Programmable logic controller ( PLC ) technology has been widely applied in metallurgy, mining, machinery, and light industry, providing a powerful tool for industrial automation and accelerating the process of mechatronics. PLC control systems can perform analog control, position control, and other functions. The most significant characteristic of PLCs is that they use computer software technology to construct traditional relay models, forming a unique, graphical programming language based on relay ladder diagrams. In other words, PLC control systems implement their control functions in program form. Therefore, PLC programming technology plays a crucial role in PLC control systems.
1 General Design Principles of PLC Control Systems
To meet the technological requirements of the controlled object and thus improve production efficiency and product quality, the following basic principles should be followed in the design of a PLC control system:
1) To meet the control requirements of the controlled object to the greatest extent possible. The primary premise of designing a PLC control system is to fully utilize the functions of the PLC and meet the control requirements of the controlled object to the greatest extent possible.
2) Ensure the PLC control system can operate safely, reliably, and stably for a long period. Comprehensive consideration should be given to system design, component selection, and software programming to strive for a safe and reliable control system.
3) The control system designed should be simple, economical, and easy to use and maintain.
4) Adapt to development needs. While meeting the above principles, the needs of future production development and process improvement should be fully considered. Appropriate margins should be reserved when selecting PLCs, input/output modules, I/O points, and memory capacity.
2. Basic Contents of PLC Control System Design
The design of a PLC control system involves many aspects, but it should first include the following basic elements:
1) Determine the design technical conditions of the control system based on the design principles.
2) Correctly select the user's input devices (buttons, operating switches, limit switches, and sensors, etc.) and output devices (including relays, contactors, indicator lights, and other actuators), as well as the controlled objects driven by the output devices (motors, solenoid valves, etc.).
3) Correctly select the PLC. The core component of a PLC control system is the CPU, which plays a crucial role in ensuring the technical and economic performance of the entire control system. Its selection also determines the selection of other related modules.
4) Prepare the I/O allocation table and wiring diagram, that is, allocate I/O points reasonably and design the I/O port wiring diagram of the PLC. When assigning I/O point numbers, try to group signals of the same type together and arrange the address numbers sequentially.
5) When designing software, write a software design specification as required, select a suitable programming language (ladder diagram is commonly used) for program design, and when programming, firstly, divide the modules reasonably; secondly, make reasonable use of instructions and pay strict attention to the definition of information names.
6) Hardware design and software design can be carried out separately and simultaneously. Hardware configuration work, such as peripheral circuits, includes the design of the main circuit, the installation and wiring of high-voltage equipment, the design and on-site installation of control consoles (cabinets), etc.
7) Make a good single-machine debugging plan and a linkage debugging plan. Linkage debugging can only be carried out after the single-machine debugging is completed without errors.
8) Carefully prepare the design specifications and user manual.
3. PLC Model Selection
With the increasing adoption of PLCs in industrial control, the variety of PLC products is growing, differing in structure, performance, capacity, instruction set, programming methods, and application scenarios. Therefore, selecting the right PLC is crucial for improving its application in control systems.
The basic principle for selecting a PLC model is: under the premise of meeting functional requirements, choose the most reliable model, the easiest to maintain and use, and the best performance-price ratio. Specifically, the following aspects should be considered:
1) Selecting an appropriate PLC: The number of input/output points is an important indicator for determining the size of a PLC. Therefore, when selecting a PLC, first ensure that there are enough I/O points, and reserve 10% to 15% as a backup. Furthermore, if the goal is only to achieve single-machine automation or mechatronics, a small PLC can be selected; if the control system is larger, requires more I/O points, and the controlled devices are more dispersed, then a large or medium-sized PLC can be selected.
2) The PLC capacity must meet the user's requirements. The following empirical formula can be used to estimate the PLC's user program storage capacity: Total memory words = (Number of digital I/O points × 10) + (Number of analog I/O points × 150). The total memory words calculated using this empirical formula should be increased by a 25% margin.
3) Competitive functionality and reasonable structure. If the controlled object contains both digital and analog signals, requiring the PLC to perform A/D and D/A conversions, arithmetic operations, and other special processing, then a PLC with corresponding functions should be selected. Furthermore, the PLC structure must be considered. Integrated structures are simpler, smaller in size, and the average price per I/O point is cheaper than modular structures.
4) The choice of input/output module type and output format depends on the type, parameter requirements, and technical requirements of the input/output signals in the control system. Output modules are categorized into three types based on their method: relay output, transistor output, and bidirectional thyristor output. For inductive loads with frequent switching and low power factor, thyristor output (AC output) or transistor output (DC output) can be used, but these modules have slightly lower overload capacity and are more expensive. Relay output modules have stronger overvoltage and overcurrent resistance and are cheaper; their disadvantage is slower response speed. They are preferred when output changes are not rapid or frequent. Furthermore, the load capacity of the input/output module should also be considered.
5) The PLC model should be uniform. That is, within the same enterprise, the PLC models used should be as uniform as possible (uniform PLC manufacturers, and uniform models from the same manufacturer) so that the application software for programming is the same, and it is also convenient for maintenance and management.
4. Conclusion
PLCs are a new generation of industrial control devices developed based on automatic control technology, microcomputer technology, and communication technology. With the development of microprocessor technology, PLCs have experienced rapid growth and are increasingly used in production across various sectors. Currently, PLCs are widely used in logic control of digital signals, analog signal control, motion control, process control, data processing, and the Internet. Looking to the future, PLCs are poised for even greater development in terms of technology, product scale, product structure compatibility, market and network growth, and will find even broader application opportunities in China.
