I. Overview
In automated systems, various types of PLCs are used; some are centrally installed in control rooms, while others are located at production sites and on machinery, often operating in harsh electromagnetic environments due to high-voltage circuits and equipment. Enhancing the reliability of a PLC control system requires not only that manufacturers improve the interference resistance of their equipment but also that engineering design, installation, construction, and maintenance are given serious consideration to collaboratively solve these issues, effectively enhancing the system's resistance to electromagnetic interference.
II. Electromagnetic Interference Sources and Their Impact on Systems
1. Classification of Interference Sources and Types of Interference
Interference sources affecting PLC control systems are similar to those impacting other industrial control devices, primarily originating from areas where current or voltage changes drastically. These areas of charge movement are the noise sources, or interference sources.
Interference types are classified based on the cause of noise, noise interference modes, and waveform characteristics. By the cause of noise, there are discharge noise, surge noise, and high-frequency oscillation noise; by waveform and nature, there are continuous noise and occasional noise; by interference mode, there are common-mode and differential-mode interferences. Common-mode and differential-mode are common classifications. Common-mode interference is the potential difference between the signal and ground, mainly caused by grid intrusion, ground potential differences, and electromagnetic radiation from space inducing common-mode (same direction) voltage on signal lines. Common-mode voltage can be quite high, especially in poorly isolated power supply rooms, where the common-mode voltage of transmitter output signals can exceed 130V. This voltage can convert into differential-mode voltage through asymmetric circuits, directly affecting control signals, potentially damaging components (this is why some systems experience high I/O module failure rates), with common-mode interference being either DC or AC. Differential-mode interference refers to interference voltage between the two poles of a signal, mainly from space electromagnetic fields coupling between signals and from common-mode interference converted by unbalanced circuits, directly impacting measurement and control accuracy.
2. Main Sources of Electromagnetic Interference in PLC Control Systems
2.1 Interference from Space Radiation
Space electromagnetic fields (EMI) are mainly produced by power networks, transient processes of electrical equipment, lightning, radio broadcasts, television, radar, high-frequency induction heating equipment, etc., known as radiation interference, which has a very complex distribution. If a PLC system is within this radiated field, it will receive interference through two main paths: direct radiation to the internal circuits of the PLC, inducing interference; or radiation to the communication network within the PLC, introducing interference through the communication lines. Radiation interference depends on the layout of site equipment and the strength, particularly the frequency, of the electromagnetic field generated. Protection is usually achieved by using shielded cables, local shielding around the PLC, and high-voltage discharge components.
2.2 Interference from External Wiring
This primarily enters through power and signal lines, known as conducted interference, which is quite severe in Chinese industrial settings.
- (1) Interference from Power Supply
Experience shows that many PLC control system failures are due to interference introduced via the power supply. In one project, switching to a power supply with better isolation resolved the issue.
The normal power supply for PLC systems comes from the grid, which, due to its wide coverage, is susceptible to all kinds of spatial electromagnetic interference, inducing voltage and current in the lines. Changes within the grid, like switch operations surges, starting/stopping of large power equipment, harmonics caused by AC/DC drive units, and transient shocks from grid short circuits, all transmit to the primary side of the power supply through transmission lines. Although PLC power supplies often use isolated power sources, the construction and manufacturing processes might not provide ideal isolation. In reality, due to distributed parameters, especially distributed capacitance, absolute isolation is unattainable.
- (2) Interference through Signal Lines
Besides transmitting valid information, various signal lines connected to PLC control systems can introduce external interference signals. This interference mainly comes from two sources: one through the power supply for transmitters or shared signal instruments, often overlooked; the other from space electromagnetic radiation inducing interference on signal lines, which is quite severe. Interference introduced by signals can cause abnormal operation of I/O signals, significantly reducing measurement accuracy, and in severe cases, damaging components. For systems with poor isolation, this can lead to signal cross-talk, causing ground loop currents in common-ground systems, resulting in logic data changes, false operations, and system crashes. The damage to I/O modules due to signal-induced interference in PLC control systems is significant, leading to numerous system failures.
- (3) Interference When Grounding System is Chaotic
Grounding is one effective means to improve electromagnetic compatibility (EMC) of electronic devices. Correct grounding can suppress electromagnetic interference and prevent equipment from radiating interference outward; however, improper grounding can introduce severe interference signals, rendering the PLC system unable to work normally.
The grounding in a PLC control system includes system ground, shield ground, AC ground, and protective ground. The interference from a chaotic grounding system mainly results from uneven potential distribution across different ground points, causing ground potential differences and ground loop currents, which affect system operation. For instance, cable shielding should be grounded at one point; if both ends A and B of a cable shield are grounded, a ground potential difference exists, leading to current flow through the shield. During abnormal conditions like lightning strikes, the ground current can increase dramatically.
Moreover, the shield, ground wire, and earth can form a closed loop, where under changing magnetic fields, induced currents might occur within the shield, coupling through the shield-core to interfere with signal circuits. If system ground and other grounds are mishandled, the resulting ground loops can cause unequal potential distribution on ground lines, affecting the normal operation of logic and analog circuits within the PLC. The logic voltage tolerance in PLCs is low; disturbances in logic ground potential can easily affect logic operations, data storage, causing data corruption, program crashes, or system freezes. Uneven analog ground potential distribution can degrade measurement accuracy, leading to significant signal distortion and false operations.
2.3 Interference from Within the PLC System
This interference mainly arises from electromagnetic radiation between components and circuits within the system, like mutual radiation between logic circuits and their impact on analog circuits, interactions between analog and logic grounds, and mismatches between components. These fall under the manufacturer's responsibility for internal EMC design, which is complex and not usually modifiable by application departments, but selecting systems with proven performance or that have been tested is advisable.
III. Anti-Interference Design in PLC Control System Engineering Applications
To ensure the system is immune or less susceptible to electromagnetic interference in industrial environments, three strategies should be implemented from the design phase: suppress interference sources; cut off or attenuate electromagnetic interference propagation paths; enhance the interference resistance of devices and systems. These are the basic principles of electromagnetic interference suppression.
Anti-interference in PLC control systems is a systemic engineering task, requiring manufacturers to produce products with strong interference resistance, and users to consider this comprehensively during system design, installation, and maintenance, tailoring the design to specific conditions to ensure electromagnetic compatibility and operational reliability. When designing anti-interference measures for specific projects, focus on:
1. Equipment Selection
When choosing equipment, prioritize products with high interference resistance, including electromagnetic compatibility (EMC), especially against external interference, like systems with floating ground technology or good isolation performance. Also, consider the manufacturer's given interference suppression specifications, such as common-mode rejection ratio, differential-mode rejection ratio, withstand voltage capability, and the ability to operate under certain electric and magnetic field strengths. Additionally, look at the product's track record in similar applications. When selecting foreign products, remember that China uses a 220V high-impedance grid, while North America uses a 110V low-impedance grid. Due to the higher impedance in China, there's greater zero-point potential drift and ground potential variation, leading to electromagnetic interference at industrial sites being at least four times higher than in North America, thus demanding higher interference resistance from systems. Foreign PLCs that work well might not perform reliably in China, so choose based on Chinese standards (GB/T13926).
2. Comprehensive Anti-Interference Design
This mainly addresses external interference with several suppression measures, including:
- Shielding PLC systems and external wiring to protect against space electromagnetic radiation.
- Isolating and filtering external wiring, particularly power cables, and organizing them in layers to prevent conducted electromagnetic interference.
- Correctly designing grounding points and systems.
Software methods should also be used to enhance system safety and reliability.
IV. Key Anti-Interference Measures
1. Using High-Quality Power Supplies to Suppress Grid-Induced Interference
In PLC control systems, the power supply plays a crucial role. Grid interference enters the system mainly through the power supplies for the CPU, I/O, and devices like transmitters and instruments with direct electrical connections. While isolation for PLC system power supplies is common, the same attention isn't always given to transmitter and shared instrument power supplies. These often lack sufficient isolation due to large distributed parameters in transformers, leading to common-mode and differential-mode interference. Choose power supplies for transmitters and shared instruments with low distributed capacitance and high suppression bandwidth (using multiple isolations, shielding, and leakage inductance techniques) to reduce interference in the PLC system.
Moreover, to ensure uninterrupted power, an online UPS can be used, enhancing power supply reliability, with the added benefit of strong interference isolation, making it an ideal power source for PLC control systems.
2. Cable Selection and Layout
To minimize electromagnetic interference from power cables, especially those feeding variable frequency drives, using copper-armored shielded power cables can significantly reduce interference, as demonstrated in one project where this approach yielded satisfactory results.
Different signals should be transmitted via different cables, with signal cables organized by signal type in layers. Never use the same cable for both power and signal transmission, and avoid running signal lines parallel to power cables to reduce electromagnetic interference.
3. Hardware Filtering and Software Anti-Interference Measures
Connect capacitors between signal lines and ground to reduce common-mode interference; add filters between signal poles to decrease differential-mode interference.
Given the complexity of electromagnetic interference, completely eliminating it is impractical, so software design for PLC systems should include anti-interference measures for enhanced reliability, like digital filtering, mains shaping sampling for periodic interference removal, periodic correction of reference point potential with dynamic zero to prevent drift, implementing information redundancy, setting software flags, and using indirect jumps with software traps to improve software reliability.
4. Correct Grounding Point Selection and Grounding System Improvement
Grounding serves two purposes: safety and interference suppression. An effective grounding system is crucial for anti-interference in PLC control systems.
Grounding methods include floating, direct, and capacitive grounding. For PLC control systems, direct grounding is preferred due to their nature as high-speed, low-voltage control devices. Due to signal cable distributed capacitance and input device filtering, signal exchange frequencies are generally below 1MHz, so grounding for PLC systems should use single-point or series single-point grounding. For centralized PLC setups, parallel single-point grounding is suitable, with each unit's cabinet center grounding point connected to a dedicated ground rod. For widely spaced units, series single-point grounding is better, with a large cross-section copper bus (or insulated cable) connecting cabinet grounding points to a single ground rod. Use copper wires over 22mm² for grounding leads, total busbars over 60mm², with grounding resistance less than 2Ω, ideally placed 10-15m away from buildings, and PLC grounding points should be more than 10m from high-voltage equipment grounding points.
When signal sources are grounded, shield grounding should be at the signal side; if not, at the PLC side. For signal lines with joints, ensure the shield is securely connected and insulated, avoiding multiple grounding points. For multiple measurement points, connect the shields of twisted pair cables together, insulate, and select an appropriate grounding point.
V. Conclusion
Interference in PLC control systems is a complex issue, requiring comprehensive consideration in anti-interference design. Effective suppression requires analyzing specific interference scenarios and applying targeted solutions to ensure the normal operation of PLC control systems.