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. This article describes the issues that should be considered when designing a PLC control system. Hardware Selection: Currently, there are many PLC products on the market. Besides domestic brands, there are foreign brands such as OMRON, MITSUBISHI, FUJJ, and Anasonic from Japan, SIEMENS from Germany, and LG from South Korea. In recent years, the price of PLC products has decreased significantly, and their cost-effectiveness has become increasingly high, which is an important reason why many technicians choose PLCs. So, how to select PLC products? 1. System Scale: First, determine whether the system will use a single PLC for control or a network of PLCs. Calculate the number of PLC input/output points accordingly. When selecting a PLC, a certain margin (10%) should be left 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, high current 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 speed: Although PLC products from various foreign manufacturers are generally similar, there are still some differences. Currently, no products from different companies are fully compatible. The development software of each company is different, and the storage capacity of the user program and the execution speed of the instructions are two important indicators. Generally, the larger the storage capacity and the faster the speed, the higher the price of the PLC. However, the PLC product should be selected reasonably according to the size of the system. 4. Programmer selection: PLC programming can be done in three ways: One is using a general handheld programmer, which can only program using statements in the manufacturer's specified statement list. This method is inefficient, but it is suitable for small systems with low usage, and it is small in size, easy to debug on-site, and has a lower cost. One method is to program using a graphical programmer. This programmer uses ladder diagrams, which is convenient and intuitive, and can be used by general electrical personnel in a short period of time. However, this programmer is relatively expensive. Another method is to program using an IBM personal computer and PLC software package. This method is the most efficient, but most companies' PLC development software packages are expensive, and this method is not easy to debug on-site. Therefore, the PLC product should be selected reasonably according to the size and complexity of the system, the length of the development cycle, and the budget. 5. Try to choose products from large companies. Their quality is guaranteed, their technical support is good, their after-sales service is generally better, and it is also conducive to your product expansion and software upgrades. I. Input Circuit Design 1. Power Supply Circuit The PLC power supply is generally AC85-240V (some are DC24V), which has a wide power supply range. However, in order to resist interference, power purification components (such as power filters, 1:1 isolation transformers, etc.) should be installed. 2. Use of DC24V power supply on PLC: Most PLC products from various companies have a DC24V power supply, but its capacity is small, ranging from tens to hundreds of milliamps. When using it to power a load, pay attention to the capacity and take precautions against short circuits (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 "-" terminal of this external power supply should not be connected to the "-" terminal or the "COM" terminal of the PLC's DC24V power supply, otherwise it will affect the operation of the PLC. 4. Input sensitivity Each manufacturer has specified the input voltage and current of the PLC. For example, the input values ​​of the Mitsubishi F7n series PLC are: DC24V, 7mA, starting current is 4.5mA, and the shutdown current is less than 1.5mA. Therefore, when there is a diode or resistor in series in the input circuit (cannot start completely), or there is a parallel resistor or leakage current (cannot be completely cut off), there will be malfunctions and the sensitivity will decrease. Measures should be taken to address this. On the other hand, when the input current of the input device is greater than the maximum input current of the PLC, it will also cause malfunctions. Weak current input devices should be used, and a PLC with common-drain input should be selected. The common point potential of the Bp input element is relatively negative, and the current flows out of the input terminal of the PLC. II. Output circuit design 1. Comparison between various output methods (1) Relay output: The advantage is that different AC and DC loads can be carried between different common points, and the voltage can also be different. The load current can reach 2A/point; however, the relay output method is not suitable for high-frequency loads, which is determined by the life of the relay. Its lifespan decreases with the increase of load current, generally between hundreds of thousands and millions of times, and some companies' products can reach more than 10 million times, with a response time of 10ms. (2) Thyristor output: The load capacity is 0.2A/point, it can only carry AC loads, it can adapt to high frequency operation, and the response time is 1ms. (3) Transistor output: The biggest advantage is that it is suitable for high frequency operation and has a short response time, generally around 0.2ms, but it can only carry DC 5-30V loads, and the maximum output load current is 0.5A/point, but it must not exceed 0.8A for every 4 points. When your system output frequency is less than 6 times per minute, you should choose relay output first, because its circuit design is simple, anti-interference and load capacity are strong. When the frequency is less than 10 times/min, you can use relay output or PLC output to drive Darlington transistor (5-10A) and then drive the load, which can greatly reduce the load. 2. Anti-interference and External Interlocking: When the PLC output is connected to an inductive load, a surge current will impact the PLC output when the load is de-energized. Therefore, a freewheeling diode should be connected in parallel next to the DC inductive load, and a surge absorption circuit should be connected in parallel for the AC inductive load to effectively protect the PLC. After the outputs of two physical quantities are software-interlocked internally by the PLC, external interlocking should also be implemented to enhance system reliability. 3. 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 2 or 1. When there are many types of loads and the current is high, a PLC product with 1-2 output points per "COM" point is used; when there are many loads but few types, a PLC product with 4-8 output points per "COM" point is used. This greatly simplifies circuit design. Add a fuse at each "COM" point; for 1-2 outputs, use a 2A fuse; for 4-8 outputs, use a 5-10A fuse, as PLCs generally don't have internal fuses. 4. External PLC Drive Circuit: When the PLC output cannot directly drive the load, an external drive circuit must be used. This can be a transistor drive, a solid-state relay, or a thyristor circuit. Protection circuits and surge absorption circuits should be used, and each channel should have an indicator diode (LED). The printed circuit board should be pluggable for easy maintenance. PLC input/output wiring also has specific requirements; please refer to the user manual of each company. III. Selection of Expansion Modules For small systems, such as those with 80 points or less, expansion is generally not needed. However, larger systems require expansion. Different companies have limitations on the total number of system points and the number of expansion modules. When expansion still cannot meet the requirements, a network structure can be used. Also, some manufacturers' products do not support expansion modules for certain instructions; therefore, this should be considered during software development. When using analog modules such as temperature sensors, manufacturers have specific guidelines; please refer to the relevant technical manuals. Various companies offer a wide range of expansion modules, such as single-input modules, single-output modules, input/output modules, temperature modules, and high-speed input modules. This modular design of PLCs facilitates product development for users. IV. PLC Network Design Designing a network using a PLC is significantly more complex than single-unit PLC control. First, you should 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 your real-time requirements, causing system crashes. Additionally, communication interfaces, communication protocols, and data transmission speeds must be considered. Finally, you should seek network design and software technical support and detailed technical documentation from the PLC vendor. The number of workstation layers to choose depends on the size of your system. V. Software Development Before developing the software, you should first familiarize yourself with the software manual of the selected PLC product. Programming should only begin after you have mastered the basics. 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 begin. This reduces errors and speeds up the process. After programming, run the air conditioning program first. Once all actions are normal, then debug it on the device.