from the pre-circuit part are blocked.

6. Optoelectronic isolation

Optoelectronic isolation refers to the use of photocouplers to transmit signals through the light emission of semiconductor light-emitting diodes (LEDs) and the light reception of photosensitive semiconductors (photoresistors, photodiodes, phototransistors, photothyristors, etc.). The input impedance of the photocoupler is smaller than that of the general interference source, so the interference voltage at the input end of the photocoupler is small, and the internal resistance of the general interference source is larger, and the current it can provide is not large, so it cannot make the light-emitting diode emit. The housing of the photocoupler is hermetically sealed, it is not affected by external light. The optocoupler has a large isolation resistance (about 1012) and a small isolation capacitance (picometer method), which prevents electromagnetic interference caused by circuit coupling. It's just that the isolation impedance of the photocoupler decreases with the increase of frequency, and the anti-interference effect will also decrease.

The optocoupler consists of a light-emitting element at the input end and a light-receiving element at the output end, and the input and output are electrically completely isolated. It is small in size, easy to use, and can form a variety of different lines to suppress common mode and differential mode interference depending on the different on-site interference conditions.

A photocoupler can be used to isolate the input signal from the internal circuitry, or to isolate the internal output signal from the external circuitry, as shown in Figure 5(a) and (b).

At present, the isolation voltage of most optocoupler devices is above ***V, and some devices have reached 8kV, both high-voltage and high-current optocoupler devices, and high-speed and high-frequency optocoupler devices (frequency up to 10MHz). The commonly used devices are 4N25, and the isolation voltage is ***V; 6N137, the isolation voltage is 3kV, and the frequency is above 10MHz.

The optocoupler application has the characteristics of simple circuit isolation at the input and output terminals, because it avoids the formation of ground loops, and the grounding point of the input and output can also be arbitrarily selected. This isolation can be used not only in digital circuits, but also in linear (analog) circuits.

The photocoupler is used to eliminate noise from two aspects: on the one hand, the noise at the input end is not transmitted to the output end, but only to transmit useful signals to the output end; On the other hand, because the signal transmission from the input to the output is realized by light, the capacitance between the poles is very small, and the insulation resistance is large, so the signal and noise at the output end will not be fed back to the input end.

When using a photocoupler, it should be noted that the photocoupler itself has a distributed capacitance of 10-30pF, so the frequency should not be too high; In addition, when the contact is input, attention should be paid to the RC filtering link to suppress the jitter of the contact. In addition, when used for low voltage, the transmission distance is limited to 100m, and the transmission rate is less than 10kbit/s.

In order to ensure reliable operation of the D/A converter and obtain accurate measurement results, the D/A converter and optocoupler can be combined to form an electrical-optical-galvanic isolation device. Place the D/A converter on the side close to the site. In order to effectively suppress the interference, two sets of photocouplers are used, so that the information exchange between the D/A converter and the PLC is converted twice by electricity-optical-electricity. As shown in Figure 6, one set of optocouplers is placed on one side of the D/A converter and one set of optocouplers is placed on the side of the PLC. There are three different ground terminals in the system: one is the common land for the PLC and I/O interface, the other is the "floating land" used for the transmission of long lines, and the third is the "on-site ground" for the D/A converter and the controlled object. The method of two photoelectric isolation is adopted, and the transmission long line is isolated and floated between the PLC and the controlled object, which not only effectively eliminates the common ground wire, suppresses the interference introduced by it, but also helps to solve the problem of long-line drive and impedance matching, and can ensure the reliable operation of the whole control system.

Hollysys PLC - the isolation technology of the input/output loop of the PLC control system

Hollysys PLC - the isolation technology of the input/output loop of the PLC control system

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