The Source of Electromagnetic Interference and Anti-interference Design of PLC Control System

With the development of industrial equipment automation control technology, programmable controllers (PLC) are increasingly used in industrial equipment control. The reliability of PLC control systems directly affects the safe production and economic operation of enterprises, and the anti-interference ability of the system is the key to the reliable operation of the entire system. This article introduces in detail the types and sources of interference that affect PLC operation, and proposes implementation strategies for anti-interference design.

Among the various types of PLCs used in automation systems, some are centrally installed in the control room, and some are installed at the production site and on various motor equipment. Most of them are in a harsh electromagnetic environment formed by strong electric circuits and strong electric equipment. To improve the reliability of PLC control systems, on the one hand, PLC manufacturers are required to improve the anti-interference ability of the equipment, and on the other hand, the application department is required to pay great attention to engineering design, installation and construction, and use and maintenance. Only with cooperation from all parties can the problem be solved perfectly and the anti-interference performance of the system be effectively enhanced.

1. Types of EMI and their impacts

The interference sources that affect PLC control systems are the same as those that generally affect industrial control equipment. Most of them are generated in areas where current or voltage changes dramatically. These areas where charges move dramatically are interference sources.

Interference types are usually divided according to the cause of interference, noise interference mode and noise waveform properties. According to the different causes of noise, it can be divided into discharge noise, surge noise, high-frequency oscillation noise, etc.; according to the different waveforms and properties of noise, it can be divided into continuous noise, occasional noise, etc.; according to different noise interference modes, it can be divided into common mode interference and differential mode interference.

Common-mode interference and differential-mode interference are a relatively common classification method. Common-mode interference is the potential difference between the signal and the ground, which is mainly formed by the superposition of the common-mode (same direction) voltage induced by the power grid series connection, the ground potential difference and the space electromagnetic radiation on the signal line. The common-mode voltage is sometimes large, especially when the power supply is a distributor with poor isolation performance. The common-mode voltage of the transmitter output signal is generally high, and some can be as high as 130V or more. The common-mode voltage can be converted into a differential-mode voltage through an asymmetric circuit, affecting the measurement and control signal and causing damage to components (this is the main reason for the high damage rate of some system I/O modules). This common-mode interference can be DC or AC. Differential-mode interference refers to the interference voltage acting between the two poles of the signal, which is mainly formed by the coupling induction of the space electromagnetic field between the signals and the conversion of the common-mode interference by the unbalanced circuit. This interference is superimposed on the signal and directly affects the measurement and control accuracy.

2. Main sources of electromagnetic interference

2.1 Radiation interference from space

Space radiated electromagnetic fields (EMI) are mainly generated by power networks, transient processes of electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., usually called radiation interference, and its distribution is extremely complex. If the PLC system is placed in its radio frequency field, it will be affected by radiation interference, and its impact is mainly through two paths: one is direct radiation to the inside of the PLC, which causes interference due to circuit induction; the other is radiation to the PLC communication network, which introduces interference due to communication line induction. Radiated interference is related to the layout of on-site equipment and the size of the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by setting shielded cables and PLC local shielding and high-voltage discharge components.

2.2 Interference from external leads of the system

It is mainly introduced through power and signal lines, usually called conducted interference. This kind of interference is more serious in my country's industrial sites, and there are mainly three types:

The first type is interference from the power supply. Practice has shown that there are many cases where PLC control system failures are caused by interference introduced by the power supply. The author encountered this during the commissioning of a certain project, and the problem was solved only after replacing the PLC power supply with higher isolation performance.

The normal power supply of the PLC system is supplied by the power grid. Due to the wide coverage of the power grid, it will be affected by all spatial electromagnetic interference and induce voltage and current on the line, especially the changes within the power grid, switching operation surges, start and stop of large power equipment, harmonics caused by AC and DC transmission devices, transient impact of power grid short circuit, etc., are all transmitted to the primary side of the power supply through the transmission line. PLC power supply usually uses isolated power supply, but its isolation is not ideal due to its structure and manufacturing process. In fact, due to the existence of distributed parameters, especially distributed capacitance, absolute isolation is impossible.

The second type is interference introduced from the signal line. In addition to transmitting effective information, all kinds of signal transmission lines connected to the PLC control system will always have external interference signals invading. There are two main ways of this interference: one is the interference of the power grid connected through the power supply of the transmitter or the power supply of the shared signal instrument, which is often ignored; the other is the interference of the signal line induced by electromagnetic radiation in space, that is, the external induction interference on the signal line, which is often very serious.

The interference introduced by the signal will cause the I/O signal to work abnormally and the measurement accuracy to be greatly reduced. In severe cases, it will cause damage to components. For systems with poor isolation performance, it will also cause mutual interference between signals, causing backflow of the common ground system bus, resulting in changes in logical data, malfunctions and crashes. The number of I/O modules damaged by the interference introduced by the signal in the PLC control system is quite serious, and there are many cases of system failures caused by this.

The third type is interference from a chaotic grounding system. Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Correct grounding can not only suppress the impact of electromagnetic interference, but also suppress the interference emitted by the equipment. On the contrary, incorrect grounding will introduce serious interference signals, making the PLC system unable to work properly.

The ground wire of the PLC control system includes system ground, shield ground, AC ground and protection ground. The interference of the chaotic grounding system on the PLC system is mainly due to the uneven distribution of potentials at each grounding point. There is a ground potential difference between different grounding points, which causes ground loop current and affects 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, there is a ground potential difference and current flows through the shielding layer. When an abnormal state occurs, such as a lightning strike, the ground wire current will be greater.

In addition, the shielding layer, grounding wire and the earth may form a closed loop. Under the action of the changing magnetic field, an induced current will appear in the shielding layer, which will interfere with the signal loop through the coupling between the shielding layer and the core wire. If the system ground and other grounding processes are messed up, the generated ground loop current may produce unequal potential distribution on the ground wire, affecting the normal operation of the logic circuit and analog circuit in the PLC. The logic voltage interference tolerance of the PLC is low, and the distribution interference of the logic ground potential can easily affect the logic operation and data storage of the PLC, causing data confusion, program runaway or crash. The distribution of the analog ground potential will lead to a decrease in measurement accuracy, causing serious distortion and malfunction of signal measurement and control.

2.3 Interference from within the PLC system

It is mainly caused by the mutual electromagnetic radiation between components and circuits within the system, such as mutual radiation between logic circuits, mutual influence between analog ground and logic ground, and mismatched use of components, etc. These are all part of the electromagnetic compatibility design of the system by PLC manufacturers, which is relatively complex and cannot be changed by the application department. You don’t need to consider it too much, but you should choose a system with more application performance or tested.

3. Anti-interference design

In order to ensure that the system is free from or reduces internal and external electromagnetic interference in the industrial electromagnetic environment, three suppression measures must be taken from the design stage: suppressing the interference source, cutting off or attenuating the propagation path of electromagnetic interference, and improving the anti-interference ability of the device and system. These three points are the basic principles of suppressing electromagnetic interference.

The anti-interference of PLC control system is a systematic project, which requires the manufacturing unit to design and produce products with strong anti-interference ability, and depends on the user department to give comprehensive consideration in engineering design, installation and construction, operation and maintenance, and to carry out comprehensive design in combination with specific conditions to ensure the electromagnetic compatibility and operational reliability of the system. When carrying out the anti-interference design of a specific project, the following two aspects should be paid attention to.

3.1 Equipment selection

When selecting equipment, first of all, we should choose products with high anti-interference ability, which includes electromagnetic compatibility, especially the ability to resist external interference, such as PLC systems that use floating ground technology and good isolation performance; secondly, we should also understand the anti-interference indicators given by the manufacturer, such as common mode rejection ratio, differential mode rejection ratio, withstand voltage, and the electric field strength and high frequency magnetic field strength environment allowed to work in; the other is to examine its application performance in similar work.

When choosing imported products, please note that my country uses a 220V high internal resistance power grid, while Europe and the United States use a 110V low internal resistance power grid. Due to the large internal resistance of my country's power grid, large zero potential drift, and large ground potential changes, the electromagnetic interference on the site of industrial enterprises is at least 4 times higher than that in Europe and the United States, and the system's anti-interference performance is required to be higher. PLC products that can work normally abroad may not be able to operate reliably in domestic industries. Therefore, when using foreign products, we must make reasonable choices according to my country's standards (GB/T13926).

3.2 Comprehensive anti-interference design

The main considerations are several suppression measures from outside the system, including: shielding the PLC system and external leads to prevent electromagnetic interference from space radiation; isolating and filtering the external leads, especially the power cables should be arranged in layers to prevent the introduction of conducted electromagnetic interference through the external leads; correctly designing the grounding points and grounding devices, and improving the grounding system. In addition, software must be used to further improve the safety and reliability of the system.

4. Main anti-interference measures

4.1 Use high-performance power supply to suppress interference introduced by the power grid

In the PLC control system, the power supply occupies an extremely important position. The power grid interference is mainly coupled into the PLC control system through the power supply of the PLC system (such as CPU power supply, I/O power supply, etc.), the power supply of the transmitter, and the power supply of the instrument with direct electrical connection to the PLC system. At present, the power supply of the PLC system generally uses a power supply with good isolation performance, while the power supply of the transmitter and the power supply of the instrument with direct electrical connection to the PLC system have not received enough attention. Although certain isolation measures have been taken, they are generally not enough. The main reason is that the isolation transformer used has large distributed parameters and poor interference suppression ability, and common mode interference and differential mode interference are coupled into the power supply. Therefore, for the power supply of the transmitter and the common signal instrument, a distributor with small distributed capacitance and large suppression band (such as multiple isolation and shielding and leakage inductance technology) should be selected to reduce the interference of the PLC system.

In addition, to ensure that the power supply from the power grid is not interrupted, an online uninterruptible power supply (UPS) can be used to improve the safety and reliability of power supply. Moreover, UPS also has strong interference isolation performance and is an ideal power supply for PLC control systems.

4.2 Correct selection and installation of cables

In order to reduce the radiated electromagnetic interference of power cables, especially the feeder cables of frequency converters, the author used copper tape armored shielded power cables in a certain project to reduce the electromagnetic interference generated by the power lines. The project achieved satisfactory results after it was put into production.

Different types of signals are transmitted by different cables. Signal cables should be laid in layers according to the type of transmitted signals. It is strictly forbidden to use different wires of the same cable to transmit power and signals at the same time. Avoid laying signal lines and power cables close to each other in parallel to reduce electromagnetic interference.

4.3 Hardware filtering and software anti-interference measures

Before the signal is connected to the computer, a capacitor is connected between the signal line and the ground to reduce common-mode interference; installing a filter between the two poles of the signal can reduce differential-mode interference.

Due to the complexity of electromagnetic interference, it is impossible to completely eliminate the interference. Therefore, when designing and configuring the software of the PLC control system, anti-interference processing should also be performed on the software to further improve the reliability of the system. Some commonly used measures to improve the reliability of the software structure include: digital filtering and power frequency shaping sampling, which can effectively eliminate periodic interference; regular correction of reference point potential and the use of dynamic zero point can prevent potential drift; the use of information redundancy technology and the design of corresponding software flags; the use of indirect jumps and the setting of software protection, etc.

4.4 Select the grounding point correctly and improve the grounding system.

There are usually two purposes for grounding: one is safety, and the other is to suppress interference. A perfect grounding system is one of the important measures for PLC control systems to resist electromagnetic interference.

There are three ways to ground the system: floating ground, direct grounding and capacitor grounding. For the PLC control system, it is a high-speed low-level control device, and the direct grounding method should be used. Due to the influence of the distributed capacitance of the signal cable and the filtering of the input device, the signal exchange frequency between the devices is generally lower than 1MHz, so the grounding wire of the PLC control system adopts one-point grounding and series one-point grounding. The centralized PLC system is suitable for the parallel one-point grounding method, and the center grounding point of the cabinet of each device is led to the grounding electrode with a separate grounding wire. If the distance between the devices is large, the series one-point grounding method should be adopted, and a large-section copper busbar (or insulated cable) should be used to connect the center grounding point of the cabinet of each device, and then the grounding busbar should be directly connected to the grounding electrode. The grounding wire uses a copper conductor with a cross-section greater than 22mm2, and the total busbar uses a copper bar with a cross-section greater than 60mm2. The grounding resistance of the grounding electrode is less than 2Ω, and the grounding electrode is preferably buried 10-15m away from the building, and the grounding point of the PLC system must be more than 10m away from the grounding point of the strong electric equipment.

When the signal source is grounded, the shielding layer should be grounded on the signal side; when 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 the shielded twisted pair cables of multiple measuring point signals are connected to the multi-core twisted pair shielded cable, the shielding layers should be connected to each other, insulated, and single-point grounding should be selected at an appropriate grounding location.

5. Conclusion

Interference in PLC control systems is a very complex issue. Therefore, various factors should be considered comprehensively in anti-interference design to suppress interference reasonably and effectively. Some interference situations require specific analysis and the right remedy should be adopted to enable the PLC control system to work normally and ensure the safe and efficient operation of industrial equipment.