Programmable Logic Controllers (PLCs) are automation control devices specifically designed for industrial environments. They can generally be used directly in most industrial settings without special measures. However, if the production environment is too harsh, electromagnetic interference is particularly strong, or if installation and use are improper, the normal and safe operation of the system cannot be guaranteed. Interference may cause the PLC to receive incorrect signals, resulting in malfunctions, or may cause data loss within the PLC ; in severe cases, it may even cause the system to malfunction. Therefore, appropriate reliability measures should be taken during system design to eliminate or reduce the impact of interference and ensure the normal operation of the system.
The reliability of control systems has always been a focus of attention in the electromechanical industry and automated production lines, as it directly affects the production safety and economic benefits of enterprises. The system's anti-interference capability is one of the important indicators for ensuring reliable operation. Most interference sources affecting PLC control systems originate in areas with drastic changes in current or voltage. This is because changes in current generate a magnetic field, which in turn produces electromagnetic radiation on the equipment. Changes in the magnetic field generate current, and these rapid electromagnetic changes produce electromagnetic waves.
In practical work, hardware and software anti-interference measures can be adopted to prevent interference. Among them, hardware anti-interference is the most basic and important anti-interference measure. It generally starts from two aspects: resistance and prevention to suppress and eliminate interference sources. In system design, corresponding reliability measures should be taken to eliminate or suppress and reduce the impact of interference, ensure the normal operation of the system, and improve the reliability of the system.
1. Measures to handle power supply interference
Power supply interference is mainly generated through impedance coupling in the power supply line, with various high-power electrical devices being the primary sources of interference. In applications with strong interference or high reliability requirements, a shielded isolation transformer and a low-pass filter can be connected to the AC power input terminal of the PLC. The isolation transformer can suppress external interference entering from the power line and improve the immunity to high-frequency common-mode interference. The shielding layer should be reliably grounded.
Low-pass filters can absorb most of the "glitches" in the power supply. L1 and L2 in the diagram suppress high-frequency differential-mode voltage. L3 and L4 use equal-length wires wound in opposite directions on the same magnetic ring. The magnetic flux generated by the 50Hz power frequency current in the magnetic ring cancels each other out, preventing the ring from saturating. The magnetic flux generated by the common-mode interference current in the two wires in the magnetic ring is superimposed, and the common-mode interference is blocked by L3 and L4 . C1 and C2 in the diagram are used to filter out common-mode interference voltage, and C3 is used to filter out differential-mode interference voltage. R is a varistor, whose breakdown voltage is slightly higher than the highest voltage during normal operation of the power supply, normally functioning as an open circuit. When encountering a spike interference pulse, it breaks down, and the interference voltage is clamped by the varistor. At this time, the voltage across the varistor equals its breakdown voltage. After the spike pulse disappears, the varistor returns to normal operation.
High-frequency interference signals are not transmitted through coupling between transformer windings, but rather through the distributed capacitance between the primary and secondary windings. Adding a shielding layer between the primary and secondary windings and grounding it along with the core can reduce the distributed capacitance between the windings and improve the ability to resist high-frequency interference. Alternatively, power supply filters can be used, as they have excellent common-mode filtering, differential-mode filtering, and high-frequency interference suppression capabilities, effectively suppressing interference between lines and between lines and ground.
2. Handling measures for inductive loads
Inductive loads have an energy storage function. When the control contacts open, the inductive load in the circuit will generate a back electromotive force several times or even tens of times higher than the power supply voltage. When the contacts close, an electric arc will be generated due to contact bounce, both of which will interfere with the system. The following measures can be taken to address this: When inductive components are connected to the input or output terminals of the PLC, for DC circuits (Figure 2 ), a freewheeling diode should be connected in parallel across them; for AC circuits, a resistor-capacitor circuit should be connected in parallel to suppress the impact of the electric arc generated when the circuit is opened on the PLC . The resistor can be 51~120 Ω, and the capacitor can be 0.1~0.47 μF . The rated voltage of the capacitor should be greater than the peak voltage of the power supply. A 1A freewheeling diode can be selected, and its rated voltage should be 2~3 times greater than the power supply voltage.
3. Measures for installation and wiring
Switching signals generally do not have strict requirements for signal cables; ordinary cables can be used. For longer signal transmission distances, shielded cables can be used. Analog signals and high-speed signals (such as signals from pulse sensors, counting encoders, etc.) should be supplied with shielded cables. Communication cables have high reliability requirements, and some communication cables have very high signal frequencies. Specialized cables (such as fiber optic cables) should generally be used. However, for less demanding requirements or lower signal frequencies, shielded multi-core cables or twisted-pair cables can also be used.
When digital I/O lines cannot be routed separately from power lines, relays can be used to isolate interference on the input / output lines. When signal line distances exceed 300m , intermediate relays should be used to convert signals, or a remote I/O module of the PLC should be used. I/O lines and power lines should be routed separately and kept at a certain distance. If it is unavoidable to route them in the same cable tray, shielded cables should be used. AC lines and DC lines should use different cables; digital and analog I/O lines should be laid separately, and the latter should use shielded cables. If analog input / output signals are far from the PLC , a current transmission method of 4~20mA or 0~10mA should be used, rather than a voltage transmission method that is susceptible to interference. Different signal lines should preferably not be connected to the same connector. If the same connector must be used, they should be separated with spare terminals or ground terminals to reduce mutual interference.
4. Isolation measures that severely disrupt the environment
PLCs internally use optocouplers, small relays in output modules, and photothyristors to isolate external switching signals. PLC analog I/O modules also typically use optocouplers for isolation. These devices not only reduce or eliminate the impact of external interference on the system but also protect the CPU module from high voltage surges entering the PLC . Therefore, it is generally unnecessary to install external interference isolation devices on the PLC . To improve anti-interference capabilities and lightning protection, the serial communication line between the PLC and the computer can consider using fiber optics or a communication interface with an optocoupler.
5. Grounding reliability measures
Grounding is one of the effective means to improve the electromagnetic compatibility of electronic equipment. The grounding wires of a PLC control system include system ground, shield ground, AC ground, and protective ground. The interference of a disordered grounding system to a PLC system is mainly due to the uneven potential distribution of various grounding points and the existence of ground potential differences between different grounding points, which causes ground loop currents and affects the normal operation of logic circuits and analog circuits in the control system.
A robust grounding system is one of the key measures for PLC control systems to resist electromagnetic interference. When the signal source is grounded, the shielding layer should be grounded on the signal side; when it is not grounded, it should be grounded on the PLC side. When there is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated to avoid multiple grounding points. When shielded twisted-pair cables for multiple measurement point signals are connected to multi-core twisted-pair shielded cables, each shielding layer should be interconnected and insulated.
When grounding, the following should be noted: the grounding wire should be as thick as possible, generally using copper core wire with a diameter greater than 2mm²; the grounding point should be as close as possible to the PLC controller; the grounding wire should avoid high-voltage circuits and main circuit wires as much as possible. If it cannot be avoided, the wires should intersect perpendicularly to minimize the length of parallel lines.
6. Reliability measures for output
When the required output power of the load exceeds the allowable value of the PLC, an external relay should be installed. The small relays inside the PLC output module have small contacts and poor arc breaking capability, and generally cannot be used directly in DC 220V circuits. An external relay must be driven by the PLC , and the contacts of the external relay are used to drive the DC 220V load to improve reliability.
In summary, interference with PLC control systems is a very complex issue. It involves not only specific input/output devices and the industrial environment, but also factors such as temperature, humidity, vibration, and shock, all of which can significantly impact the PLC . Therefore, in application, identifying the problem, understanding the source of interference, and comprehensively considering various factors are essential to reasonably and effectively suppress interference and ensure the normal operation of the PLC control system.
