Interference sources and anti-interference measures for PLC control systems
I. Interference and its sources in PLC control systems
Electromagnetic interference in the field is one of the most common and easily detrimental factors to the reliability of PLC control systems. As the saying goes, to treat the symptoms, one must first address the root cause; only by identifying the problem can a solution be proposed. Therefore, it is essential to know the source of the field interference.
(1) Interference sources and general classification
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 electromagnetic waves are produced at high speeds. Electromagnetic interference is typically classified into common-mode interference and differential-mode interference based on its interference mode.
Common-mode interference is the potential difference between a signal and ground, mainly formed by the superposition of common-mode (same-direction) voltages induced on the signal line by grid interference, ground potential difference, and spatial electromagnetic radiation.
Common-mode voltage can be converted into differential-mode voltage through asymmetrical circuits, directly affecting the measurement and control signals and causing damage to components (this is the main reason why some system I/O modules have a high failure rate). This common-mode interference can be DC or AC.
Differential-mode interference refers to the interference voltage acting between two poles of a signal. It is mainly formed by the coupling induction of spatial electromagnetic fields between signals and the voltage formed by the conversion of common-mode interference by unbalanced circuits. This interference is superimposed on the signal and directly affects the measurement and control accuracy.
(2) Main sources and pathways of interference in PLC systems
strong electrical interference
The normal power supply for PLC systems is provided by the power grid. Due to the wide coverage of the power grid, it is susceptible to electromagnetic interference from all directions, which can induce voltage on the lines. In particular, changes within the power grid, such as surges from knife switch operations, the start-up and shutdown of large power equipment, harmonics caused by AC/DC drives, and transient impacts from power grid short circuits, are all transmitted to the primary side of the power source through the transmission lines.
cabinet interference
High-voltage electrical appliances, large inductive loads, and messy wiring inside the control cabinet can all easily cause some degree of interference to the PLC.
Interference introduced from signal lines
In addition to transmitting valid information, various signal transmission lines connected to the PLC control system are always subject to external interference signals.
There are two main pathways to this interference: one is grid interference introduced through the power supply of the transmitter or the power supply of the shared signal instrument, which is often overlooked; the other is interference induced by spatial electromagnetic radiation on the signal line, i.e., external induced interference on the signal line, which is very serious.
Interference introduced by signals can cause abnormal operation of I/O signals and a significant reduction in measurement accuracy, and in severe cases, it can damage components.
Interference from grounding system disorder
Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Proper grounding can suppress the effects of electromagnetic interference and prevent the equipment from emitting interference; while incorrect grounding can introduce serious interference signals, making the PLC system unable to work properly.
Interference from within the PLC system
It is mainly generated by the mutual electromagnetic radiation between internal components and circuits, such as the mutual radiation of logic circuits and its impact on analog circuits, the mutual influence between analog ground and logic ground, and the mismatch between components.
Inverter interference
First, the harmonics generated during the startup and operation of the frequency converter cause conducted interference to the power grid, resulting in voltage distortion and affecting the power supply quality. Second, the output of the frequency converter generates strong electromagnetic radiation interference, affecting the normal operation of surrounding equipment.
II. Main Anti-interference Measures of PLC Control System
(1) Proper handling of power supply to suppress interference introduced by the power grid.
To mitigate grid interference introduced by the power supply, a shielded isolation transformer with a 1:1 turns ratio can be installed to reduce interference between the equipment and ground. Alternatively, an LC filter circuit can be connected in series at the power input.
(2) Installation and wiring
● Power lines, control lines, and PLC power and I/O lines should be wired separately. The isolation transformer should be connected to the PLC and I/O lines using double-insulated wires. PLC I/O lines and high-power lines should be routed separately. If they must be in the same cable tray, AC and DC lines should be bundled separately. If conditions permit, separate cable trays are best, as this maximizes spatial distance and minimizes interference.
● The PLC should be kept away from strong interference sources such as welding machines, high-power silicon rectifiers, and large power equipment, and should not be installed in the same switch cabinet as high-voltage electrical appliances. Inside the cabinet, the PLC should be kept away from power lines (the distance between them should be greater than 200mm). Inductive loads installed in the same cabinet as the PLC, such as the coils of high-power relays and contactors, should be connected in parallel with an RC arc suppression circuit.
● PLC inputs and outputs should ideally be wired separately, and digital and analog signals should also be wired separately. Shielded cables should be used for transmitting analog signals, and the shielding layer should be grounded at one or both ends. The grounding resistance should be less than 1/10 of the shielding layer resistance.
● Do not use the same cable for AC output lines and DC output lines. Output lines should be kept as far away as possible from high-voltage lines and power lines, and should not run in parallel.
(3) Wiring of I/O terminals
Input wiring
● Input wiring should generally not be too long. However, if there is little environmental interference and the voltage drop is not significant, the input wiring can be appropriately longer.
● Input and output lines must not use the same cable; input and output lines must be separate.
●Use normally open contacts to connect to the input terminal whenever possible, so that the ladder diagram is consistent with the relay schematic diagram and easy to read.
Output connection
● Output wiring is divided into independent output and common output. Different types and voltage levels of output voltage can be used in different groups. However, outputs in the same group can only use the same type and voltage level of power supply.
●Since the output components of the PLC are encapsulated on a printed circuit board and connected to a terminal block, short-circuiting the load connected to the output components will burn out the printed circuit board.
●When using relay output, the size of the inductive load it bears will affect the lifespan of the relay. Therefore, when using inductive loads, appropriate selection should be made, or an isolation relay should be added.
● The output load of the PLC may generate interference, so measures must be taken to control it, such as freewheeling tube protection for DC output, RC snubber circuit for AC output, and bypass resistor protection for transistor and bidirectional thyristor output.
(4) Correctly select the grounding point and improve the grounding system.
Proper grounding is essential for ensuring the reliable operation of a PLC, preventing damage from accidental voltage surges. Grounding typically serves two purposes: safety and interference suppression. A well-designed grounding system is one of the key measures for PLC control systems to resist electromagnetic interference.
The grounding wires of a PLC control system include system ground, shield ground, AC ground, and protective ground. A chaotic grounding system interferes with the PLC system mainly because of uneven potential distribution at various grounding points. Potential differences exist between different grounding points, causing ground loop currents and affecting normal system operation. For example, a cable shield must be grounded at one point. If both ends (A and B) of the cable shield are grounded, a potential difference exists, and current flows through the shield. In abnormal conditions such as lightning strikes, the ground current will be even greater.
Furthermore, the shielding layer, grounding wire, and earth may form a closed loop. Under the influence of a changing magnetic field, induced currents will appear in the shielding layer, interfering with the signal loop through the coupling between the shielding layer and the core wire. If the system grounding is not handled properly, the resulting ground loop current may create unequal potential distributions on the ground wire, affecting the normal operation of the logic and analog circuits in the PLC.
PLCs have low tolerance for logic voltage interference. Distributed interference from logic ground potential can easily affect PLC logic operations and data storage, causing data corruption, program crashes, or system freezes. Simulated ground potential distribution will lead to decreased measurement accuracy, causing serious distortion and malfunctions in signal measurement and control.
●Safely grounded or power supply grounded
Connecting the power cord's grounding terminal to the cabinet's grounding point constitutes a safety ground. In the event of a power leak or the cabinet becoming energized, the current can be conducted to the ground through this safety ground, preventing harm to people.
●System grounding
The PLC controller is grounded to ensure it is at the same potential as all the devices it controls; this is called system grounding. The grounding resistance value must not exceed 4Ω. Generally, the PLC device system ground and the negative terminal of the switching power supply in the control cabinet should be connected together as the control system ground.
●Signal and shielding grounding
Generally, signal lines must have a unique reference ground. Shielded cables must also be uniquely grounded locally or in the control room in situations where conducted interference may occur, to prevent the formation of "ground loops." When the signal source is grounded, the shielding layer should be grounded on the signal side; if 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, and multiple grounding points must be avoided. When shielded twisted-pair cables for multiple measurement point signals are connected to a multi-core twisted-pair shielded cable, each shielding layer should be interconnected and insulated, and a suitable single-point grounding point should be selected.
(5) Suppression of inverter interference
Interference handling for frequency converters generally involves the following methods:
Adding an isolation transformer is mainly for dealing with conducted interference from the power source, and can block most of the conducted interference before it reaches the isolation transformer.
Using filters provides strong anti-interference capabilities and prevents interference from the device itself from being conducted to the power supply. Some filters also have peak voltage absorption functions.
Using an output reactor, adding an AC reactor between the frequency converter and the motor, is mainly to reduce the electromagnetic radiation generated by the frequency converter output during energy transmission, which could affect the normal operation of other equipment.
To improve the reliability of PLC control systems, PLC manufacturers need to enhance the anti-interference capabilities of their equipment. Furthermore, close attention must be paid to the design, installation, and maintenance processes; multi-party cooperation is essential to effectively address problems and strengthen the system's anti-interference performance.
