1. Overview
With the development of science and technology, PLCs are increasingly widely used in industrial control. The reliability of PLC control systems directly affects the safe production and economic operation of industrial enterprises, and the system's anti-interference capability is crucial to the reliable operation of the entire system. Various types of PLCs used in automation systems are installed either centrally in the control room or on production sites and various motor equipment. They are mostly located in harsh electromagnetic environments created by high-voltage circuits and equipment. To improve the reliability of PLC control systems, designers must understand various interferences in advance to effectively ensure reliable system operation.
2. Sources of electromagnetic interference and their interference to the system
Interference affecting PLC control systems originates from the same sources as interference affecting general industrial control equipment. They mostly occur in areas where current or voltage changes drastically. These areas where charge moves drastically are noise sources, i.e., interference sources.
Interference types are typically classified according to their causes, interference modes, and waveform characteristics. Specifically: based on the cause, they are classified as discharge noise, surge noise, and high-frequency oscillation noise; based on waveform and characteristics, they are classified as continuous noise and intermittent noise; and based on the mode, they are classified as common-mode interference and differential-mode interference. Common-mode interference and differential-mode interference are commonly used classification methods. Common-mode interference is the potential difference between the signal and the ground, mainly formed by the common-mode (same-direction) voltage induced on the signal line by grid interference, ground potential difference, and spatial electromagnetic radiation. Common-mode voltage can sometimes be large, especially in electrical power supply rooms with poor isolation performance, where the common-mode voltage of the transmitter output signal is generally high, sometimes exceeding 130V. Common-mode voltage can be converted into differential-mode voltage through asymmetrical circuits, directly affecting the measurement and control signals and causing component damage (this is the reason for the high failure rate of some system I/O modules). This common-mode interference can be DC or AC. Differential-mode interference refers to the interference voltage 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 voltage is directly superimposed on the signal and directly affects the measurement and control accuracy.
3. Main sources of electromagnetic interference in PLC control systems
(1) Radiation interference from space
Radiated electromagnetic interference (EMI) in space is mainly generated by power networks, transient processes of electrical equipment, lightning, radio broadcasts, television, radar, high-frequency induction heating equipment, etc., and is commonly referred to as radiated interference, the distribution of which is extremely complex. If a PLC system is placed within the radio frequency field, it will receive radiated interference, the effects of which are mainly through two paths: one is direct radiation into the PLC's internal circuitry, causing interference through circuit induction; the other is radiation into the PLC's communication network, introducing interference through communication line induction. Radiated interference is related to the layout of field equipment and the magnitude of the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by installing shielded cables, local shielding of the PLC, and high-voltage discharge components.
(2) Interference from external system leads
This interference is mainly introduced through power and signal lines and is commonly referred to as conducted interference. This type of interference is quite serious in industrial settings in my country.
(3) Interference from the power supply
Practice has shown that PLC control system failures are often caused by interference introduced by the power supply. I encountered this during the commissioning of a certain project, and the problem was only solved after replacing the PLC power supply with one that had higher isolation performance.
The PLC system is normally powered by the power grid. Because the power grid has a wide coverage area,
The circuit will be affected by all forms of electromagnetic interference, inducing voltage and disturbances on the lines. In particular, changes within the power grid, such as switching surges, starting and stopping of large electrical equipment, harmonics from AC/DC rotating devices, and transient impacts from power grid short circuits, all reach the power source through transmission lines. PLC power supplies typically employ isolated power supplies, but their isolation is not ideal due to their structure and manufacturing processes. In reality, absolute isolation is impossible due to the presence of distributed parameters, especially distributed capacitance.
(4) Interference introduced from signal lines
In addition to transmitting valid signals, various signal transmission lines connected to a PLC control system are always subject to external interference. This interference primarily occurs through two pathways: first, interference from the power supply of transmitters or shared signal instruments, which is often overlooked; second, interference induced by electromagnetic radiation from the surrounding environment, i.e., external induced interference on the signal lines, which is quite serious. Interference introduced by signals can cause abnormal I/O signal operation and a significant reduction in measurement accuracy, and in severe cases, damage to components. For systems with poor isolation performance, it can also lead to mutual interference between signals, causing backflow on the common ground bus, resulting in changes in logic data, malfunctions, and system crashes. The number of I/O module damages caused by signal-introduced interference in PLC control systems is quite high, and many system failures are also caused by this.
(5) Interference from a disordered grounding system
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; conversely, incorrect grounding can introduce serious interference signals, causing the PLC system to malfunction. The grounding wires of a PLC control system include system ground, shield ground, AC ground, and protective ground. The interference caused by a chaotic grounding system to the PLC system is mainly due to uneven potential distribution at various grounding points. Potential differences exist between different grounding points, causing ground loop currents and affecting the normal operation of the system. For example, the cable shielding layer must be grounded at one point. If both ends A and B of the cable shielding layer are grounded, a potential difference exists, and current flows through the shielding layer. When an abnormal condition occurs, such as a lightning strike, 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 may appear within the shielding layer, interfering with the signal loop through coupling between the shielding layer and the core wire. If the system grounding is inconsistent with other grounding methods, the resulting ground loop currents may create unequal potential distributions on the ground wire, affecting the normal operation of the logic and analog circuits within the PLC. PLCs have low logic voltage interference tolerance; interference from logic ground potential distributions can easily affect PLC logic operations and data storage, causing data corruption, program crashes, or system freezes. Analog ground potential distributions will lead to decreased measurement accuracy, causing serious distortion and malfunctions in signal measurement and control.
(6) Interference from within the PLC system
It is mainly generated by the mutual electromagnetic radiation between internal components and circuits of the system, such as logic circuits.
Mutual radiation and its impact on analog circuits, the mutual influence between analog ground and logic ground, and the mismatch between components are all aspects of the electromagnetic compatibility design within the PLC system by the manufacturer. These are quite complex and cannot be changed by the application department, so they can be disregarded. However, it is advisable to choose systems with extensive application experience or proven track records.
4. How can interference in PLC systems be resolved more effectively and simply ?
1) Selecting equipment with better isolation performance, using high-quality power supplies, and making more reasonable routing of power and signal lines can solve interference problems, but it is more complicated, difficult to operate, and more expensive.
2) Use signal isolators to solve interference problems. Simply add such a product between the input and output terminals at the location of interference to effectively solve the interference problem.
5. Why are signal isolators the preferred solution for interference in PLC systems ?
1) It is simple, convenient, reliable, and inexpensive to use.
2) It can greatly reduce the workload of designers and system debugging personnel. Even complex systems will become very simple and reliable in the hands of ordinary designers.
6. Working principle of signal isolators
First, the signal received by the PLC is modulated and transformed by semiconductor devices, then isolated and converted by optical or magnetic sensors, and finally demodulated and transformed back to the original signal or a different signal before isolation. Simultaneously, the power supply to the isolated signal is also isolated. This ensures absolute independence between the transformed signal, power supply, and ground.
7. Selection of Isolators
Since isolators are located between two system channels, the first step in selecting an isolator is to determine its input/output functions, while ensuring that the isolator's input/output modes (voltage type, current type, loop-powered type, etc.) are compatible with the interface modes of the front and back channels. In addition, many other important parameters affect product performance, such as accuracy, power consumption, noise, insulation strength, and bus communication capabilities. For example, noise is related to accuracy, and power consumption and heat are related to reliability; these require careful selection by the user. In short, suitability, reliability, and cost-effectiveness are the main principles for selecting isolators.
