Brief Analysis of PLC and DCS System Interference Problems and Solutions

1. Overview
With the development of science and technology, PLC is increasingly used in industrial control. The reliability of PLC control systems directly affects the safe production and economic operation of industrial enterprises. The anti-interference ability of the system is the key to the reliable operation of the entire system. Various types of PLCs used in automation systems are installed in the control room, or installed on the production site and various motor equipment. Most of them are in a harsh electromagnetic environment formed by high-voltage circuits and high-voltage equipment. To improve the reliability of PLC control systems, designers can only effectively ensure the reliable operation of the system if they understand various interferences in advance.

2. Electromagnetic interference sources and interference to the system
The interference that affects the PLC control system comes from the same source of interference that generally affects industrial control equipment. Most of them are generated in the parts where the current or voltage changes violently. The parts where these charges move violently are noise sources, that is, interference sources.
The types of interference are usually divided according to the causes of interference, the interference mode of the noise and the waveform properties of the noise. Among them: according to the different causes of noise, it is divided into discharge noise, surge noise, high-frequency oscillation noise, etc.; according to the waveform and nature of the noise, it is divided into continuous noise, occasional noise, etc.; according to the different sound interference modes, it is 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 common mode (same direction) voltage induced by the power grid series, ground potential difference and space electromagnetic radiation on the signal line. The common mode voltage is sometimes large, especially in the electrical power supply room 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. Common-mode voltage can be converted into differential-mode voltage through asymmetric circuits, directly affecting the measurement and control signals and causing damage to components (this is why some system I/O modules have a high damage rate). This common-mode interference can be DC or AC. Differential-mode interference refers to the interference voltage between the two poles of a signal, which is mainly caused by the coupling induction of the electromagnetic field between the signals and the voltage formed by the conversion of common-mode interference by the unbalanced circuit. This interference is directly superimposed on the signal, directly affecting the measurement and control accuracy.

3. What are the main sources of electromagnetic interference in PLC control systems?

(1) Radiation interference from space

The radiated electromagnetic field (EMI) in space is mainly generated by the transient process of power grid, electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., usually called radiated interference, and its distribution is extremely complex. If the PLC system is placed in the radio frequency field, radiated interference will be recovered, and its influence mainly passes through two paths; one is the direct radiation inside the PLC, which causes interference by circuit induction; the other is the radiation to the network inside the PLC communication, which introduces interference by the induction of the communication line. Radiated interference is related to the layout of the 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) Interference from external leads of the system is mainly introduced through power and signal lines, usually called

Conducted interference. This type of interference is more serious in my country's industrial sites.

(3) Interference from power supply

Practice has proved 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. The problem was solved 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 circuit on the line. In particular, changes within the power grid, switching operation surges, start-up and stop of large power equipment, harmonics caused by AC and DC rotating devices, transient impact of power grid short circuit, etc., all pass through the transmission line to the power supply side. PLC power supply usually uses isolated power supply, but its structure and manufacturing process factors make its isolation not ideal. In fact, due to the existence of distributed parameters, especially distributed capacitance, absolute isolation is impossible.

(4) Interference introduced from signal lines

In addition to transmitting effective signals, all kinds of signal transmission lines connected to the PLC control system will always have external interference signals intruding. 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 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 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, and in severe cases, it will cause damage to components. For systems with poor isolation performance, it will also cause mutual interference between signals, causing the common ground system bus to reflux, resulting in changes in logical data, malfunctions and crashes. The PLC control system has suffered serious damage to the I/O modules due to the interference introduced by the signal, and there are also many cases of system failures caused by this.

(5) Interference from 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; while incorrect grounding will introduce serious interference signals, making the PLC system unable to work properly. The ground wires of the PLC control system include system ground, shielding ground, AC ground and protective 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 and lightning strikes, 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 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 is likely to 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.

(6) 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 of logic circuits and its influence on analog 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 the PLC manufacturer, which is relatively complex and cannot be changed as an application department. You don’t need to consider it too much, but you should choose a system with more application performance or tested.

4. How can we solve PLC system interference better and more simply?

1) Selecting equipment with better isolation performance, selecting excellent power supplies, and more reasonable routing of power and signal lines can solve interference, but it is more cumbersome, difficult to operate, and more expensive.

2) Use signal isolators to solve interference problems. As long as this product is added between the input and output ends where there is interference, the interference problem can be effectively solved.

5. Why is the signal isolator the first choice to solve the interference of PLC system?

1) Simple, convenient, reliable and low cost.

2) It can greatly reduce the workload of designers and system debuggers, even if complex systems are

In the hands of ordinary designers, it will become very simple and reliable.

6. What is the working principle of the signal isolator?

First, the signal received by the PLC is modulated and transformed through semiconductor devices, then
isolated and converted through optical or magnetic induction devices, and then demodulated and transformed back to the original signal before isolation or a different signal, and the power supply of the isolated signal is isolated at the same time. Ensure that the converted signal, power supply, and ground are absolutely independent.

7. There are so many brands of isolators on the market now, with varying prices. How should we choose?

The isolator is located between two system channels, so when choosing an isolator, the input and output functions must be determined first, and the isolator input and output modes (voltage type, current type, loop power supply type, etc.) must be adapted to the front-end and back-end channel interface modes. In addition, there are many important parameters related to product performance, such as accuracy, power consumption, noise, insulation strength, bus communication function, etc. For example, noise is related to accuracy, power consumption and heat are related to reliability, and these require users to choose carefully. In short, applicability, reliability, and product cost performance are the main principles for selecting isolators.