I. Overview
The various types of PLCs utilized in automation systems are either centrally installed in control rooms or distributed across production sites and motor equipment. Most of these PLCs operate in harsh electromagnetic environments created by high-voltage circuits and equipment. To enhance the reliability of PLC control systems, two primary aspects require attention: firstly, PLC manufacturers must improve the equipment's anti-interference capabilities; secondly, significant emphasis must be placed on engineering design, installation, construction, operation, and maintenance, with multi-faceted cooperation to comprehensively address issues and effectively bolster the system's anti-interference performance.
II. Electromagnetic Interference Sources and Their Impact on Systems
1. Interference Sources and General Classification
The interference sources affecting PLC control systems are similar to those impacting general industrial control equipment. These primarily originate from locations where current or voltage undergoes significant fluctuations, where rapid charge movements create noise sources, i.e., interference sources.
Interference types are typically categorized based on their causes, noise interference modes, and waveform properties. According to their causes, they can be divided into discharge noise, surge noise, high-frequency oscillation noise, etc. Based on waveform and properties, they can be classified as continuous noise and sporadic noise. Additionally, they are categorized as common-mode interference and differential-mode interference based on interference modes. Common-mode and differential-mode interference are commonly used classifications. Common-mode interference refers to the potential difference between the signal and ground, primarily caused by grid penetration, ground potential differences, and common-mode (same-direction) voltage superposition induced on signal lines by spatial electromagnetic radiation. Common-mode voltages can be substantial, especially in power supply rooms with poorly isolated power distributors, where transducer output signals often exhibit high common-mode voltages, sometimes exceeding 130V. Common-mode voltages can convert into differential-mode voltages through asymmetric circuits, directly affecting measurement and control signals, potentially damaging components (a primary reason for high I/O module failure rates in some systems). This common-mode interference can be either DC or AC. Differential-mode interference refers to the interference voltage acting between the two poles of a signal, mainly induced by spatial electromagnetic field coupling between signals and converted from common-mode interference by unbalanced circuits. It directly superimposes on signals, directly impacting measurement and control accuracy.
2. Main Sources of Electromagnetic Interference in PLC Control Systems
2.1 Radiation Interference from Space
Electromagnetic interference in space (EMI) primarily originates from power networks, transient processes in electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., commonly known as radiation interference, with complex distribution. If a PLC system is placed within the radio frequency field, it will receive radiation interference, which primarily affects the system through two paths: direct radiation within the PLC internals, causing interference through circuit induction; and radiation towards the PLC communication network, introducing interference through communication line induction. Radiation interference is related to the layout of field equipment, the electromagnetic field generated, and particularly its frequency. Typically, shielding cables, local PLC shielding, and high-voltage discharge components are employed for protection.
2.2 Interference from External Leads of the System
This primarily enters through power and signal lines, commonly referred to as conducted interference, which is severe in Chinese industrial settings.
(1) Interference from Power Supply
Practical experience demonstrates that power supply-induced interference frequently causes PLC control system failures. The author encountered such an issue during a project commissioning, which was resolved by replacing the PLC power supply with one featuring higher isolation performance.
PLC systems are typically powered by the grid, which covers a vast area and is susceptible to all forms of spatial electromagnetic interference, inducing voltages and currents on lines. In particular, internal grid changes, such as switching surges, startups and shutdowns of large electrical equipment, harmonics generated by AC/DC drive devices, and grid short-circuit transients, are transmitted to the primary side of the power supply through transmission lines. Although PLC power supplies often employ isolation transformers, their isolation is imperfect due to factors like construction and manufacturing processes. Absolute isolation is impractical due to the presence of distributed parameters, particularly distributed capacitance.
(2) Interference from Signal Lines
Signal transmission lines connected to PLC control systems, besides transmitting valid information, are also prone to external interference signal intrusion. This interference primarily enters through two pathways: grid interference coupled into the transducer power supply or shared signal instrument power supply, often overlooked; and spatial electromagnetic radiation induction on signal lines, known as external induction interference on signal lines, which can be severe. Signal-induced interference can cause abnormal I/O signal operation, significantly reducing measurement accuracy, and in severe cases, damaging components. For systems with poor isolation performance, it can also lead to mutual interference between signals, causing ground loop currents in common ground system buses, resulting in logical data changes, misoperations, and system crashes. PLC control systems frequently experience significant I/O module damage due to signal-induced interference, causing numerous system failures.
(3) Interference from Chaotic Grounding Systems
Proper grounding is an effective means of enhancing the electromagnetic compatibility (EMC) of electronic equipment. Correct grounding can both suppress electromagnetic interference and prevent the equipment from emitting interference. Conversely, improper grounding can introduce severe interference signals, rendering the PLC system inoperable.
The grounding system of PLC control systems includes system grounds, shield grounds, AC grounds, and protective grounds. Chaotic grounding systems primarily interfere with PLC systems by causing uneven potential distribution among grounding points, resulting in ground potential differences between different grounding points, which generate ground loop currents and affect system operation. For example, cable shields must be grounded at a single point; if both ends A and B of a cable shield are grounded, a ground potential difference will exist, allowing current to flow through the shield. In abnormal conditions like lightning strikes, the ground current will be even larger.
Furthermore, the shield, grounding wire, and earth can potentially form a closed loop, and under the influence of a changing magnetic field, induced currents can arise within the shield, interfering with signal loops through coupling between the shield and core wires. If the system ground is mixed with other grounding treatments, the resulting ground loop currents can create unequal potential distributions on the ground wire, affecting the normal operation of logical and analog circuits within the PLC. PLCs have low tolerance for logical voltage interference, and disturbances in logical ground potential can easily impact PLC logic operations and data storage, leading to data corruption, program runaway, or system crashes. Analog ground potential disturbances will degrade measurement accuracy, causing severe distortion and misoperations in signal measurement and control.
2.3 Internal Interference in PLC Systems
This primarily arises from mutual electromagnetic radiation between internal components and circuits, such as radiation between logic circuits and their impact on analog circuits, interactions between analog and logic grounds, and mismatched component usage. These belong to the PLC manufacturer's electromagnetic compatibility design within the system, which is complex and unalterable by application departments. Therefore, excessive consideration is unnecessary, but selecting systems with proven track records or extensive testing is recommended.
III. Anti-Interference Design for PLC Control System Engineering Applications
To ensure that the system is protected from or minimizes internal and external electromagnetic interference (EMI) in industrial electromagnetic environments, three aspects of suppression measures must be implemented from the design stage: suppressing the sources of interference, cutting off or attenuating the transmission paths of EMI, and enhancing the anti-interference capabilities of the devices and systems. These three points constitute the fundamental principles for suppressing EMI.
The anti-interference of PLC control systems is a systematic project, requiring manufacturing units to design and produce products with strong anti-interference capabilities. Additionally, the comprehensive consideration of engineering design, installation, operation, and maintenance by the using departments, combined with specific situations for comprehensive design, is essential to ensure the electromagnetic compatibility and operational reliability of the system. When designing anti-interference measures for specific projects, the following two main aspects should be focused on:
- Equipment Selection
When selecting equipment, products with high anti-interference capabilities should be prioritized, including those with good electromagnetic compatibility (EMC), particularly resistance to external interference, such as PLC systems employing floating ground technology and excellent isolation performance. Furthermore, it is crucial to understand the anti-interference indicators provided by the manufacturer, such as common-mode rejection ratio, differential-mode rejection ratio, withstand voltage capability, and the maximum electric field intensity and magnetic field intensity environments in which the equipment can operate. Additionally, examining the equipment's performance in similar applications is essential. When selecting imported products, it's important to note that China uses a 220V high-internal-resistance power grid system, while Europe and the Americas use 110V low-internal-resistance grids. Due to the high internal resistance of China's power grid, resulting in large zero-point potential drift and ground potential variation, industrial sites in China experience at least four times more EMI than in Europe and America, necessitating stricter anti-interference performance requirements. PLC products that function normally abroad may not operate reliably in domestic industries. Therefore, when adopting foreign products, it is necessary to make rational selections based on China's standards (GB/T13926).
- Comprehensive Anti-Interference Design
This primarily involves considering several suppression measures from external sources. The main contents include: shielding the PLC system and external leads to prevent spatial radiation EMI; isolating and filtering external leads, especially power cables, to prevent the introduction of conducted EMI through them by layered arrangement; correctly designing grounding points and grounding devices to improve the grounding system. Furthermore, software measures must be utilized to further enhance the safety and reliability of the system.
IV. Main Anti-Interference Measures
- Employing High-Performance Power Supplies to Suppress Grid-Induced Interference
Power supplies play a crucial role in PLC control systems. Grid interference enters PLC control systems primarily through the power supplies (e.g., CPU power supply, I/O power supply) of the PLC system, transmitter power supplies, and instrument power supplies that have direct electrical connections with the PLC system. Currently, for PLC system power supplies, power supplies with good isolation performance are commonly used. However, less attention is paid to the power supplies for transmitters and instruments directly connected to the PLC system, despite some isolation measures being implemented. These measures are generally insufficient, mainly due to the use of isolation transformers with large distribution parameters and poor interference suppression capabilities, allowing common-mode and differential-mode interference to couple through the power supply. Therefore, for transmitters and shared signal instruments, power supply distributors with small distribution capacitance and large suppression bands (e.g., using multiple isolations, shielding, and leakage inductance techniques) should be selected to reduce interference in the PLC system.
Additionally, to ensure uninterrupted power supply, online uninterruptible power supplies (UPS) can be used to improve power supply safety and reliability. UPS also possesses strong interference isolation capabilities, making it an ideal power source for PLC control systems.
- Cable Selection and Layout
To reduce electromagnetic interference radiated by power cables, especially those feeding frequency converters, copper tape armored shielded power cables can be used, as demonstrated in a project where this measure effectively reduced EMI generated by power lines, yielding satisfactory results after commissioning.
Different types of signals should be transmitted through separate cables, and signal cables should be layered according to signal types. It is strictly prohibited to use different conductors within the same cable to simultaneously transmit power and signals. Signal lines should be kept away from parallel layouts with power cables to minimize EMI.
- Hardware Filtering and Software Anti-Interference Measures
Before signals are connected to the computer, capacitors can be connected in parallel between the signal line and ground to reduce common-mode interference. Additionally, filters installed between the two poles of the signal can reduce differential-mode interference.
Due to the complexity of EMI, it is impossible to eliminate all interference completely. Therefore, during software design and configuration of PLC control systems, anti-interference measures should also be implemented in software to further enhance system reliability. Commonly used measures include digital filtering and power frequency shaping sampling to effectively eliminate periodic interference; regularly calibrating reference point potentials and adopting dynamic zero points to prevent potential drift; employing information redundancy technology to design corresponding software flag bits; and improving software structure reliability through indirect jumps and setting software traps.
- Proper Selection of Grounding Points and Improvement of the Grounding System
Grounding serves two primary purposes: safety and interference suppression. A well-designed grounding system is one of the essential measures for PLC control systems to resist EMI.
There are three types of grounding methods: floating ground, direct ground, and capacitive ground. For PLC control systems, which are high-speed, low-level control devices, the direct grounding method should be adopted. Due to the influence of signal cable distributed capacitance and input device filtering, the signal exchange frequency between devices is generally below 1MHz. Therefore, the grounding of PLC control systems adopts single-point grounding or series-single-point grounding. Centrally arranged PLC systems are suitable for parallel single-point grounding, with each device's cabinet center grounding point connected to the grounding electrode via a separate grounding wire. If the device spacing is large, series-single-point grounding should be used, connecting the cabinet center grounding points of each device with a large-section copper busbar (or insulated cable), and then directly connecting the grounding busbar to the grounding electrode. The grounding wire should be a copper conductor with a cross-section greater than 22mm2, and the main busbar should be a copper bar with a cross-section greater than 60mm2. The grounding resistance of the grounding electrode should be less than 2Ω, and it is best buried more than 10-15m away from the building. Additionally, the grounding point of the PLC system must be at least 10m away from the grounding point of strong electrical equipment.
When grounding the signal source, the shielding layer should be grounded at the signal side; if not grounded, it should be grounded at the PLC side. If there are connectors in the middle of the signal line, the shielding layer should be securely connected and insulated to avoid multiple grounding points. When connecting the shielded twisted pairs of multiple measurement points to the multi-core twisted-pair shielded main cable, the shielding layers should be well connected and insulated, with a properly selected single grounding point.
V. Conclusion
Interference in PLC control systems is a highly complex issue. Therefore, in anti-interference design, various factors should be considered comprehensively to effectively suppress interference. Specific analysis and targeted solutions are required for some interference scenarios to ensure the normal operation of PLC control systems.