Anti-interference measures for PLCs and touch screens - Mitsubishi PLC

1. Select the power supply with excellent performance and press the disturbance introduced by the power grid

The power supply of the PLC controller is powered by a non-power line, and is powered by a special line directly led by the main bus of the low-voltage distribution room. The isolation transformer should be selected, and the transformer capacity should be 1.2-1.5 times larger than the actual need, and the filter can be added before the isolation transformer.

The power supply of the transmitter and the surface of the common signal should be a power distributor with small distributed capacitance, multiple isolation, shielding and leakage inductance skills. The operator and I/O system are each powered by their own isolation transformers and separated from the main circuit power supply. The 24V DC power supply of the PLC manipulator should not supply power to various peripheral sensors as much as possible, so as to reduce the disturbance of the PLC manipulator caused by the internal short circuit fault of the peripheral sensor or power line.

In addition, in order to ensure that the power grid feed is not interrupted, the online uninterruptible power supply can be selected for power supply, and the UPS has the functions of over-voltage, under-voltage maintenance, software monitoring, and isolation from the power grid, which can improve the safety and reliability of power supply. For some important equipment, the communication power circuit can choose a dual power supply system.

Anti-interference measures for PLCs and touch screens - Mitsubishi PLC

Anti-interference measures for PLCs and touch screens - Mitsubishi PLC

2. Correctly select the connecting address and improve the grounding system

Good grounding is an important condition to ensure the reliable operation of the PLC manipulator, which can prevent accidental voltage impulse damage and press disturbance. The perfect grounding system is one of the important ways for PLC controllers to resist electromagnetic disturbance.

The PLC manipulator is a high-speed low-level control device and should be directly grounded. In order to press the disturbance of the power supply, input and output terminals, a special grounding wire should be connected to the PLC controller, and the connection address should be separated from the connection address of the power supply equipment. If this requirement cannot be met, it must also be grounded together with other equipment, and it is forbidden to connect it in series with other equipment. The connection address should be as close as possible to the PLC manipulator, and the PLC manipulator arranged in the assembly is suitable for the parallel grounding method, and the central grounding address of each equipment is led to the grounding electrode with a unique grounding wire.

When the signal source is grounded, the shield should be grounded on the signal side; When the signal source is not grounded, it should be grounded on the PLC manipulator side. When there is a joint in the center of the signal line, the shielding layer should be firmly connected and insulated, and the shielding layer should be well connected with each other. Select the appropriate grounding for single-point grounding, and prevent multi-point grounding.

3. Reasonable selection of products and equipment

When selecting equipment, we should first understand the anti-disturbance indicators given by domestic PLC manufacturers, such as common mode press ratio, differential mode press ratio, withstand voltage capacity, allowable electric field strength and high-frequency magnetic field strength environment, and products with strong anti-disturbance ability should be selected.

4. Anti-disturbance method of input and output channels of PLC controller

The filtering of the input module reduces the differential mode disturbance between the lines of the input signal. In order to reduce common-mode turbulence between the input signal and the ground, the PLC manipulator should be well grounded. When there is an inductive load at the input terminal, the communication input signal can be connected to the capacitor and resistor in parallel at both ends of the load, and the DC input signal can be connected to the current diode in parallel. RC surge absorbers are used to compress the parasitic capacitance between the input signal lines, the parasitic capacitance between other lines, or the induced electromotive force generated by coupling.

The output is a communication inductive load, and the RC surge absorber can be connected in parallel at both ends of the load. For switching outputs, surge absorbers or thyristor output modules can be used. In addition, the output point of the PLC controller can be directly connected to the electrical control circuit by series center relay or photoelectric coupling.

5. Software method for PLC manipulator to resist disturbance

Due to the complexity of electromagnetic disturbance, it is not enough to adopt hardware anti-disturbance methods. The software anti-disturbance technology of the PLC controller is selected to further improve the reliability of the system. Digital filtering, power frequency shaping sampling and reference point potential punctuality correction are used to effectively eliminate periodic disturbances and prevent potential drift. Using the information redundancy skills, the corresponding software flags were designed. Direct jumps and software maintenance are available.

The anti-disturbance problem of programmable manipulators and human-machine interfaces in field applications is a very complex system engineering, involving specific input and output equipment and specific environments in industrial sites. It requires us to take into account various factors.

According to the actual situation of the site, it is necessary to consider from the aspects of reducing the disturbance source and blocking the disturbance channel, make full use of various anti-disturbance methods, and design programmable controllers and human-machine interfaces. In order to truly improve the anti-disturbance ability of programmable controllers and HMIs in field applications, and ensure the safe and stable operation of the system.

Anti-interference measures for PLCs and touch screens - Mitsubishi PLC

Sections viewed

Contact

working hours