In the design of a PLC system, the first step is to determine the overall system solution. Once this is established, the next step involves selecting the appropriate PLC model. Choosing a PLC requires considering factors such as the specific model of the manufacturer and the type of PLC that best fits the application. There are different types of system solutions, including distributed systems and remote I/O systems, and network communication requirements must also be taken into account. So, how should one go about choosing the right PLC?
First, the choice of PLC manufacturer plays a crucial role. The decision should be based on the user's equipment requirements, the designer’s familiarity with various PLC brands, design habits, compatibility of supporting products, and technical support availability. In terms of reliability, most PLCs from well-known foreign manufacturers are generally reliable.
In general, for control of independent equipment or simple systems, Japanese PLCs often offer better cost-performance. However, for large-scale systems requiring advanced network communication capabilities, open and distributed control systems, or remote I/O configurations, PLCs from European or American manufacturers tend to have more robust communication features. Additionally, in specialized industries like metallurgy or tobacco, it’s advisable to choose a proven and reliable PLC system that has been successfully used in similar applications.
Second, determining the number of input/output (I/O) points is essential. This number should be based on all the required I/O points for the system. It’s common practice to add an extra 10% to 20% margin to accommodate future expansion. However, the final number should be adjusted according to the specific product characteristics of the chosen PLC.
Third, memory capacity is another important consideration. The memory size refers to the hardware storage available, while the program capacity is the amount of memory used by the user's application. During the design phase, since the program hasn't been written yet, the program capacity is estimated based on the memory capacity. A rough estimate is typically 10 to 15 times the number of digital I/O points plus 100 times the number of analog I/O points. A 25% buffer is usually added to this total.
Fourth, the control functions of the PLC should align with the application needs. These include computing functions, control functions, communication functions, programming functions, diagnostic capabilities, and processing speed. For example, basic PLCs may only require logic operations, timing, and counting functions, while more complex systems may need advanced arithmetic, data transfer, PID control, and more.
Fifth, the communication function is vital, especially for larger systems. PLCs should support various fieldbus protocols and standard communication methods like TCP/IP. Communication interfaces such as serial, parallel, industrial Ethernet, and RIO ports are commonly used. Redundant configurations are recommended for critical systems to ensure reliability.
Sixth, the programming function is essential. PLCs can be programmed using offline or online modes. Offline programming is cost-effective but less flexible, while online programming allows for real-time debugging and is preferred in larger systems. Standardized programming languages like Ladder Diagram, Function Block Diagram, and Structured Text are widely used.
Seventh, diagnostic functions help identify and resolve issues quickly. Both hardware and software diagnostics are important for maintaining system performance and reducing downtime.
Eighth, the processing speed of the PLC affects its ability to handle real-time tasks. Faster processing ensures that signals are not missed during scanning, which is crucial for high-speed control applications.
Finally, the type of PLC—integral or modular—should be selected based on the system size. Integral PLCs are compact and suitable for small systems, while modular PLCs offer greater flexibility for larger and more complex setups.
When selecting modules, considerations include digital I/O, analog I/O, and function modules like communication, positioning, and PID control. Redundancy options should also be considered for critical applications.
In summary, when choosing a PLC, it's important to consider economics, convenience, universality, and compatibility. These factors ensure a reliable, efficient, and scalable control system that meets both current and future needs.
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