1. Use power with excellent performance to restrain the interference introduced by the power grid

In PLC control system, the power supply occupies a very important position. Power grid interference into PLC control system mainly through the PLC system power supply (such as power supply, I/O power supply, etc.), transmitter power supply and PLC system with direct electrical connection of the instrument power supply, etc. Now, for the PLC system power supply, generally use good isolation performance power supply, and for the transmitter power supply and PLC system with direct electrical connection of the instrument power supply, and not enough attention, although a certain isolation measures have been taken, but generally not enough, mainly the use of isolation transformer distribution parameters, inhibit interference ability is poor, Common mode interference and differential mode interference through power coupling. Therefore, for the transmitter and shared signal instrument power supply should choose small distribution capacitance, inhibition belt (such as the use of multiple isolation and shielding and leakage technology) of the distributor, in order to reduce the interference of PLC system.

To ensure that the power grid feed point is not interrupted, the online uninterruptible power supply (UPS) can be used to supply power, improving the power supply security and reliability. And UPS also has strong interference isolation performance, is an ideal power supply for PLC control system.

2 Cable selection

In order to reduce the power cable radiation electromagnetic interference, especially the frequency conversion device feed cable. In a certain project, the author used copper tape armored shielded power cable, so as to reduce the electromagnetic interference of power line production, and achieved satisfactory results after the project put into operation.

3. Hardware filtering and software anti-failure measures

Before the signal is connected to the computer, a capacitor is connected between the signal line and the ground to reduce common-mode interference. The differential mode interference can be reduced by adding a filter between the poles of the signal.

Due to the complexity of electromagnetic interference, it is impossible to fundamentally eliminate the impact of interference, so in the software design and configuration of PLC control system, should also be anti-interference processing in software, further improve the reliability of the system. Some commonly used measures: digital filtering and power frequency shaping sampling, can effectively eliminate periodic interference; The reference point potential is corrected regularly and the dynamic zero point is adopted to prevent potential drift effectively. Using information redundancy technology, the corresponding software flag bit is designed. Indirect jump and software trap are adopted to improve the reliability of software structure.