When designing anti-interference measures for specific engineering projects, products with high anti-interference capabilities should be selected, and measures such as suppressing interference sources, cutting off or attenuating the propagation path of electromagnetic interference, and using software methods should be adopted to improve the anti-interference capabilities of devices and systems.
1. Use a high-performance power supply to suppress interference introduced by the power grid.
For the power supply of the PLC controller, a non-power line should be used, directly connected to the main busbar of the low-voltage distribution room via a dedicated line. An isolation transformer should be selected, and its capacity should be approximately 1.2 to 1.5 times larger than the actual requirement. A filter can also be added before the isolation transformer. For the power supply of transmitters and shared signal instruments, a power distribution unit with low distributed capacitance, multiple isolation and shielding technologies, and leakage inductance should be selected.
The controller and I/O system are each powered by their own isolation transformers, and are separate from the main circuit power supply. The 24V DC power supply of the PLC controller should avoid powering external sensors as much as possible to reduce interference from short circuits in the sensors or power lines. Furthermore, to ensure uninterrupted power supply from the mains, an online uninterruptible power supply (UPS) can be used. UPS systems offer overvoltage and undervoltage protection, software monitoring, and grid isolation, improving power supply safety and reliability. For critical equipment, a dual-power supply system can be used for the AC power circuit.
2. Correctly select and implement cable laying procedures to eliminate spatial radiation interference from programmable logic controllers (PLCs) and human-machine interfaces (HMIs).
Different types of signals are transmitted via separate cables, employing distance reduction technology. Signal cables are laid in layers according to the type of signal transmitted, and signal lines of the same type are twisted together. It is strictly forbidden to transmit power and signals simultaneously using different conductors on the same cable. Signal lines and power cables should not be laid close together or parallel; the angle between the cables should be increased to reduce electromagnetic interference. To reduce radiated electromagnetic interference from power cables, especially those used as feeder cables for frequency converters, shielded power cables must be used to block interference from entering through the interference path.
3. Anti-interference measures for PLC controller input/output channels
Input module filtering can reduce differential-mode interference between input signal lines. To reduce common-mode interference between the input signal and ground, the PLC controller must be properly grounded. When there is an inductive load at the input, for AC input signals, a capacitor and resistor can be connected in parallel across the load; for DC input signals, a freewheeling diode can be connected in parallel. To suppress parasitic capacitance between input signal lines, and induced electromotive force generated by parasitic capacitance or coupling with other lines, an RC surge absorber can be used.
For AC inductive loads, an RC surge absorber can be connected in parallel across the load. For DC loads, a freewheeling diode can be connected in parallel, and it should be placed as close to the load as possible. For switching outputs, surge absorbers or thyristor output modules can be used. Additionally, using an intermediate relay or optocoupler connected in series at the output point can prevent the PLC controller output point from being directly connected to the electrical control circuit, ensuring complete electrical isolation.
4. 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 techniques of the PLC controller must be used in conjunction to further improve system reliability. Measures such as digital filtering, power frequency shaping sampling, and timed correction of the reference point potential are employed to effectively eliminate periodic interference and prevent potential drift. Information redundancy technology is used, and corresponding software flags are designed; indirect jumps are employed, and software protection is implemented. For example, for switch input signals, multiple reads are performed using a timer delay; only when the results are consistent is the validity confirmed, thus improving software reliability.
5. Select the correct grounding point and improve the grounding system.
Proper grounding is essential for ensuring the reliable operation of a PLC controller. It can prevent damage from accidental voltage surges and suppress interference. A well-designed grounding system is also one of the important measures for PLC controllers to resist electromagnetic interference.
PLC controllers are high-speed, low-level control devices and should be directly grounded. To suppress interference applied to the power supply, input terminals, and output terminals, a dedicated ground wire should be connected to the PLC controller, and the grounding point should be separate from the grounding point of the power equipment. If this requirement cannot be met, it must at least be grounded together with other equipment, and series grounding with other equipment is prohibited. The grounding point should be as close as possible to the PLC controller.
Centrally located PLC controllers are suitable for parallel single-point grounding, with each unit's cabinet's central grounding point led to a grounding electrode via a separate grounding wire. Distributed PLC controllers should use series single-point grounding. The grounding electrode's resistance should be less than 2Ω, and it is best to bury the grounding electrode 10-15m away from the building. Furthermore, the PLC controller's grounding point must be at least 10m away from the grounding point of any high-voltage equipment. If expansion units are used, their grounding points should be connected together with the basic unit's grounding point.
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.
6. Equipment Selection
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, and the maximum electric field strength and magnetic field strength allowed to operate in the environment. You should choose products with high anti-interference capabilities, such as programmable controllers with floating ground technology and good isolation performance, and human-machine interfaces (HMIs).
The anti-interference issue in the field application of programmable logic controllers (PLCs) and human-machine interfaces (HMIs) is complex and nuanced. Anti-interference design is a highly complex systematic engineering project, involving specific input/output devices and the specific industrial environment. It requires comprehensive consideration of various factors, taking into account the actual site conditions, including reducing interference sources and cutting off interference paths. Only by fully utilizing various anti-interference measures in the design of PLCs and HMIs can we truly improve their anti-interference capabilities in field applications and ensure the safe and stable operation of the system.
