I. Problem Statement Programmable logic controller (PLC) technology is primarily applied in automation control engineering. This paper introduces the general methods for constructing a PLC control system, focusing on how to comprehensively utilize previously learned knowledge points and rationally combine them according to actual engineering requirements. II. Basic Steps in PLC Control System Design 1. Main Contents of System Design (1) Determine the technical conditions for the control system design. Technical conditions are generally determined in the form of design task book, which is the basis of the whole design; (2) Select the form of electrical drive and actuators such as motors and solenoid valves; (3) Select the model of PLC; (4) Compile the input/output allocation table of PLC or draw the input/output terminal wiring diagram; (5) Write the software specification according to the requirements of system design, and then use the corresponding programming language (commonly ladder diagram) to design the program; (6) Understand and follow the user cognitive psychology, pay attention to the design of human-machine interface, and enhance the friendly relationship between people and machines; (7) Design the operating table, electrical cabinet and non-standard electrical components; (8) Write the design specification and user manual; The above contents can be adjusted appropriately according to the specific task. 2. Basic steps of system design The main steps of programmable controller application system design and debugging are shown in Figure 1. [align=center] Figure 1 Main steps of programmable controller application system design and debugging[/align] (1) Thoroughly understand and analyze the process conditions and control requirements of the controlled object a. The controlled object is the controlled machinery, electrical equipment, production line or production process. b. Control requirements mainly refer to the basic control method, the actions to be completed, the composition of the automatic work cycle, necessary protection and interlocking, etc. For more complex control systems, the control tasks can be divided into several independent parts, which can simplify the complex and is conducive to programming and debugging. (2) Determine the I/O devices According to the functional requirements of the controlled object for the PLC control system, determine the user input and output devices required by the system. Commonly used input devices include buttons, selector switches, limit switches, sensors, etc., and commonly used output devices include relays, contactors, indicator lights, solenoid valves, etc. (3) Select the appropriate PLC type According to the determined user I/O devices, count the number of required input signals and output signals, and select the appropriate PLC type, including the selection of machine type, capacity, I/O module, power supply module, etc. (4) Allocate I/O points Allocate the input and output points of the PLC, compile the input/output allocation table or draw the wiring diagram of the input/output terminals. Next, PLC programming can be carried out, and the design and on-site construction of control cabinets or operating consoles can also be carried out. (5) Design the ladder diagram program of the application system. The ladder diagram is designed based on the working function chart or state flow diagram, i.e., programming. This step is the core work of the entire application system design and is also a relatively difficult step. To design a good ladder diagram, you must first be very familiar with the control requirements and have some practical experience in electrical design. (6) Input the program into the PLC. When using a simple programmer to input the program into the PLC, the ladder diagram needs to be converted into instruction mnemonics for input. When using the auxiliary programming software of the programmable controller to program on the computer, the program can be downloaded to the PLC through the connection cable between the upper and lower computers. (7) Perform software testing. After the program is input into the PLC, testing should be carried out first. Because there will inevitably be omissions in the program design process. Therefore, before connecting the PLC to the field equipment, software testing must be carried out to eliminate errors in the program and lay a good foundation for overall debugging and shorten the overall debugging cycle. (8) Overall debugging of the application system After the PLC hardware and software design and control cabinet and on-site construction are completed, the online debugging of the entire system can be carried out. If the control system consists of several parts, the local debugging should be carried out first, and then the overall debugging should be carried out. If the control program has many steps, the segmented debugging can be carried out first, and then the whole system can be connected together for debugging. Problems found during debugging should be eliminated one by one until the debugging is successful. (9) Compilation of technical file system Technical files include instruction manual, electrical schematic diagram, electrical layout diagram, electrical component list, and PLC ladder diagram. III. PLC hardware system design 1. Selection of PLC model Before making a decision on the system control scheme, the control requirements of the controlled object should be understood in detail to determine whether to use PLC for control. When the logic relationship of the control system is relatively complex (requiring a large number of intermediate relays, time relays, counters, etc.), the process flow and product modification are frequently changed, data processing and information management are required (data calculation, analog quantity control, PID regulation, etc.), the system requires high reliability and stability, and the factory is to be networked for automation, it is necessary to use PLC control. At present, many manufacturers at home and abroad provide a variety of PLC products with different functions, which makes users dazzled and at a loss. Therefore, only by comprehensively weighing the advantages and disadvantages and rationally selecting the model can we achieve the goal of economic practicality. Generally, the model selection should be based on meeting the functional needs of the system. Do not blindly pursue large and comprehensive models, so as to avoid wasting investment and equipment resources. The model selection can be considered from the following aspects. (1) Selection of input/output points Blindly selecting a model with more points will cause a certain waste. First, we need to find out the total number of I/O points of the control system, and then reserve 15 to 20% of the actual required total number of points as a spare (to leave room for system modification, etc.) before determining the required number of PLC points. In addition, it should be noted that some high-density input point modules have a limit on the number of input points that can be connected at the same time. Generally, the number of input points that can be connected at the same time should not exceed 60% of the total input points. The driving capability (A/point) of each output point of the PLC is also limited. The output current of each point of some PLCs varies with the applied load voltage. The allowable output current of a PLC generally decreases with the increase of ambient temperature. These issues should be considered when selecting a model. The output points of a PLC can be divided into several connection methods: common point, grouped, and isolated. Different voltage types and voltage levels can be used between the output points of the isolated type, but the average price per point of this type of PLC is higher. If isolation is not required between output signals, the PLC with the first two output methods should be selected. (2) Selection of storage capacity The user's storage capacity can only be roughly estimated. In systems that only control switching quantities, the total number of input points can be multiplied by 10 words/point + the total number of output points multiplied by 5 words/point for estimation; counters/timers are estimated at (3~5) words/each; when there is arithmetic processing, it is estimated at (5~10) words/quantity; in systems with analog inputs/outputs, it can be estimated at approximately (80~100) words of storage capacity per input/(or output) analog quantity; when there is communication processing, it is roughly estimated at more than 200 words per interface. Finally, a margin of 50~100% of the estimated capacity is generally reserved. For designers who lack experience, a larger margin should be reserved when selecting the capacity. (3) Selection of I/O response time The I/O response time of PLC includes input circuit delay, output circuit delay and time delay caused by scanning operation mode (generally 2~3 scan cycles), etc. For systems with switch control, the response time of PLC and I/O can generally meet the requirements of actual engineering, and there is no need to consider the I/O response problem. However, for systems with analog control, especially closed-loop systems, this problem must be considered. (4) Select the output mode of PLC according to the characteristics of the output load. Different loads have corresponding requirements for the output mode of PLC. For example, for inductive loads that are frequently switched on and off, transistor or thyristor output type should be selected, rather than relay output type. However, relay output type PLC has many advantages, such as small on-state voltage drop, isolation function, relatively low price, strong ability to withstand instantaneous overvoltage and overcurrent, flexible load voltage (AC or DC) and large voltage range. Therefore, AC and DC loads with infrequent operation can choose relay output type PLC. (5) Selection of online and offline programming. Offline programming means that the host and programmer share a CPU. The programming, monitoring and operation status of PLC is selected by the programmer's selection switch. In programming mode, the CPU only serves the programmer and does not control the field. Programming with a dedicated programmer falls into this category. Online programming refers to a process where the host and the programmer each have a CPU. The host's CPU completes the control of the field and communicates with the programmer at the end of each scan cycle. The programmer sends the modified program to the host, and in the next scan cycle, the host will control the field according to the new program. Computer-aided programming can achieve both offline and online programming. Online programming requires the purchase of a computer and the configuration of programming software. The choice of programming method should be determined according to the needs. (6) Selection based on whether network communication is required: If the PLC-controlled system needs to be connected to the factory automation network, the PLC needs to have communication networking capabilities, that is, the PLC should have interfaces for connecting to other PLCs, host computers, and CRTs. Large and medium-sized machines have communication functions, and most small machines also have communication functions at present. (7) Selection of PLC structure: Under the same function and the same I/O point data, the integrated type is cheaper than the modular type. However, the modular type has the advantages of flexible function expansion, convenient maintenance (replacing modules), and easy fault diagnosis. The PLC structure should be selected according to actual needs. 2. Assigning Input/Output Points Generally, input points and input signals, and output points and output controls are one-to-one. After assignment, assign each input signal and output signal according to the channel and contact number configured in the system, i.e., number them. In some cases, two signals may use one input point. In such cases, the wires should be connected according to the logical relationship before connecting to the input point (e.g., the two contacts should be connected in series or in parallel first), and then connected to the input point. (1) Determining the I/O Channel Range Different models of PLC have different input/output channel ranges. The corresponding programming manual should be consulted according to the selected PLC model. It is absolutely not allowed to "mismatch". The relevant operation manual must be consulted. (2) Internal Auxiliary Relays Internal auxiliary relays do not output externally and cannot be directly connected to external devices. Instead, they are used for data storage or data processing when controlling other relays, timers/counters. Functionally, internal auxiliary relays are equivalent to intermediate relays in traditional electrical control cabinets. The input/output relay area of ​​the unassigned module and the link relay area when not using 1:1 link can all be used as internal auxiliary relays. According to the needs of program design, the internal auxiliary relays of PLC should be arranged reasonably. The purpose of each internal auxiliary relay in the program should be listed in detail in the design specification to avoid repeated use. Refer to the relevant operation manual. (3) Assign timers/counters The number of timers/counters of PLC can be found in the relevant operation manual. 7.3 PLC software system design method and steps 7.3.1 PLC software system design method After understanding the PLC program structure, the program needs to be written in detail. There are many ways to write PLC control programs. Here, we mainly introduce several typical programming methods. 1. Graphical programming The graphical method is to design PLC programs by drawing diagrams. The common ones are ladder diagram method, logic flowchart method, timing flowchart method and step sequence control method. (1) Ladder diagram method: The ladder diagram method is to write PLC programs using ladder diagram language. This is a programming method that imitates the relay control system. Its graphics and even component names are very similar to those of relay control circuits. This method can easily transplant the original relay control circuit into the ladder diagram language of PLC. This is the most convenient programming method for people familiar with relay control. (2) Logic flowchart method: The logic flowchart method uses a logic block diagram to represent the execution process of the PLC program and reflects the relationship between input and output. The logic flowchart method uses a logic block diagram to represent the process flow of the system to form the logic flowchart of the system. The PLC control program compiled by this method has a clear logical thinking, and the causal relationship between input and output and the interlocking conditions are clear. The logic flowchart makes the entire program structure clear, which is convenient for analyzing the control program, finding fault points, debugging the program and repairing the program. Sometimes, for a complex program, it may be difficult to start by directly using statement lists and ladder diagrams. In this case, you can first draw a logic flowchart, and then use statement lists and ladder diagrams to compile PLC application programs for each part of the logic flowchart. (3) Timing Flowchart Method: The timing flowchart method involves first drawing the timing diagram of the control system (i.e., the control timing diagram of which control should be performed at a certain time), then drawing the corresponding control task flowchart according to the timing relationship, and finally writing the flowchart into a PLC program. The timing flowchart method is very suitable for programming control systems based on time. (4) Step Sequential Control Method: The step sequential control method is to design complex control programs with the help of sequential control instructions. Generally, more complex programs can be divided into several program segments with relatively simple functions. A program segment can be regarded as a step in the entire control process. From the perspective of the whole, the control process of a complex system is composed of such steps. The task of system control can actually be regarded as completing the control of each step at different times or in different processes. For this reason, many PLC manufacturers have added step sequential control instructions to their PLCs. After drawing the state flowchart of each step, the control program can be easily written using the step sequential control instructions. 2. Experience-based Programming Experience-based programming is to design using one's own or others' experience. Most people choose programs that closely match their process requirements before designing, treating these programs as "experimental programs." These "experimental programs" are then modified one by one to suit the specific engineering requirements. The experience mentioned here may come from personal experience or from the design experience of others; it requires accumulation and careful summarization over time. 3. Computer-Aided Design Programming Computer-aided design involves using PLC programming software to design programs on a computer, perform offline or online programming, offline simulation, and online debugging. Programming software makes it very convenient to program offline or online and debug online on a computer. It also makes it very convenient to access, encrypt, and create EXE executable files for programs on a computer. 7.3.2 Steps in PLC Software System Design After understanding the program structure and programming methods, the next step is to actually write the PLC program. Writing a PLC program, like writing any other computer program, involves the following process: 1. System Task Segmentation The purpose of segmentation is to break down a complex project into several simpler smaller tasks. This transforms a complex problem into several simpler ones. This facilitates program development. 2. Develop the logic diagram of the control system. The logic diagram shows the result of a certain logic relationship and the actions derived from it. This logic relationship can be based on the sequence of various control activities or the time cycle of the entire activity. The logic diagram reflects the control action and the activities of the controlled object during the control process, and also reflects the relationship between input and output. 3. Draw various circuit diagrams. The purpose of drawing various circuits is to link the addresses and names of the system's inputs and outputs. This is a crucial step. When drawing the PLC input circuit, it is necessary to consider not only whether the signal connection points are consistent with the names, but also whether the voltage and current at the input terminals are appropriate, as well as the reliability and stability under special conditions. In particular, it is necessary to consider whether high voltage can be introduced to the PLC input terminals, as introducing high voltage into the PLC input terminals will cause significant damage to the PLC. When drawing the PLC output circuit, it is necessary to consider not only whether the output signal connection points are consistent with the names, but also the load-carrying capacity and voltage withstand capability of the PLC output module. In addition, the output power and polarity of the power supply must also be considered. In the entire circuit design process, design principles must be considered to improve stability and reliability. Although PLC control is convenient and flexible, circuit design still requires caution and comprehensiveness. Therefore, when drawing the circuit diagram, thorough consideration is needed; the placement of buttons and switches must be meticulously planned. 4. Programming the PLC and Performing Simulation Debugging After completing the circuit diagram, you can begin programming the PLC. The methods described above can be used. When programming, in addition to ensuring the program is correct and reliable, it should also be concise, time-saving, easy to read, and easy to modify. Simulation experiments should be conducted after each program block is completed to facilitate problem identification and timely modification. It's best not to perform a final check after the entire program is finished. 5. Constructing the Control Console and Control Cabinet After completing the electrical design and programming, you can proceed to construct the control console and control cabinet. When time is limited, this work can be done concurrently with programming. When constructing the control console and control cabinet, pay attention to the quality and specifications of switches, buttons, relays, and other components; they must meet the requirements. The installation of the equipment must be safe and reliable. For example, issues such as shielding, grounding, and high-voltage isolation must be properly addressed. 6. On-site Commissioning: On-site commissioning is a crucial step in completing the entire control system. It's rare for a program design to be usable without on-site commissioning. Only through on-site commissioning can we identify areas where control loops and programs fail to meet system requirements; only through on-site commissioning can we discover inconsistencies between control circuits and programs; only through on-site commissioning can we conduct final field tests and adjustments to the control circuits and programs to meet the requirements of the control system. 7. Writing Technical Documents and On-site Trial Run: After on-site commissioning, the control circuits and programs are basically finalized, and the hardware and software of the entire system are essentially problem-free. At this point, a comprehensive review of the technical documents is necessary, including compiling circuit diagrams, PLC programs, user manuals, and help files. This essentially completes the work.