Plc designers must know the five insider - Solutions - Huaqiang Electronic Network

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When selecting a PLC, there are several key factors to consider. First, it's important to choose a model that you're familiar with, especially if you're new to programmable logic controllers. Many users find it challenging to decide which PLC is the best option due to the variety of manufacturers available. In general, if you've worked with a specific brand before or have experience with its programming software, it’s wise to stick with that type. This approach helps in leveraging existing knowledge and resources effectively.

Another consideration is whether to go with an imported or domestic PLC. Some joint-venture manufacturers in China produce PLCs that are comparable in performance to imported models, while also offering better after-sales support and easier access to spare parts. Additionally, domestic PLCs are typically about one-third cheaper than their imported counterparts. However, for large-scale projects or critical applications, high-end imported PLCs from well-known international brands may still be the preferred choice.

If the required functionality is similar, it’s not always necessary to go for the most advanced model. For example, if your project only requires a dozen digital inputs and outputs, using a more basic PLC would save costs without compromising performance. It’s also important to stay updated with new PLC models as technology evolves rapidly. I once encountered a situation where an old F1-20MR PLC was replaced by a FXON-20MR model, which was more cost-effective and equally functional.

Installation and wiring are crucial steps in ensuring reliable PLC operation. Avoid placing the PLC in the same cabinet as high-voltage equipment or components like relays and contactors. These should be connected with RC snubber circuits to reduce electrical noise. Keep the PLC away from strong interference sources such as high-power thyristor devices or high-frequency welders.

For wiring, use standard cables for simple switch signals, but opt for shielded cables when signals need to travel long distances or when dealing with analog or high-speed signals. Separate power lines from signal lines and install them in different conduits. If signal lines exceed 300 meters, consider using intermediate relays or remote I/O modules. Always ensure that power and I/O cables are routed separately, and if they must share a conduit, use shielded cables.

Signal transmission should follow best practices. When analog signals are far from the PLC, use current-based transmission (4–20 mA or 0–10 mA) instead of voltage-based methods. Shielded cables for analog signals should be grounded at one end, while digital signal cables should have their shielding connected to a potential equalization line and grounded at both ends to reduce interference.

Power supply stability is essential for reliable PLC operation. To minimize interference, consider using a separate power supply for the PLC. In environments with high electromagnetic interference, adding a shielded isolation transformer and a low-pass filter can significantly improve system reliability. Ensure that power lines have sufficient cross-section to reduce voltage drop and maintain stable operation.

Many failures in PLC systems originate from external sensors, especially mechanical switches and limit switches, which tend to fail more frequently than the PLC itself. To enhance system reliability, consider replacing mechanical switches with high-reliability proximity switches. This small change can greatly improve the overall performance of the control system.

Finally, protecting the user program is crucial. While many PLCs store programs in RAM with a backup battery, this method isn't foolproof in high-interference environments. A common practice is to write the program into an EPROM using a PLC ROM writer, then protect it during runtime to prevent accidental overwriting. This ensures that your program remains intact even under challenging conditions.