In the design of a PLC system, the first step is to determine the overall system solution. Once this is established, the next phase involves selecting the appropriate PLC model and manufacturer. When choosing a PLC, it's essential to consider not only the specific requirements of the equipment but also the designer's familiarity with different manufacturers' products, the consistency of available support systems, and the level of technical service provided. In terms of reliability, as long as the PLC comes from a reputable foreign company, its reliability should generally not be an issue.
For independent equipment or simple control systems, Japanese PLCs often offer a better cost-performance ratio. However, for large-scale systems that require advanced network communication capabilities, such as open and distributed control systems or remote I/O setups, European and American PLCs tend to have more robust communication features. Additionally, in specialized industries like metallurgy or tobacco, it's crucial to select a PLC that has proven performance within that sector.
The number of input and output (I/O) points is one of the fundamental parameters when selecting a PLC. This number should be based on the total number of I/O points required by the equipment. It's advisable to add a 10–20% margin to account for future expansion. During the ordering process, adjustments may be necessary depending on the manufacturer’s product specifications.
Memory capacity is another important factor. The memory refers to the hardware storage space available, while the program capacity is the amount of memory used by the user's application. Since the program capacity is unknown during the design phase, it is typically estimated using the memory capacity. A common estimation method is to multiply the number of digital I/O points by 10–15 and the number of analog I/O points by 100, then add 25% for safety.
Control functions are also critical. These include computing, communication, programming, diagnostics, and processing speed. Simple PLCs usually handle logic operations, timing, and counting, while more advanced models can perform complex arithmetic, PID control, and data transfer. Communication functions are especially important in larger systems, where multiple PLCs may need to connect via fieldbus or industrial Ethernet.
Programming functions vary between offline and online modes. Offline programming is cost-effective but less flexible, while online programming allows for real-time debugging and is commonly used in larger systems. Standardized programming languages like Ladder Diagram, Function Block Diagram, and Structured Text are widely supported.
Diagnostic functions help identify hardware and software issues, which directly affects maintenance time and operator skill requirements. Processing speed is also vital, as slow response times can lead to missed signals. Modern PLCs operate quickly, with scan times ranging from 0.2ms/K for large systems to under 0.5ms/K for smaller ones.
PLCs come in two main types: integral and modular. Integral PLCs are compact and suitable for small systems, while modular PLCs offer greater flexibility and are ideal for larger applications. Various modules, such as digital and analog I/O, function modules, and communication units, can be selected based on specific needs.
Redundancy is another consideration, especially in critical processes. Redundant configurations for control units and I/O interfaces ensure system reliability. Finally, when selecting components, economic factors, convenience, universality, and compatibility should all be taken into account to ensure a well-balanced and efficient system.
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