Here are some points to note when designing:
1. Use a high-performance power supply to suppress interference introduced by the power grid.
The PLC controller is powered by a non-power line supply, directly drawn from the main busbar of the low-voltage distribution room via a dedicated line. An isolation transformer should be selected, with a capacity 1.2 to 1.5 times larger than the actual requirement, and a filter can be added before the isolation transformer.
The power supplies for transmitters and general signal instruments should be selected using power distributors with low distributed capacitance, multiple isolation, shielding, and leakage inductance technology. The controller and I/O system should be powered by their own isolation transformers, separate from the main circuit power supply. The 24V DC power supply of the PLC controller should, as far as possible, avoid supplying power to various external sensors to reduce interference from short circuits in external sensors or power lines.
In addition, to ensure uninterrupted power supply from the grid, an online uninterruptible power supply (UPS) can be used. UPS systems have overvoltage and undervoltage protection, software monitoring, and grid isolation functions, which can improve the safety and reliability of power supply. For some critical equipment, a dual-power supply system can be used for the AC power circuit.
2. Correctly select the grounding point and improve the grounding system.
Proper grounding is essential for the reliable operation of a PLC controller, preventing damage from accidental voltage surges and suppressing interference. A well-designed grounding system is also a crucial measure for PLC controllers to resist electromagnetic interference.
A PLC controller is a high-speed, low-level control device and should be directly grounded. To suppress interference to the power supply, input, and output terminals, a dedicated grounding wire should be connected to the PLC controller, and the grounding point should be separate from the grounding point of the power supply equipment. If this requirement cannot be met, it must share a common ground with other equipment and should not be connected in series with other equipment. The grounding point should be as close as possible to the PLC controller. For centrally located PLC controllers, a parallel single-point grounding method is suitable, with the central grounding point of each device's cabinet led to the grounding electrode by a separate grounding wire.
When the signal source is grounded, the shielding layer should be grounded on the signal side; when the signal source is not grounded, it should be grounded on the PLC controller side. When there are joints in the signal lines, the shielding layers should be securely connected and insulated, and all shielding layers should be interconnected. Choose an appropriate single-point grounding point to avoid multiple grounding points.
3. Select appropriate product equipment
When selecting equipment, you should first understand the anti-interference indicators provided by domestic PLC manufacturers, such as common mode rejection ratio, differential mode rejection ratio, withstand voltage, allowable electric field strength and high-frequency magnetic field strength environment, and choose products with strong anti-interference capabilities.
4. Anti-interference measures for PLC controller input/output channels
Input module filtering can reduce differential-mode interference between input signals. To reduce common-mode interference between input signals and ground, the PLC controller should be properly grounded. When there is an inductive load at the input, a capacitor and resistor can be connected in parallel across the load for AC input signals, and a current diode can be connected in parallel for DC input signals. To suppress induced electromotive force generated by parasitic capacitance between input signal lines, parasitic capacitance between other lines, or coupling, an RC surge absorber can be used.
The output is an AC inductive load, and an RC surge absorber can be connected in parallel across the load. For switching output applications, surge absorbers or thyristor output modules can be used. Alternatively, the PLC controller's output points can be directly connected to the electrical control circuit via series intermediate relays or optocouplers.
5. Software measures for PLC controller anti-interference
Due to the complexity of electromagnetic interference, hardware-based anti-interference measures alone are insufficient. Software-based anti-interference technology using a PLC controller further improves system reliability. Digital filtering, power frequency shaping sampling, and reference point potential timing correction effectively eliminate periodic interference and prevent potential drift. Information redundancy technology is utilized, and corresponding software flags are designed; indirect switching and software protection are employed.
The interference immunity problem in the field application of programmable logic controllers (PLCs) and human-machine interfaces is a very complex systems engineering problem, involving specific input/output devices and the specific environment of industrial sites. It requires us to comprehensively consider various factors.
Based on the actual site conditions, it is essential to comprehensively consider factors such as reducing interference sources and cutting off interference channels, and to fully utilize various anti-interference measures in the design of programmable logic controllers (PLCs) and human-machine interfaces (HMIs). Only in this way can the anti-interference capabilities of PLCs and HMIs be truly improved in field applications, ensuring the safe and stable operation of the system.
