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; and their application areas have expanded from small to large, achieving a leap from simple control of single devices to being capable of handling 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, automobiles, light textiles, transportation, environmental protection, and cultural entertainment. Their usage can be mainly divided into the following categories: 1. Switching logic control replaces traditional relay circuits, realizing 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 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. These are widely used in various machinery, machine tools, robots, elevators, and other applications. 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. They can 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. [b]III. Application Characteristics of PLC[/b] 1. High Reliability and Strong Anti-interference Ability 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 with relay contactor systems of the same scale, the electrical wiring and switch contacts in a control system constructed using PLCs are reduced to hundreds or even thousands of times less, significantly reducing the risk of failure. In addition, PLCs have a hardware fault self-detection function, which can promptly issue alarm information when a fault occurs. In the 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. In this way, the entire system will have extremely high reliability. 2. Complete Support, 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, 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, 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 representations 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 is simple, easy to maintain, and easy to modify. PLC uses stored logic instead of wiring logic, which greatly reduces the external wiring of the control equipment, shortens the cycle of control system design and construction, and makes daily maintenance easier. More importantly, it makes it possible to change the production process by changing the program of the same equipment. This is especially suitable for multi-variety, small-batch production. (2) Installation and wiring ● Power lines, control lines, and PLC power lines and I/O lines should be wired separately. The isolation transformer should be connected to the PLC and I/O with double-insulated wire. The PLC IO lines and high-power lines 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. Inside 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, outputs within the same group can only use power supplies of the same type and voltage level. ● Since the PLC's output components are packaged on a printed circuit board and connected to a terminal block, short-circuiting the load connected to the output component will burn out the printed circuit board. ● When using relay outputs, the size of the inductive load will affect the relay's lifespan. Therefore, inductive loads should be selected appropriately or an isolation relay should be added. ● PLC output loads may generate interference, so measures must be taken to control it, such as freewheeling diode protection for DC outputs, RC snubber circuits for AC outputs, and bypass resistor protection for transistor and bidirectional thyristor outputs. [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 the high reliability and strong anti-interference ability mentioned above, when the production environment is too harsh, the electromagnetic interference is particularly strong, or the installation and use are improper, program errors or calculation errors may occur, resulting in erroneous inputs and outputs. This will cause the equipment to malfunction and operate incorrectly, 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 are required to improve the anti-interference ability of the equipment; on the other hand, high attention should be paid to the design, installation and use maintenance, and multi-party cooperation is needed to solve the problem and effectively enhance the anti-interference performance of the system. 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 with high heat generation, and the surrounding ventilation and heat dissipation space 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 use environment, vibration reduction measures must be taken, such as using vibration damping rubber. (4) Avoid air containing corrosive and flammable gases, such as hydrogen chloride and hydrogen sulfide. In environments with a lot of dust or corrosive gases, 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 severe power interference, an isolation transformer with a shielding layer can be installed to reduce interference between the equipment and ground. Generally, PLCs have a DC 24V output to provide to the input terminal. When the input terminal uses an external DC power supply, a DC regulated power supply should be selected. Because ordinary rectifier and filter power supplies are prone to causing the PLC to receive incorrect information due to the influence of ripple. 2. Interference in the control system and its sources: Field electromagnetic interference is one of the most common and most likely factors to affect the reliability of the PLC control system. The so-called treatment of symptoms should start with treating 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 classifications Interference sources affecting PLC control systems are mostly 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 spatial electromagnetic radiation. Common-mode voltage can be converted into differential-mode voltage through asymmetrical circuits, which directly affects the measurement and control signals 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 the signal, mainly formed by the coupling induction of spatial electromagnetic field 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 paths of interference in PLC systems Strong electrical interference The normal power supply of PLC systems is supplied by the grid. Due to the wide coverage of the power grid, it is susceptible to electromagnetic interference from all directions, resulting in induced voltage on the lines. In particular, changes within the power grid, such as surges from knife switch operations, starting and stopping of large power equipment, harmonics from AC/DC drives, and transient impacts from power grid short circuits, are all transmitted to the primary side of the power source through transmission lines. In-cabinet interference: High-voltage electrical appliances, large inductive loads, and messy wiring within the control cabinet can easily cause interference to the PLC. Interference introduced from signal lines: Various signal transmission lines connected to the PLC control system, besides transmitting valid information, will always be subject to external interference signals. This interference mainly occurs through two pathways: one is grid interference introduced through the power supply of transmitters or shared signal instruments, which is often overlooked; the other is 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 I/O signal operation and a significant reduction in measurement accuracy, and in severe cases, damage to components. Interference from a messy grounding system: Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Proper 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 inside the PLC system is mainly generated by mutual electromagnetic radiation between internal components and circuits, such as mutual radiation of logic circuits and their impact on analog circuits, mutual influence between analog ground and logic ground, and mismatch between components. Inverter interference is caused by harmonics generated during inverter startup and operation, which conducts interference to the power grid, causing voltage distortion and affecting the power supply quality of the power grid; secondly, the output of the inverter will generate 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 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. (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. Grounding typically serves two purposes: safety and interference suppression. A robust grounding system is a crucial measure for PLC control systems to resist electromagnetic interference. The grounding wires of a PLC control system include system ground, shield ground, AC ground, and protective ground. A chaotic grounding system primarily interferes with the PLC system due to uneven potential distribution at various grounding points. Potential differences between different grounding points create ground loop currents, affecting normal system operation. For example, a cable shield must be grounded at one point. If both ends (A and B) of the cable shield are grounded, a potential difference exists, causing current to flow through the shield. In abnormal conditions such as lightning strikes, the ground current will be even greater. Furthermore, the shield, grounding wire, and earth may form a closed loop. Under the influence of a changing magnetic field, induced currents will appear within the shield, interfering with the signal loop through the coupling between the shield and the core wire. If the system ground and other grounding methods are chaotic, 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 tolerance for logic voltage interference. Distributed interference from the logic ground potential can easily affect PLC logic operations and data storage, causing data corruption, program crashes, or system freezes. Distributed analog ground potential will lead to decreased measurement accuracy, causing serious distortion and malfunctions in signal measurement and control. ● Safety Ground or Power Ground: Connecting the power line grounding terminal to the cabinet grounds the PLC for safety grounding. If the power supply leaks or the cabinet becomes energized, the current can be conducted to the ground through the safety ground, preventing harm to people. ● 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 in 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 must also be uniquely grounded locally or in the control room in situations where conducted interference is possible 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 is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multiple grounding points must be avoided. When the shielded twisted-pair cable of multiple measurement point signals is connected to the multi-core twisted-pair shielded cable, each shielding layer should be connected to each other and insulated, and a suitable single-point grounding point should be selected. 5) Suppression of inverter interference Inverter interference is generally handled in the following ways: Adding an isolation transformer, mainly to deal with conducted interference from the power supply, can block most of the conducted interference before the isolation transformer. Using filters, filters have strong anti-interference capabilities and can also prevent the interference of the equipment itself from being conducted to the power supply. Some also have peak voltage absorption functions. Using output reactors, adding AC reactors between the inverter and the motor mainly reduces the electromagnetic radiation generated by the inverter output line during energy transmission, which affects the normal operation of other equipment. [b]V. Conclusion[/b] Interference in PLC control systems is a highly complex issue. Therefore, anti-interference design should comprehensively consider various factors and effectively suppress interference to ensure the normal operation of the PLC control system. With the continuous expansion of PLC application areas, how to use PLC efficiently and reliably has become an important factor in its development. In the 21st century, PLCs will see even greater development, with a richer 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.