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 barrier transformer should be selected, and the transformer capacity should be 1.2-1.5 times larger than the actual need, and a filter can be added before the barrier transformer.
The power supply of the transmitter and the general signal surface should be selected with small distributed capacitance, multiple barriers, shielding and leakage inductance skills. The controller and I/O system are powered by their own barrier transformers and are separate from the main circuit power. 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 used for power supply, and the UPS has the functions of over-voltage, under-voltage maintenance, software monitoring, and blocking from the power grid, which can improve the safety and reliability of power supply. For some important equipment, the AC power supply circuit can choose a dual power supply system.
2. Correctly select the docking address and improve the grounding system
Excellent grounding is an important condition to ensure the reliable operation of the PLC manipulator, which can avoid accidental voltage shock damage and press disturbance. The perfect grounding system is one of the important ways for PLC manipulators to resist electromagnetic disturbance.
The PLC manipulator is a high-speed low-level control device and should be directly grounded. In order to suppress the disturbance of the power supply, input and output terminals, a special grounding wire should be connected to the PLC manipulator, 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 that is assembled and placed is suitable for the parallel point grounding method, and the center of the cabinet of each equipment is led to the grounding electrode with a separate 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 middle of the signal line, the shielding layer should be firmly connected and insulated, and the shielding layer should be connected to each other. Select the appropriate grounding point for single-point grounding, and avoid 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 grounded. When there is a rational load at the input, the AC 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. In order to suppress the induced electromotive force generated by parasitic capacitance between input signal lines, parasitic capacitance or coupling with other wires, RC surge absorbers are used.
The output is an AC rational 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 intermediate relay or photoelectric coupling.
5. Software method for PLC manipulator to resist disturbance
Because of the complexity of electromagnetic interference, it is not enough to simply adopt hardware anti-interference 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 calibration are used to effectively eliminate periodic disturbance and avoid potential drift. Using information redundancy skills, the corresponding software flags were planned; 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 of industrial sites. It requires us to consider all aspects of the elements.
According to the actual situation of the site, it is necessary to comprehensively consider the aspects of reducing the disturbance source and blocking the disturbance channel, and make full use of various anti-disturbance methods to carry out the planning of programmable manipulators and human-machine interfaces. In order to truly improve the anti-disturbance ability of programmable controllers and HMI in field applications, and ensure the safe and stable operation of the system.
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