Anti-interference and measures for programmable controller systems

PLC programmable controller (hereinafter referred to as PLC) is a device used for industrial production automation control. Although the manufacturer may take some measures to improve its stability, there are also many external factors that can cause interference, causing program errors or arithmetic errors, resulting in incorrect input and output, which will cause equipment to lose control and malfunction. To improve the stability of PLC control systems, on the one hand, it is required for PLC manufacturers to improve the anti-interference ability of their equipment; On the other hand, it is required to attach great importance to engineering design, installation, construction, and maintenance, and to cooperate with multiple parties in order to effectively solve problems and enhance the anti-interference performance of the system. And with the increasingly widespread application of PLC, its anti-interference problem is also becoming increasingly important. This article proposes some anti-interference measures for this issue.
Interference and its sources in control systems
1. The main sources and ways of interference in PLC systems
(1) Radiation interference from space
Radiated electromagnetic fields (EMI) in space are mainly generated by transient processes of power networks, electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., commonly known as radiation interference, and their distribution is complex. If the PLC system is placed in the RF field, radiation interference is recovered, and its impact mainly passes through two paths: first, it directly radiates inside the PLC, causing interference through circuit induction; Instead, it radiates the network within PLC communication, causing interference through the induction of communication lines. Radiation interference is related to the layout of on-site equipment and the magnitude of the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by setting shielded cables, PLC local shielding, and high-voltage relief components.
(2) Interference from external leads of the system
Mainly introduced through power and signal lines, commonly referred to as conducted interference. This interference is more severe in industrial sites in China.
Interference from power supply
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 subjected to spatial electromagnetic interference and induce voltage and circuits on the line. Especially the changes within the power grid, such as surges during switch operation, start-up and shutdown of large power equipment, harmonics caused by AC and DC transmission devices, and transient impacts of power grid short circuits, are all transmitted to the power source through transmission lines. PLC power supply usually adopts isolated power supply, but its isolation is not ideal due to its structure and manufacturing process factors. In fact, isolation is impossible due to the presence of distributed parameters, especially distributed capacitors.
Interference introduced by Line B
The various signal transmission lines connected to the PLC control system, in addition to transmitting effective information, will always have external interference signals invading. There are two main ways for this interference: firstly, it is caused by the power supply of the transmitter or the power supply of the shared signal instrument connected in series to the power grid, which is often overlooked; The second is that the signal line is interfered by the induction of spatial electromagnetic radiation, that is, external induced interference on the signal line, which is very serious. Introducing interference from signals can cause abnormal operation of I/O signals and greatly reduce measurement accuracy, and in severe cases, it can cause component damage. For systems with poor isolation performance, it will also lead to mutual interference between signals, causing backflow of the common ground system bus, causing logical data changes, misoperation, and crashes. The number of I/O module damage caused by signal interference in the PLC control system is quite serious, and there are also many cases of system failures caused by this.
Interference from chaotic grounding system
Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Proper grounding can not only suppress the impact of electromagnetic interference but also suppress external interference from equipment; However, incorrect grounding can actually introduce serious interference signals, making the PLC system unable to function properly.
The grounding wire of the PLC control system includes system grounding, shielding grounding, AC grounding, and protection grounding. The interference of the chaotic grounding system on the PLC system is mainly due to the uneven distribution of potential at various grounding points, and the existence of ground potential differences between different grounding points, causing ground loop current 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, there is a ground potential difference, and there is current flowing through the shielding layer. When abnormal conditions such as lightning strikes occur, the ground wire current will be greater.
2. Interference sources and general classification
The interference sources that affect the PLC control system, like those that affect industrial control equipment in general, are mostly generated in areas where the current or voltage changes sharply. These areas where the charges move violently are noise sources, i.e. interference sources.
The types of interference are usually divided according to the reasons for the interference, the noise interference mode, and the different waveform properties of the noise. Among them, according to the different causes of noise generation, it can be divided into discharge noise, surge noise, high-frequency oscillation noise, etc; According to the waveform and nature 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 commonly used classification methods. Common mode interference is the potential difference between a signal and ground, mainly formed by the superposition of the common state (in the same direction) voltage induced by the grid connection, ground potential difference, and spatial electromagnetic radiation on the signal line. The common mode voltage can be converted into differential mode voltage through asymmetric circuits, directly affecting the measurement and control signal, causing component damage (which 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 between two signals, mainly formed by the coupling induction of spatial electromagnetic fields between signals and the conversion of common mode interference by unbalanced circuits. This interference is superimposed on the signal and directly affects the measurement and control accuracy.