I. Overview Over the years, Programmable Logic Controllers (PLCs) have achieved a leap from wiring logic to stored logic; their functions have evolved from weak to strong, realizing the progress from logic control to digital control; their application areas have expanded from small to large, achieving a leap from simple control of single devices to being capable of various tasks such as motion control, process control, and distributed control. Today's PLCs have greatly improved their capabilities in handling analog signals, digital calculations, human-machine interfaces, and networks, becoming the mainstream control equipment in the industrial control field and playing an increasingly important role in various industries. [b]II. Application Areas of PLCs[/b] Currently, PLCs are widely used in various industries at home and abroad, including steel, petroleum, chemical, electric power, building materials, machinery manufacturing, automobile, light textile, transportation, environmental protection, and cultural entertainment. The usage can be mainly divided into the following categories: 1. Switching logic control replaces traditional relay circuits to realize logic control and sequential control. It can be used for the control of single devices, as well as for multi-machine group control and automated production lines. Examples of applications include injection molding machines, printing presses, staplers, combination machine tools, grinding machines, packaging production lines, and electroplating production lines. 2. Industrial Process Control: In industrial production processes, there are continuously changing quantities (i.e., analog quantities) such as temperature, pressure, flow rate, liquid level, and speed. PLCs use corresponding A/D and D/A conversion modules and various control algorithms to process these analog quantities and complete closed-loop control. PID control is a commonly used method in general closed-loop control systems. Process control has wide applications in metallurgy, chemical engineering, heat treatment, and boiler control. 3. Motion Control: PLCs can be used for controlling circular or linear motion. Dedicated motion control modules are generally used, such as single-axis or multi-axis position control modules that can drive stepper motors or servo motors, widely used in various machinery, machine tools, robots, elevators, etc. 4. Data Processing: PLCs have functions such as mathematical operations (including matrix operations, function operations, and logical operations), data transmission, data conversion, sorting, table lookup, and bit manipulation, enabling them to complete data acquisition, analysis, and processing. Data processing is generally used in large-scale control systems in industries such as papermaking, metallurgy, and food processing. 5. Communication and Networking PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of factory automation networks, modern PLCs all have communication interfaces, making communication very convenient. III. Application Characteristics of PLCs 1. High Reliability and Strong Anti-interference Capability High reliability is a key performance characteristic of electrical control equipment. PLCs, due to their use of modern large-scale integrated circuit technology and strict manufacturing processes, employ advanced anti-interference technology in their internal circuits, resulting in high reliability. Compared to relay contactor systems of the same scale, using PLCs to construct control systems reduces electrical wiring and switch contacts to hundreds or even thousands of times less, significantly reducing the risk of failure. Furthermore, PLCs have hardware fault self-detection functions, promptly issuing alarm information when a fault occurs. In application software, users can also program fault self-diagnosis programs for peripheral devices, enabling circuits and equipment other than the PLC in the system to also receive fault self-diagnosis protection. This results in extremely high reliability for the entire system. 2. With complete supporting components, comprehensive functions, and strong applicability, PLCs have evolved into a series of products of various sizes, suitable for industrial control applications of all scales. Besides logic processing functions, most PLCs possess robust data processing capabilities, making them applicable to various digital control fields. The proliferation of diverse functional units has allowed PLCs to penetrate various industrial control applications such as position control, temperature control, and CNC. Furthermore, the enhanced communication capabilities of PLCs and the development of human-machine interface technology have made it very easy to build various control systems using PLCs. 3. Easy to learn and use, highly popular among engineering technicians. PLCs are industrial control equipment designed for industrial and mining enterprises. Their interfaces are easy to use, and their programming languages ​​are readily accepted by engineering technicians. The graphical symbols and expressions of ladder diagram language are quite similar to relay circuit diagrams, opening a convenient door for those unfamiliar with electronic circuits, computer principles, and assembly language to engage in industrial control. 4. The system design requires minimal workload, is easy to maintain, and is readily modifiable. PLCs use stored logic instead of wired logic, significantly reducing external wiring for control equipment, greatly shortening the design and construction cycle of the control system, and simplifying daily maintenance. More importantly, it makes it possible to change the production process by modifying the same equipment's program. This is particularly suitable for multi-variety, small-batch production. [b]IV. Issues to Note in PLC Applications[/b] A PLC is a device used for automated control in industrial production. Generally, it can be used directly in industrial environments without any special measures. However, despite its high reliability and strong anti-interference capabilities as mentioned above, when the production environment is too harsh, electromagnetic interference is particularly strong, or improper installation and use may cause program errors or calculation errors, resulting in incorrect inputs and outputs. This will cause equipment malfunctions and misoperations, thus failing to guarantee the normal operation of the PLC. To improve the reliability of the PLC control system, on the one hand, PLC manufacturers need to improve the anti-interference capabilities of the equipment; on the other hand, high attention must be paid to design, installation, and maintenance. Multi-party cooperation is necessary to effectively solve problems and enhance the system's anti-interference performance. Therefore, the following issues should be noted during use: 1. Working Environment (1) Temperature The PLC requires an ambient temperature of 0-55°C. When installing, it should not be placed under components that generate a lot of heat. The surrounding space for ventilation and heat dissipation should be large enough. (2) Humidity In order to ensure the insulation performance of the PLC, the relative humidity of the air should be less than 85% (no condensation). (3) Vibration The PLC should be kept away from strong vibration sources to prevent frequent or continuous vibration with a vibration frequency of 10-55Hz. When vibration is unavoidable in the operating environment, vibration reduction measures must be taken, such as using vibration damping rubber. (4) Air Avoid corrosive and flammable gases, such as hydrogen chloride and hydrogen sulfide. For environments with a lot of dust or corrosive gases in the air, the PLC can be installed in a well-sealed control room or control cabinet. (5) Power Supply The PLC has a certain ability to resist interference from power lines. In environments with high reliability requirements or particularly serious power interference, an isolation transformer with a shielding layer can be installed to reduce interference between the equipment and the ground. Generally, PLCs have a DC 24V output to provide input terminals. When an external DC power supply is used at the input terminal, a DC regulated power supply should be selected. Because ordinary rectifier and filter power supplies are prone to receiving incorrect information from PLC due to the influence of ripple. 2. Interference and its sources in the control system. Field electromagnetic interference is one of the most common and most likely factors to affect the reliability of PLC control systems. The so-called treatment of symptoms should start with addressing the root cause. Only by finding the problem can a solution be proposed. Therefore, it is necessary to know the source of field interference. (1) Interference sources and general classification. Most interference sources affecting PLC control systems are generated in parts where current or voltage changes drastically. The reason is that the change in current generates a magnetic field, which generates electromagnetic radiation on the equipment; the change in magnetic field generates current, and electromagnetic high speed generates electromagnetic waves. Electromagnetic interference is usually divided into common-mode interference and differential-mode interference according to different interference modes. Common-mode interference is the potential difference between the signal and the ground, mainly formed by the superposition of common-mode (same direction) voltage induced on the signal line by the grid, the ground potential difference, and the electromagnetic radiation in space. Common-mode voltage can be converted into differential-mode voltage through asymmetrical circuits, which directly affects the measurement and control signal and causes damage to components (this is the main reason why some system I/O modules have a high failure rate). This common-mode interference can be DC or AC. Differential-mode interference refers to the interference voltage acting between the two poles of a signal. It is mainly caused by the coupling induction of spatial electromagnetic fields between signals and the voltage formed by the conversion of common-mode interference by unbalanced circuits. This interference is superimposed on the signal and directly affects the measurement and control accuracy. (2) Main sources and pathways of interference in PLC systems: Strong electrical interference: The normal power supply of PLC systems is supplied by the power grid. Due to the wide coverage of the power grid, it will be affected by all spatial electromagnetic interference and induce voltage on the lines. In particular, changes within the power grid, such as surges from knife switch operation, start-up and shutdown of large power equipment, harmonics caused by AC and DC transmission devices, and transient impacts from power grid short circuits, are all transmitted to the primary side of the power supply through the transmission lines. Cabinet interference: High-voltage electrical appliances, large inductive loads, and messy wiring in the control cabinet can easily cause a certain degree of interference to the PLC. Interference introduced from signal lines: In addition to transmitting various types of effective information, various signal transmission lines connected to the PLC control system will always have external interference signals intruding. This interference mainly occurs through two pathways: first, interference from the power grid introduced through the transmitter's power supply or the power supply of shared signal instruments, which is often overlooked; second, interference induced by spatial electromagnetic radiation on the signal lines, i.e., external induced interference on the signal lines, which is very serious. Interference introduced by signals can cause abnormal operation of I/O signals and a significant reduction in measurement accuracy, and in severe cases, can damage components. Interference from a disordered grounding system: Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Correct grounding can suppress the effects of electromagnetic interference and also suppress the interference emitted by the equipment; while incorrect grounding will introduce serious interference signals, making the PLC system unable to work properly. Interference from within the PLC system is mainly generated by mutual electromagnetic radiation between internal components and circuits, such as mutual radiation between logic circuits and its impact on analog circuits, the mutual influence between analog ground and logic ground, and the mismatch between components. Inverter interference: first, the harmonics generated during inverter startup and operation cause conducted interference to the power grid, causing voltage distortion and affecting the power supply quality; second, the inverter output generates strong electromagnetic radiation interference, affecting the normal operation of surrounding equipment. 3. Main anti-interference measures (1) Reasonable handling of power supply to suppress interference introduced by the power grid. For power grid interference introduced by the power supply, an isolation transformer with a shielded layer and a transformation ratio of 1:1 can be installed to reduce the interference between the equipment and the ground. An LC filter circuit can also be connected in series at the power input terminal. As shown in Figure 1 (2) Installation and wiring ● Power lines, control lines, and power lines and I/O lines of PLC should be wired separately. The isolation transformer should be connected to PLC and I/O with double-insulated wire. The IO lines and high-power lines of PLC should be run separately. If they must be in the same cable tray, AC lines and DC lines should be bundled separately. If conditions permit, it is best to run them in separate trays. This will not only give them the largest possible space distance, but also minimize interference. ● PLC should be kept away from strong interference sources such as welding machines, high-power silicon rectifiers and large power equipment. It should not be installed in the same switch cabinet as high-voltage electrical appliances. In the cabinet, PLC should be kept away from power lines (the distance between the two should be greater than 200mm). Inductive loads installed in the same cabinet as the PLC, such as the coils of high-power relays and contactors, should be connected in parallel with RC arc suppression circuits. ● The input and output of the PLC should preferably be wired separately, and the digital and analog signals should also be laid separately. The transmission of analog signals should use shielded wires, and the shield should be grounded at one or both ends. The grounding resistance should be less than 1/10 of the shield resistance. ● AC output lines and DC output lines should not use the same cable. Output lines should be kept as far away as possible from high-voltage lines and power lines, and parallel connections should be avoided. (3) I/O terminal wiring Input wiring ● Input wiring should generally not be too long. However, if the environmental interference is small and the voltage drop is not large, the input wiring can be appropriately longer. ● Input/output lines should not use the same cable and should be separated. ● Normally open contacts should be used to connect to the input terminals as much as possible so that the ladder diagram is consistent with the relay schematic diagram and easy to read. Output connection ● Output terminal wiring is divided into independent output and common output. Different types and voltage levels of output voltage can be used in different groups. However, the outputs in the same group can only use the same type and voltage level of power supply. ● Since the output components of the PLC are encapsulated on the printed circuit board and connected to the terminal block, if the load connected to the output component is short-circuited, the printed circuit board will be burned. ● When using relay output, the size of the inductive load will affect the service life of the relay. Therefore, when using inductive load, it should be selected reasonably or an isolation relay should be added. ● The output load of the PLC may generate interference, so measures should be taken to control it, such as the freewheeling tube protection of DC output, the RC snubber circuit of AC output, and the bypass resistor protection of transistor and bidirectional thyristor output. (4) Correctly select the grounding point and improve the grounding system. Good grounding is an important condition for ensuring the reliable operation of the PLC and can avoid the damage caused by accidental voltage surges. The purpose of grounding is usually twofold: one is for safety, and the other is to suppress interference. A complete grounding system is one of the important measures for the PLC control system to resist electromagnetic interference. The ground wire of the PLC control system includes system ground, shield ground, AC ground and protective ground. The interference of a chaotic grounding system on a PLC system is mainly due to uneven potential distribution at various grounding points. Potential differences exist between different grounding points, causing ground loop currents and affecting normal system operation. For example, cable shielding must be grounded at one point. If both ends (A and B) of the cable shielding are grounded, a ground potential difference exists, and current flows through the shielding. In abnormal conditions such as lightning strikes, the ground current will be even greater. Furthermore, the shielding, grounding wire, and earth may form a closed loop. Under the influence of a changing magnetic field, induced currents will appear within the shielding, interfering with the signal loop through the coupling between the shielding and the core wire. If the system grounding is chaotic and inconsistent with other grounding methods, the resulting ground loop currents may create unequal potential distributions on the ground wires, affecting the normal operation of the logic and analog circuits within the PLC. PLCs have low logic voltage interference tolerance; interference from logic ground potential distribution can easily affect PLC logic operations and data storage, causing data corruption, program crashes, or system freezes. Analog ground potential distribution will lead to decreased measurement accuracy, causing serious distortion and malfunctions in signal measurement and control. ● Safety Grounding or Power Grounding: Connecting the power cord grounding terminal to the cabinet grounding point constitutes safety grounding. If the power supply leaks or the cabinet becomes energized, the current can be conducted to the ground through the safety grounding, preventing harm to personnel. ● System Grounding: The PLC controller is grounded to ensure it is at the same potential as all controlled devices. This is called system grounding. The grounding resistance value must not exceed 4Ω. Generally, the PLC equipment system ground and the negative terminal of the switching power supply inside the control cabinet should be connected together as the control system ground. ● Signal and Shielding Grounding: Signal lines must have a unique reference ground. Shielded cables, when encountering situations where conducted interference may occur, should also be uniquely grounded locally or in the control room to prevent the formation of "ground loops." When the signal source is grounded, the shielding layer should be grounded on the signal side; if not grounded, it should be grounded on the PLC side. When there are joints in the signal line, the shielding layer should be securely connected and insulated, avoiding multiple grounding points. When connecting shielded twisted-pair cables for multiple measurement points to a multi-core twisted-pair shielded cable, each shielding layer should be interconnected and insulated, with a single-point grounding point selected. (5) Suppression of Inverter Interference Inverter interference can generally be handled in the following ways: Adding an isolation transformer, mainly targeting conducted interference from the power supply, can block most of the conducted interference before the isolation transformer. Using filters, which have strong anti-interference capabilities and also prevent the equipment's own interference from being conducted to the power supply; some also have peak voltage absorption functions. Using output reactors, adding an AC reactor between the inverter and the motor mainly reduces electromagnetic radiation generated by the inverter output during energy transmission, affecting the normal operation of other equipment. V. Conclusion Interference in PLC control systems is a very complex problem. Therefore, in anti-interference design, various factors should be comprehensively considered to reasonably and effectively suppress interference, so that the PLC control system can operate normally. With the continuous expansion of PLC application fields, how to use PLC efficiently and reliably has also become an important factor in its development. In the 21st century, PLCs will see greater development, with a wider variety of products and more complete specifications. Through perfect human-machine interfaces and comprehensive communication equipment, they will better adapt to the needs of various industrial control applications. As an important component of automation control networks and internationally used networks, PLCs will play an increasingly important role in the field of industrial control.