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Source: Changhui Instruments http://yunrun.com.cn/ With the development of science and technology, PLCs are being used more and more widely in industrial control. The reliability of PLC control systems has a direct impact on the safe production and economic operation of industrial enterprises, and the system’s ability to resist interference is key to its reliable operation. The various types of PLCs used in automation systems are either installed centrally in control rooms or in the production area as well as on various motorized devices; most of them operate in a harsh electromagnetic environment created by high-voltage circuits and high-voltage equipment. To improve the reliability of PLC control systems, designers must be aware in advance of various types of interference in order to ensure the system operates reliably. Electromagnetic interference sources and their impact on systems: The disturbances that affect PLC control systems stem from the same sources of interference that affect industrial control equipment. These disturbances generally occur in areas where there are sharp changes in current or voltage; it is in these areas of rapid charge movement that noise sources, or in other words, interference sources, exist. Interference types are usually classified based on the cause of the interference, the pattern of the noise interference, and the waveform characteristics of the noise. Among them: based on the different causes of noise, it is divided into discharge noise, sporadic noise, etc.; based on the different patterns of sound interference, it is divided into common-mode interference and differential-mode interference – this is a relatively common method of classification. Common-mode interference is the potential difference between the signal and ground, primarily resulting from interference from the power grid, ground potential differences, and common-mode voltages induced on the signal lines by spatial electromagnetic radiation. The common-mode voltage can sometimes be quite high, especially in rooms powered by electrical equipment with poor isolation performance; the common-mode voltage of the transmitter’s output signal is generally high, with some values reaching over 130V. The common-mode voltage can be converted into another common-mode voltage through asymmetric circuits, which directly affects the measurement and control signals and can cause damage to components. This kind of common-mode interference can be either DC or AC in nature. Common-mode interference refers to the interference voltage that exists between two levels of a signal; it is primarily caused by the coupling and induction of spatial electromagnetic fields between the signals, as well as by voltages generated by unbalanced circuits. This interference is added directly to the signals, affecting the accuracy of measurement and control. What are the main sources of electromagnetic interference in PLC control systems? 1. Radiative interference from space: The radiative electromagnetic fields in space are primarily generated by power grids, transient processes in electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc.; this type of interference is commonly referred to as radiative interference. Its distribution is extremely complex; if a PLC system is placed within a certain frequency field, it will experience radiative interference. This interference affects the system in two main ways: first, through radiation generated within the PLC itself, where interference arises from electrical circuit interactions; and second, through radiation from the PLC’s communication network, with interference being introduced due to interactions along the communication lines. Radiation interference is related to the magnitude of the electromagnetic fields generated by on-site equipment, especially the frequency; protection is generally achieved by using shielded cables, local PLC shielding, and high-voltage discharge components. 2. Interference from external leads of the system is mainly introduced through power and signal lines, and is commonly referred to as conductive interference. This kind of interference is severe in industrial sites in our country. 3. Interference from the power supply: Experience has shown that many control system failures are caused by interference introduced by the power supply; this was encountered during engineering adjustments, and later a PLC power supply with better isolation capabilities was used. Only then can the problem be solved. The normal power supply for PLC systems comes from the electrical grid. Due to the wide coverage of the grid, it is subject to various electromagnetic interferences, which generate induced voltages and currents along the lines. In particular, changes within the grid itself, switching operations that cause surges, the start-up and shutdown of large electrical equipment, harmonics generated by AC/DC rotating devices, and transient impacts resulting from grid short circuits—all of these can affect the PLC power supply connected to the grid. Isolated power supplies are typically used for such purposes, but factors related to their structure and manufacturing process result in less than ideal isolation performance. In fact, absolute isolation chambers are impossible due to the presence of distributed parameters, especially distributed capacitance. 4. Interference introduced due to a chaotic grounding system. The various signal transmission lines connected to the PLC control system not only transmit valid signals, but are also subject to external interference. While it is possible to suppress interference emitted by the equipment itself, an incorrect grounding method can introduce severe interference signals, preventing the PLC control system from functioning properly. The ground wires in a PLC control system include the system ground, shield ground, AC ground, and protection ground, etc. Interference to the PLC system caused by a chaotic grounding system arises mainly from uneven distribution of the various grounding points; voltage differences exist between these different grounding points, which lead to ground loop currents and affect the proper operation of the system. For example, the cable shielding layer must be grounded at one point only; if both ends of the cable shielding layer are grounded, a voltage difference will exist, and current will flow as a result of this voltage difference. In the event of abnormal conditions such as lightning strikes, the current in the ground wires will be even greater. Furthermore, the shielding layer, ground wire, and earth may form a closed circuit; under the influence of a changing magnetic field, induced currents can appear within the shielding layer, and through the coupling between the shielding layer and the core wire, these currents interfere with the signal circuit. If it becomes systematically confused with other grounding treatments, the resulting ground loop currents may cause uneven potential distributions along the ground wires, affecting the proper operation of the logical and analog circuits within the PLC. PLCs have a low tolerance to logical voltage interference; variations in the logical ground potential can easily affect their logical operations and data storage, leading to data corruption, abnormal program execution, or system crashes. Variations in the analog ground potential result in reduced measurement accuracy, causing serious distortions and incorrect readings in signal monitoring and control. 5. Interference introduced from signal lines: Various signal transmission lines connected to PLC control systems, in addition to transmitting the intended signals, are always subject to the intrusion of external interference signals. There are mainly two ways such interference occurs: one is grid interference that enters through the power supply of transmitters or shared signal instruments. This is often overlooked; secondly, the signal lines are disturbed by spatial electromagnetic radiation, that is, external induced interference on the signal lines, and this is quite serious. Interference introduced by signals can cause abnormal operation of the signals and a **reduction in measurement accuracy; in severe cases, it can lead to damage to the components. In systems with poor isolation performance, this also leads to interference between signals, resulting in backflow in the common-ground system bus, which causes changes in logical data, erroneous operations, and system crashes. PLC control systems suffer from quite serious damage to their modules due to interference introduced by signals, which in turn leads to numerous system failures. 6. Interference from within the PLC is primarily caused by the mutual electromagnetic radiation between various components and circuits within the system, such as radiation between logic circuits and its impact on analog circuits, the interaction between analog grounds and logic grounds, as well as improper pairing of components. This relates to the electromagnetic compatibility design carried out by PLC manufacturers within their systems; it is quite complex. As the application department, there is nothing that can be changed in this regard, so there is no need to worry too much about it. However, it is important to choose systems that have a track record of successful applications or have been proven reliable. When a PLC system is interfered with, the following main interference phenomena often occur: 1. The motor rotates irregularly when the system issues commands; 2. The numerical display values fluctuate wildly when the signal is zero; 3. When sensors are in operation, the signals collected by the PLC do not match the values corresponding to the actual parameters, and the error values are random and irregular; 4. Normal operation is not possible when it shares the same power supply as an AC servo system. How can PLC system interference be addressed more effectively and simply? 1. Ideally, it is necessary to use devices with good isolation capabilities, select high-quality power supplies, properly route power and signal cables, and ensure proper grounding of the power supply. However, this requires cooperation among different equipment manufacturers, which is difficult to achieve and comes at a high cost. 2. By using analog signal isolators, which are also known as signal transmitters and fall under the category of signal conditioning, the main purpose is to reduce interference. It is precisely because of its strong anti-interference capability that it is widely used in automated control systems. Especially in complex industrial environments, the control programs become more and more complex as they age. Signal isolators (http://yunrun.com.cn/product/250.html) provide isolation for input, output, and power supply of various analog signals, and they are indeed one of the effective measures against interference in modern automation control systems. Why is a signal isolator the preferred choice for eliminating interference in PLC systems? 1. It is simple and easy to use, reliable, and cost-effective, and can address multiple types of interference at the same time. 2. It can significantly reduce the workload of designers and system testers; even complex systems become highly stable and reliable in the hands of ordinary designers. What is the working principle of a signal isolator? First, the signal received by the PLC is modulated and transformed using semiconductor devices; then it is isolated and converted through optical or magnetic sensors. Subsequently, it is demodulated to revert to the original signal before isolation or to a different signal, while the power supply for the isolated signal is also isolated. Ensure that the transformed signal, power supply, and ground are absolutely independent of each other. There are so many brands of isolators available on the market today, with prices varying widely. How should one make a choice? Isolators are located between two system channels; therefore, when selecting an isolator, it is first necessary to determine its input and output functions. Additionally, the input and output mode of the isolator (voltage type, current type, loop-powered type, etc.) must be compatible with the interface modes of the upstream and downstream channels. There are also many other important parameters that affect the performance of the product, such as accuracy, power consumption, noise level, insulation strength, and bus communication capabilities. For example, noise level is related to accuracy, while power consumption is associated with reliability. These factors require careful consideration by the user when making a selection. In short, trial use, reliability, and cost-performance are the main criteria for choosing an isolator. Working principle: First, the semiconductor device is modulated and transformed, then isolated conversion is carried out using optical or magnetic sensors, followed by demodulation to restore the original signal to its state before isolation. At the same time, the power supply for the signal after isolation is also isolated, ensuring that the power supply for the transformed signal is isolated as well. Ensure that the transformed signal, power supply, and ground are absolutely independent of each other. The function of a signal isolator: ① Protect the lower-level control circuits. ②Reduce the impact of environmental noise on the test circuit. ③Robust protection against interference to the common ground, frequency converters, solenoid valves, as well as the input/output ports and communication interfaces of PLC/DCS systems. ④Standard series guide rail structure, easy to install, and provides effective isolation ； The potentials between the input, output, solenoid valves, and ground can overcome various high- and low-frequency pulsation disturbances in the noise level of the frequency converter. What are the main types of signal isolators? 1. Isolators: In industrial production, they are used to increase the load capacity of instruments, ensure that instruments connected to the same signal do not interfere with each other, and improve the performance of electrical installations. The input signals such as voltage, current, frequency, resistance, etc. need to be collected, amplified, processed, and subjected to anti-interference treatment, before isolated voltage and current signals are output for safe use by secondary instruments and PLC/DCS systems. 2. Power Distributor: In industrial settings, a two-wire transmission system is generally used. It is necessary to provide a 24V power supply for primary instruments such as transmitters, while also collecting, amplifying, processing the incoming current signals, and dealing with interference before outputting isolated current and voltage signals for use by subsequent secondary instruments or other devices. 3. Safety barriers: Some special industrial environments require not only two-wire transmission, but also a power supply for distribution as well as signal isolation. Additionally, they need to have explosion-proof properties against safe sparks, in order to reliably restrict the power of the supply and prevent ignition between the power supply, signals, and ground. Current limiting and voltage reduction are used to control both the signal and power circuits, thereby keeping the energy entering hazardous areas within safe limits. What considerations should be taken into account when installing and maintaining signal isolators? Although different manufacturers use varying production processes and wiring standards for these isolators, the applications in which they are used are generally similar; therefore, the requirements regarding product protection and maintenance remain largely the same. 1. Read the instructions carefully before use. 2. When used for signal isolation, the input terminal should be connected in series into the loop circuit, while the output terminal is connected to the sampling loop. 3. When used as an isolation power distribution unit, the input terminal should be connected to the power supply circuit, while the output terminal should be connected to the transmitter. 4. If it is not working properly, first check whether the wiring is correct, and pay attention to the presence of power supply as well as its polarity. Why do the signal errors received by a PLC sometimes be large and its stability poor? There are many reasons for this phenomenon, with the potential difference between the reference points of different instrument signals being an important factor. Due to this discrepancy, interference currents are generated between the instrument signals, resulting in large errors and poor stability in the PLC. Therefore, it is optimal for the signals from different devices and instruments to have a common reference point representing the best condition. The isolator ensures complete electrical isolation between input and output, while the interface board on the PLC creates a common reference point, thereby resolving the issue of interference. The 4-20mA channels need to be isolated, but there is no space in the cabinet for a power supply. What to do? A passive signal isolator that can isolate 4-20mA signals without the need for an external power supply. YR9101A is one such product. There are so many brands of isolators available on the market today, with prices varying greatly. How should one make a choice? Isolators are located between two system channels; therefore, when selecting an isolator, it is first necessary to determine its input and output functions. The input and output modes of the isolator must also be compatible with those of the front-end and back-end channel interfaces. In addition, there are many other important parameters that affect the performance of the product, such as accuracy, power consumption, noise level, insulation strength, and bus communication capabilities. For example, noise level is related to accuracy, while power consumption is related to reliability. Users need to choose these carefully. In short, suitability, reliability, and cost-performance are the main principles for choosing an isolator. The signals from field two-wire pressure transmitters received by the DCS are unstable – how to resolve this issue? Two-wire transmitters are widely used in the field of industrial automation, and like other industrial field devices, they are also subject to interference and have challenges related to resistance to such interference. Based on the interface mode of the DCS simulation board, isolating power distribution units with different functions are selected; in principle, such units should be able to provide isolated power to the transmitters, ensuring that each transmitter has its own independent power supply, while also transmitting the transmitter signals to the DCS in an isolated manner. The interface of PLC simulation boards uses a two-wire circuit for power supply, and signal isolation is required. How should one choose a product in such cases? The two-wire circuit power supply method is a common type of interface for simulation boards. Products suitable for this interface are known as the two-wire loop isolation series. The isolation components used within such passive signal isolators, which obtain energy from the output side, are all of the transformer type; they transmit signals while also delivering electrical power from the power supply side to the input section, enabling the various circuits in that input section to function properly. Such as YR9201A, etc. How many common two-wire distributors are there? There are two series of common two-wire isolated distributors available for selection. What the two series of products have in common is that they are both capable of providing an isolated power supply to the two-wire transmitters through an external power source after isolation; the power distribution to the transmitters depends on the analog electrical signals provided by the PLC/DCS installed on site.