Interference types and sources of PLC operation and its 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 in the production site and 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 equipment, and on the other hand, application departments are required to pay great attention to engineering design, installation and construction, and use and maintenance. Only by cooperating with multiple parties can the problem be perfectly solved 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.

2. Main sources of electromagnetic interference

2.1 Radiation interference from space

Space radiated electromagnetic fields (EMI) are mainly generated by transient processes of power networks and 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 capability, 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.