Five things plc designers should know

1. Model selection

1. Choose a model with familiar programming software. I often receive some letters, especially some users who use programmable controllers (PLC) for the first time, asking which model of PLC to choose? Model selection is an old problem. Since there are many manufacturers producing PLCs, sometimes you feel that you have no idea how to solve this problem. Generally speaking, if you know more about which company's PLC model, especially if you are familiar with its instructions and programming software, it is better to choose the company's PLC. Because the products of various companies are similar in terms of reliability and performance indicators. If a certain model of PLC has been used on your equipment (or product) or imported equipment, if you want to use PLC to develop new products, it is recommended to choose the PLC you have already used, provided that the process conditions are met, so that you can achieve the benefits of resource sharing. 2. Choose a model of a joint venture factory. Is it better to choose an imported PLC or a domestic PLC? Some domestic PLC manufacturers, especially some joint venture PLC manufacturers, have the same performance as imported PLCs, and domestic PLC manufacturers have easy after-sales service and spare parts.

The price of domestic PLC is about 1/3 cheaper than that of imported PLC. Of course, imported PLCs, especially those produced by some internationally renowned large companies, especially large or super large PLCs, are still the first choice for major projects. 3. Choose a model with equivalent performance Another important issue in PLC selection is that the performance should be equivalent. If there are only a dozen switch input and output projects, and a PLC model with analog output and input is selected, it is a waste of resources. At this time, as long as a PLC with equivalent performance is selected, its price can be greatly reduced. 4. Choose a new model Since PLC products are updated very quickly, it is necessary to choose the corresponding new model. I once encountered a PLC user who used an F1-20MR PLC on a device imported in the 1980s. The PLC broke down. The factory tried every means to buy a PLC of the same model as a replacement, but failed to buy it. In fact, it can be completely replaced by a FXON-20MR PLC, and of course other models can also be used.

2. Installation and wiring

Different cables should be used for AC circuits and DC circuits. 4. Signal transmission Usually, when analog input and output signals are far away from the PLC, 4-20mA or 0-10mA current transmission should be used instead of voltage transmission. The shielding layer of the shielded wire that transmits analog signals should be grounded at one end. In order to discharge high-frequency interference, the shielding layer of the digital signal line should be connected in parallel with the potential equalization line, and both ends of the shielding layer should be grounded.

3. Power supply

Ensuring that PLC works reliably is one of the goals pursued by the project. When designing the system, measures to improve reliability should be taken to eliminate the influence of interference and ensure the normal operation of the system. Power supply interference is mainly generated by impedance coupling of the power supply line. Power supply is one of the main ways for interference to enter PLC. If conditions permit, a separate power supply circuit can be used for PC to avoid interference to PLC caused by the start and stop of other equipment. In situations where interference is strong or reliability is required to be high, a shielded isolation transformer and low-pass filter can be added to the AC power input end of PLC. The isolation transformer can suppress external interference from the power line, and the low-pass filter can absorb most of the "burr" interference in the power supply. The power part, control part, PLC and I/O power supply should be wired separately, see the attached figure for details. Twisted pair cables are used to connect the isolation transformer to PLC and I/O power supply. The power line of the system should have a sufficient cross-section to reduce the line voltage drop.

4. External Sensor

A large number of practices have shown that a considerable part of the failure rate of PLC control systems is caused by external sensor failures. In particular, the failure rate of some mechanical travel switches and limit switches is often much higher than the failure rate of the PLC itself, so corresponding measures should be taken when designing a PLC control system. For example, using a high-reliability proximity switch instead of a mechanical travel limit switch can ensure the high reliability of the PLC control system.

5. Protection and backup of user programs

The debugged PLC user program is often placed in the RAM with a battery as a backup power supply. Generally, this method is safe, but in a strong interference environment, the user program in the BAM may be rewritten or flushed. At present, the common practice is to use the PLC ROM writer to write the debugged program to the EPROM for long-term storage. When the system is running, the EPROM should be write-protected to prevent it from being rewritten.