In modern industrial production equipment, there are numerous digital and analog control devices, such as motor start/stop, solenoid valve opening/closing, product counting, and temperature, pressure, and flow setting and control. For these automatic control problems in industrial settings, programmable logic controllers (PLCs) have become one of the most effective tools for solving them. The following points should be considered when designing a PLC control system: I. Selection of Programmable Logic Controllers (PLCs) and Programmers : Currently, there are many PLC products on the market. Besides domestic brands, foreign brands include: OMRON, MITSUBISHI, FUJI, IDEC, HITACHI, and Panasonic from Japan; Siemens from Germany; and LG from South Korea. How to select a PLC product? 1. The system should first determine whether to use a standalone PLC control or a network of PLCs. Based on this, calculate the number of input/output (I/O) points, and when selecting a PLC, reserve a 10% margin based on the actual required number of points. 2. Determine the load type: Based on whether the load connected to the PLC output is DC or AC, whether it has a high or low current, and the frequency of the PLC output point operation, determine whether to use relay output, transistor output, or thyristor output. Choosing different output methods for different loads is crucial for stable system operation. 3. Storage capacity and instruction execution speed are important indicators for PLC selection. Generally, the larger the storage capacity and the faster the speed, the higher the price. Although products from different foreign manufacturers are largely similar, there are still some differences. 4. Selection of "COM" points: Different PLC products have different numbers of "COM" points. Some have 8 output points per "COM" point, some have 4, and some have 1 or 2. When there are many types of loads with high current, use a product with 1-2 output points per "COM" point; when there are few types of loads but a large number of them, use a product with 4-8 output points per "COM" point. 5. Because different manufacturers use different development software, system compatibility is a key consideration when purchasing. Currently, no fully compatible products have been found; therefore, PLC products should be selected based on the specific system requirements. 6. Programmer Selection: PLC programming can be done in three ways: First, using a handheld programmer, which can only program using statements from the manufacturer's specified statement list. This method is easy for on-site debugging and is small and low-cost, but it is inefficient and adaptable to fewer machine types, making it more suitable for systems with small capacity and low usage. Second, using a graphical programmer. This method uses a graphical interface, which is convenient and intuitive, and can generally be mastered by electrical personnel in a short time, but the programmer is more expensive. Third, using IBM or compatible personal computers + PLC software packages. This is the most efficient and commonly used method, but most software packages are expensive. 7. It is advisable to choose products from large companies, as their product quality is guaranteed, technical support is better, and after-sales service is generally better, which is beneficial for future product expansion and software/hardware upgrades. II. Input and Output Circuit Design 1. Power Supply Circuit: PLCs are generally powered by AC85-240V (some also use DC24V), offering a wide power range. However, to prevent interference, power purification components (such as power filters, 1:1 isolation transformers, etc.) should be added. 2. Use of DC24V Power Supply on the PLC: Most PLC products from various companies have a DC24V power supply. However, this power supply has a small capacity, ranging from tens to hundreds of milliamps. When using it to power a load, attention should be paid to its capacity, and short-circuit protection measures should be taken (because overload or short circuit of this power supply will affect the operation of the PLC). 3. External DC24V Power Supply: If the input circuit has proximity switches, photoelectric switches, etc., powered by DC24V, and the DC24V power supply capacity on the PLC is insufficient, an external DC24V power supply must be provided. However, the "one" terminal of this external power supply should not be connected to the "one" or "COM" terminal of the PLC's DC24V power supply; otherwise, it will affect the operation of the PLC. 4. Input Sensitivity: Each PLC manufacturer specifies the input voltage and current. When the input current of the input element exceeds the PLC's maximum input current or there is leakage current, malfunctions will occur, reducing sensitivity. Therefore, weak current input should be used, leakage current protection measures should be taken, and a PLC with a leakage-type input should be selected. For two-wire sensors (photoelectric switches, contactless switches) with LED limit switches, input leakage current will cause incorrect input or the light to illuminate. The countermeasure is to connect a bleeder resistor to reduce the input impedance. The resistance value is derived from the formula in Figure 1. When transistors or bidirectional thyristors output, if connected to a device with a large inrush current, protection for the transistors and thyristors must be considered. Transistors and thyristors can withstand inrush currents up to 10 times their rated current. 5. Handling Inductive Loads: When connecting inductive loads to the input and output terminals, an impulse suppressor or diode should be connected in parallel across the load. The cathode of the diode should be connected to the voltage (10) side. 6. External Interlocking and Grounding: When using a PLC to control the forward and reverse rotation of a motor, an external interlocking circuit should be established to prevent accidents and mechanical damage caused by abnormal PC operation. Grounding: The GR terminal is the earth grounding terminal. Use a dedicated grounding wire (cross-sectional area of ​​2mm² or more) to prevent induced current, employing the third grounding method (grounding resistance below 100Ω). LG is the neutral terminal of the noise filter. If malfunction occurs due to high noise, or to prevent electric shock, short-circuit LG and GR, using the third grounding method. The length of the grounding wire should preferably be within 20m. Sharing the grounding wire with other equipment or connecting it to the metal structure of a building will have the opposite effect and cause adverse effects. 7. PLC External Drive Circuit: When the PLC output cannot directly drive the load, an external drive circuit must be used. Solid-state relays or thyristor circuits can be used for driving, and protection circuits and surge absorption circuits should be used. III. Selection of Expansion Modules: For small systems, such as systems with fewer than 80 points, expansion is generally not required; when the system is larger, expansion is necessary. Different companies have limitations on the total number of system points and expansion modules. When expansion still cannot meet the needs, a network structure can be used. Also, some manufacturers' products do not support expansion modules for certain instructions; therefore, this must be considered during software programming. When using simulation modules such as temperature sensors, each manufacturer has its own regulations; please refer to the relevant technical manuals. IV. PLC Network Design Designing a network using a PLC is much more difficult than single-unit PLC control. First, choose a PLC model you are familiar with, have a thorough understanding of its basic and function instructions, and carefully understand the instruction execution speed and user program storage capacity. Otherwise, it will not meet real-time requirements and may cause system crashes. Additionally, communication interfaces, communication protocols, and data transmission speeds must be considered. Finally, seek network design and software support, as well as detailed technical information, from the PLC manufacturer. The number of workstation layers to use depends on the system size. V. Software Development Before developing the software, you should first familiarize yourself with the PLC product manual. Programming should only begin after you are familiar with it. If using a graphical programmer or software package, programming can be done directly. If using a handheld programmer, the ladder diagram should be drawn first, and then programming should be performed. This can reduce errors and speed up the process. After programming, run the machine without load first. After all actions are normal, then debug it on the device.