The basic method of PLC control for addressing on-site signal interference in industrial environment
1. Overview
With the development of science and technology, the application of PLC in industrial control has become increasingly widespread. The reliability of PLC control systems directly affects the safe production and economic operation of industrial enterprises, and the interference resistance of the system is the key to the reliable operation of the entire system. Various types of PLC used in the automation system are mostly installed in the control room, on-site production equipment, and various motor devices. They are mostly located in the harsh electromagnetic environment formed by strong electrical circuits and equipment. To improve the reliability of PLC control systems, designers can only effectively ensure the reliable operation of the system by understanding various interferences in advance.
2. Electromagnetic interference sources and their interference to the system
The interference that affects PLC control systems is similar to that which affects industrial control equipment in general. It mostly occurs at the locations where current or voltage undergoes drastic changes. These are the areas where the charges move vigorously, and they are the noise sources, that is, the interference sources.
The types of interference are usually classified according to the causes of the interference, the interference patterns of noise and the waveform properties of noise. Among them: According to the different causes of noise generation, they are divided into discharge noise, sporadic noise, etc.; According to the different interference patterns of sound, they are divided into common-mode interference and differential-mode interference, which is a commonly used classification method. Common-mode interference is the potential difference between the signal and the ground, mainly formed by the grid series connection, ground potential difference and the common-mode voltage induced on the signal line by space electromagnetic radiation. Common-mode voltage is sometimes large, especially in the case of electrical power supply rooms with poor isolation performance, the common-mode voltage of the transmitter output signal is generally high, and some can even reach above 130V. Common-mode voltage can be converted into common-mode voltage through asymmetric circuits, directly affecting the measured and controlled signals, causing component damage. This common-mode interference can be either DC or AC. Common-mode interference refers to the interference voltage between signal levels, mainly formed by the coupling induction of space electromagnetic fields between signals and the voltage formed by the conversion of common-mode interference in unbalanced circuits. This kind of interference directly superimposes on the signal and directly affects the measurement and control accuracy.
3. What are the main sources of electromagnetic interference in PLC control systems?
Radiation interference from space
The electromagnetic fields of space radiation mainly originate from power networks, transient processes of electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc. They are usually referred to as radiation interference. The distribution of such interference is extremely complex. If a PLC system is placed within the specified frequency field, it will be affected by radiation interference. The impact mainly occurs through two paths: one is the direct radiation to the internal components of the PLC, which is generated by circuit induction and causes interference; the other is the radiation to the communication network within the PLC, which is introduced by the induction of the communication lines and the discharge of high-voltage suppressors. The magnitude of radiation interference is related to the electromagnetic fields generated by on-site equipment, especially the frequency. Protection is usually achieved by setting shielded cables, local shielding of PLC, and high-voltage discharge elements.
2) Interference from external leads of the system
It is mainly introduced through power supply and signal lines, and is usually referred to as conducted interference. This kind of interference is extremely serious in industrial sites in our country.
3) Interference from the power supply
Practice has proved that there are many cases where faults in the control system are caused by interference from the power supply. This has been encountered during engineering debugging. Only after replacing the PLC power supply with one that has better isolation performance could the problem be solved.
The normal power supply of PLC systems is all provided by the power grid. Due to the wide coverage of the power grid, it will be affected by all kinds of space electromagnetic interference and induced voltages and circuits on the lines, especially the changes within the power grid itself, surge currents caused by switch operations, harmonics generated by the start and stop of large power equipment, AC-DC conversion devices, and transient impacts of power grid short circuits, etc., all of which are transmitted through the power grid lines to the power supply side of the PLC. PLC power supplies usually adopt isolated power supplies, but due to their structural and manufacturing process factors, their isolation performance is not ideal. In fact, due to the existence of distributed parameters, especially distributed capacitors, absolute isolation is impossible.
4) Interference introduced when the grounding system is in disorder
The various signal transmission lines connected to the PLC control system, apart from transmitting valid signals, will always be affected by external interference. Moreover, they can suppress the emission of interference from the equipment outward. However, incorrect grounding will instead introduce serious interference signals, causing the PLC control system to fail to work normally. The grounding of the PLC control system includes system ground, shield ground, AC ground, and protection ground, etc. The confusion of the grounding system will mainly cause uneven distribution of each grounding point, and there is a potential difference in ground potential between different grounding points, resulting in ground loop current and affecting the normal operation of the system. For example, the cable shielding layer must be grounded at one point. If both ends of the cable shielding layer are grounded, there will be a potential difference, and current will flow through the potential difference. When an abnormal state occurs or lightning strikes, the ground current will be even greater.
Furthermore, the shielding layer, grounding wire and the earth may form a closed circuit. Under the influence of the changing magnetic field, there will be induced current within the shielding layer. Through the coupling between the shielding layer and the core wire, interference signal circuits will be formed. If the grounding treatment of the system is chaotic and inconsistent with other grounding methods, the ground circulation current generated may have uneven potential distribution on the ground wire, which will affect the normal operation of the logic circuits and analog circuits in the PLC. The interference voltage of the PLC's logic operation is relatively low. The distribution of the logic ground potential interference is likely to affect the logic operation and data storage of the PLC, causing data confusion, program runaway or system crash. The distribution of the analog ground potential will lead to a decrease in measurement accuracy and serious distortion and misoperation in signal measurement and control.
5) Interference introduced by the introduction of the feeder line
The various signal transmission lines connected to the PLC control system, in addition to transmitting valid signals, will always have external interference signals invading. This interference mainly comes in two ways: Firstly, it is through the power supply of the transducer or the shared signal instrument that it is introduced into the power grid through the series connection. This is often overlooked; secondly, it is the interference induced by the spatial electromagnetic radiation on the signal lines, that is, the external induced interference on the signal lines. This is very serious. Interference introduced by signals can cause abnormal signal operation and a significant reduction in measurement accuracy. In severe cases, it can even cause damage to components. For systems with poor isolation performance, it will also lead to mutual interference between signals, causing the common ground system bus to return, resulting in changes in logical data, malfunctions and system crashes. The damage to modules caused by signal introduction interference in PLC control systems is quite serious, and the situations causing system failures due to this are also many.
6) Interference from within the PLC
It is mainly generated by the mutual electromagnetic radiation between the internal components and circuits of the system, such as the mutual radiation between logic circuits and its influence on analog circuits, the mutual influence between analog ground and logic ground, and the mismatched use of components. This belongs to the content of electromagnetic compatibility design for the system inside by the PLC manufacturing factory. It is rather complex and as an application department, it cannot be changed. Therefore, there is no need to consider it too much. However, a system with more application experience or that has been tested should be selected.
4. When the system is disturbed, the following main interference phenomena are often encountered:
When the system issues instructions, the motor rotates irregularly.
2) When the signal is zero, the digital display table shows erratic fluctuations in values.
3) When the sensor is operating, the signal values collected by the PLC do not match the corresponding actual parameter values. Moreover, the error values are random and irregular.
4) The system does not work properly when sharing the same power supply with the AC servo system.
5. How can we better and more simply solve the interference problem in PLC systems?
Under ideal conditions, one should choose equipment with better isolation performance, use high-quality power supply, power lines and signal lines for wiring, and ensure reasonable power-grounding. However, this requires joint efforts from different equipment manufacturers to achieve, which is difficult to accomplish and comes with high costs.
2) By using analog signal isolators, there is a device called signal transducer, which belongs to the category of signal conditioning and mainly serves to resist interference. Precisely because it has an exceptionally strong ability to resist interference, it is widely applied in automation control systems. Especially for complex industrial sites where control programs become increasingly complex with time, signal isolators provide three-terminal isolation for various analog signals for input, output, and power supply. It is indeed one of the effective measures for resisting interference in current automation control systems.
6. Why is signal isolator the preferred solution for eliminating interference in PLC systems?
It is simple and convenient to use, reliable, cost-effective and capable of simultaneously addressing multiple interferences.
2) It can significantly reduce the workload of designers and system debuggers. Even for complex systems, they will become very stable and reliable when handled by ordinary designers.
What is the working principle of a signal isolator?
Firstly, the signals received by the PLC are modulated and transformed through semiconductor devices, then isolated and converted by photo-sensing or magnetic-sensing devices, and then demodulated to restore the original signals or different signals before isolation. At the same time, the power supply for the isolated signals is isolated and processed. Ensure that the signals, power supply and ground after the transformation are absolutely independent.
Nowadays, there are so many brands of isolators available in the market, and their prices vary greatly. How should one make a choice?
The isolator is located between the two system channels. Therefore, when choosing an isolator, one must first determine the input and output functions. At the same time, the input and output mode of the isolator (voltage type, current type, loop power supply type, etc.) should be adapted to the interface modes of the front-end and rear-end channels. Besides, there are many important parameters such as accuracy, power consumption, noise, insulation strength, and bus communication that are related to the product performance. For example, noise is related to accuracy, and power consumption energy is related to reliability. These require users to make careful selections. In conclusion, trial use, reliability, and product cost performance are the main principles for choosing an isolator.
Working principle: Firstly, the semiconductor device is modulated and transformed. Then, through the photo-sensor or magnetic sensor, the isolation conversion is carried out. Subsequently, the demodulation is performed to restore the original signal before isolation. Meanwhile, the power supply of the isolated signal is isolated for processing to ensure that the power supply of the transformed signal is isolated. Ensure that the power supply, signal and ground of the transformed signal are absolutely independent.
One: Protect the control loop of the subordinate units.
2. Weaken the influence of environmental noise on the test circuit.
III: Implement reliable protection for public grounding, frequency converter, solenoid valve, PLC/DCS input/output and communication interfaces.
Standard series guide rail structure, easy to install, can effectively isolate the potentials between input, output, solenoid valves and earth, and can overcome various high and low frequency pulsation interference of the noise level of the frequency converter.
What are the main types of signal isolators?
Isolator
In industrial production, in order to enhance the load capacity of instruments, ensure that instruments connected to the same signal do not interfere with each other, and improve the electrical installation performance, it is necessary to collect, amplify, process and perform anti-interference treatment on the input signals such as voltage, current or frequency, resistance, etc., and then output isolated current and voltage signals for safe use by secondary instruments and PLC/DCS.
2) Distribution panel
In industrial sites, two-wire transmission mode is generally adopted. It not only provides 24V power supply for primary instruments such as transmitters, but also collects, amplifies, processes and filters the input current signals, and then outputs isolated current and voltage signals for secondary instruments or other instruments to use.
3) Safety barrier
Some special industrial sites not only require two-wire transmission, but also need to provide power distribution and signal isolation functions. At the same time, they need to have the performance of explosion-proof against sparks in safety conditions, reliably suppressing power supply power, preventing ignition between power supply, signal and ground, limiting current and voltage for dual restrictions on signal and power circuits, and limiting the energy entering the hazardous areas within the safe quota range.
What are the matters needing attention when installing and maintaining signal isolators?
Due to the differences in manufacturers, the production processes and wiring definitions of the isolators are not all the same. However, the application scenarios are basically the same, so the protection requirements and maintenance for the products are also basically the same.
Before use, please read the instructions carefully.
When used as a signal isolator, the input end should be connected in series to the loop circuit and the output end should be connected to the sampling circuit.
3. When used for isolation power distribution, the input terminals should be connected to the power circuit and the output terminals should be connected to the transmitter.
4. If the device does not work properly, first check whether the wiring is correct. Also, pay attention to whether the power supply is available and the polarity is correct.
Why is it that sometimes the phenomenon signals received by PLC have large errors and poor stability?
There are many reasons for this kind of imagination. The potential difference between different reference points of the instrument signals is an important factor. Due to this difference, interference currents are generated between the instrument signals, resulting in large PLC errors and poor stability. Therefore, it is the best situation for the signals of different equipment and instruments to have a common reference point. The isolator electrically isolates the input/output completely, and forms a common reference point with the analog interface board on the PLC to solve the problem of ideal anti-interference.
Isolating the 4-20mA channel, but there is no space left in the cabinet for installing the power supply. What should we do?
A passive signal isolator is available, which can isolate 4-20mA signals without the need for an external power supply. PH1033 is such a product.
Nowadays, there are so many brands of isolators on the market, and their prices vary greatly. How should one make a choice?
The isolator is located between the two system channels. Therefore, when choosing an isolator, one must first determine the input and output functions. At the same time, the input and output mode of the isolator should be adapted to the interface modes of the front-end and back-end channels. Besides, there are many important parameters such as precision, power consumption, noise, insulation strength, and bus communication function that are related to the product performance. For example, noise is related to precision, and power consumption energy is related to reliability. These require users to make careful selections. In conclusion, applicability, reliability, and product cost performance are the main principles for choosing an isolator.
The signals from the field two-wire pressure transmitters received by DCS are unstable. How to solve this problem?
Two-wire transmitters are frequently used in the field of industrial automation. Similar to other industrial field devices, two-wire transmitters also have issues of being susceptible to interference and resisting interference. According to the interface mode of the analog board of DCS, different types of isolation transformers with different functions should be selected. In principle, it is required that they can not only provide isolation power for the transmitters to ensure that each transmitter has an independent power supply, but also isolate and transmit the signals of the transmitters to the DCS.
The interface of the PLC simulation board adopts a two-wire loop power supply method and requires signal isolation. How to select the products?
The two-wire loop power supply method is a common interface for analog boards. Products compatible with this interface are called two-wire loop isolation series products. The isolation devices inside these isolation transmitters all adopt transformer methods. On the one hand, they transmit signals; on the other hand, they also transfer the electrical energy from the power supply end to the input part, enabling various circuits in the input part to work normally. For example, products such as ph2217 and other interface devices.
How many types of two-wire circuit breakers are commonly used?
There are two series of products available for the commonly used two-wire isolation transmitters. The common feature of these two series is that they can both provide an independent isolated power supply for the two-wire transmitters through an external power supply after isolation: the power distribution for the transmitters depends on the PLC/DCS analog quantity configured at the site.
Application of Safety Barrier
The Safety Barrier was originally an auxiliary unit of the electrically combined instrument unit. Its main function was to serve as an isolation device for the explosion-proof system. The Safety Barrier isolated the signals from the hazardous zone, transformed them, and output the isolated current signals to the safe zone. It was applied in the intrinsically safe explosion-proof system, where the energy sent to the field circuit was limited by current-limiting and voltage-limiting circuits, thereby preventing the dangerous energy from non-intrinsically safe circuits from entering the intrinsically safe circuit. The Safety Barrier is widely used in automation systems such as DCS/PLC/PCS in industries like petrochemicals.
Common safety barriers are classified by their structural forms into zener type and isolation type.
Application of Zener Diode Safety Barrier:
In the circuit, fast fuses, current-limiting resistors or voltage-limiting diodes are adopted to limit the input electrical energy, thereby ensuring that the energy output to the hazardous area is within the limit. However, due to the inherent flaws in the principle, its reliability in application has been compromised, and its application scope has been restricted. The reasons are as follows:
The installation site should have a very reliable grounding system. The grounding resistance of the zener safety barrier must be less than 1 ohm; otherwise, it will lose its protective safety function. Obviously, such a requirement is extremely strict and difficult to guarantee in engineering applications.
2. It is required that the field instruments in the hazardous area must be of isolated type. Otherwise, when the grounding terminal of the zener safety barrier is connected to the earth, the signal cannot be transmitted correctly. Moreover, due to the signal grounding, the anti-interference ability of the signal is directly reduced, which affects the stability of the system.
3. The Zener safety barrier has a significant impact on the power supply and is also prone to damage due to fluctuations in the power supply.
Application of Isolated Safety Barrier:
Adopt a circuit structure that electrically isolates the input, output and power supply from each other, and at the same time meets the requirements of explosion-proof type for limiting energy.
By adopting the three-party isolation method, there is no need for grounding lines of the system, which brings great convenience to the design and on-site construction.
2. For the on-site instruments in the hazardous area, isolated instruments are not necessary to be adopted.
3. The signal lines do not need to be grounded in common, which significantly enhances the stability and anti-interference capability of the detection and control circuit signals, thereby improving the reliability of the entire system.
4. Isolated safety barriers possess stronger input signal processing capabilities and can accept and process signals such as thermocouples, thermoresistors, and frequencies, which is something that zener diode safety barriers cannot achieve.
5. The isolated safety barrier can output two mutually isolated signals, which can be provided to two devices using the same signal source. This ensures that the signals of the two devices do not interfere with each other and simultaneously enhances the electrical safety insulation performance between the connected devices.
Therefore, after comparing the characteristics and performances of Zener-type and isolation-type safety barriers, it can be seen that isolation-type safety barriers have more prominent advantages and are more widely used. In engineering sites with higher requirements, isolation-type safety barriers are almost invariably adopted as the main intrinsically safe explosion-proof instruments, and they have gradually replaced Zener-type safety barriers.