Anti-interference Technology in Computer Control System

- Jan 17, 2019-

1. Overview of Anti-interference

Because the working environment of industrial field is often very bad, the computer control system is inevitably subject to a variety of interference. These disturbances may affect the accuracy of the measurement and control system, reduce the performance index of the system, reduce the reliability of the system, and even lead to chaotic operation or failure of the system, thus causing production accidents. Disturbance may come from the outside or from the inside; it acts on the control system through different ways, and its degree of action and consequences are related to the nature of the interference and the intensity of the interference and other factors. Interference exists objectively. To study anti-interference technology is to distinguish the sources of interference and explore measures to suppress or eliminate interference, so as to improve the reliability and stability of computer control system.

2. The Concept of Interference

Interference refers to noise other than useful signals or the destructive factors that cause computer equipment to fail to work properly. The cause of interference signal is called interference source. The device or system affected by the interference source through the propagation path is called the interference object. The source, route and object of interference constitute three factors of interference system. Anti-jamming technology is achieved by taking measures to one or more of these three elements. In order to effectively suppress and eliminate interference, we must first distinguish the source of interference, the way of transmission and the mode of interference.

3. Sources of interference

The sources of interference in computer control systems are various, sometimes even complicated. In general, according to the source, interference can be divided into internal interference and external interference. External interference is related to the environment and operating conditions of the system, and has nothing to do with the internal structure of the system. Internal interference is introduced by system structure layout and manufacturing process.

External interference: It has nothing to do with the system structure and is determined by the use conditions and external environment. External interference mainly includes: astronomical interference, such as interference wave caused by lightning or atmospheric ionization; celestial interference, such as electromagnetic wave radiated by the sun or other planets; electromagnetic wave interference emitted by surrounding electrical equipment; power frequency interference of power supply; interference caused by meteorological conditions, such as humidity and temperature; geomagnetic field interference; spark discharge, arc discharge, glow discharge and so on. Electromagnetic wave, etc.

Internal interference: is caused by the structural layout of the system, circuit design, changes in the nature of components and drifts, mainly: coupling induction caused by capacitance and distributed inductance; electromagnetic field radiation induction; wave reflection of long-line transmission; interference caused by potential difference caused by multi-point grounding; interference caused by parasitic oscillation and thermal noise, flicker noise, spike noise, etc. 

4. The Way of Dissemination of Interference

In the field of computer control system, there are many strong electric equipments, which will produce electromagnetic interference during starting and production. In addition, there are electromagnetic wave and lightning interference from space, as well as the influence of alternating magnetic field around high voltage transmission line. The main ways of interference propagation are electric field coupling, magnetic field coupling and common impedance coupling.

Electric field coupling: also known as electrostatic coupling, through capacitive coupling into other lines. Distributed capacitance will be formed between two wires. Distributed capacitance will be formed between printed wires on PCB, transformer turns and winding wires. The existence of these distributed capacitors can provide the interference reactance channel for the interference signal, and the electric field interference can enter the system from the channel to form interference.

Magnetic field coupling: The magnetic field will be generated around any current-carrying conductor. When the current changes, the alternating magnetic field will be generated. The magnetic field will inevitably cause interference in the closed circuit around it. It is through mutual inductance coupling between conductors. Within the equipment, magnetic leakage of coil or transformer will also cause interference; outside the equipment, two parallel wires will also cause interference. Electromagnetic radiation can cause magnetic coupling interference, such as electromagnetic waves propagating into space around a conductor when high-frequency current flows through it. These disturbances are easily coupled to the computer through power lines and long signal lines. In addition, long-line interference has antenna effect, that is, it can radiate and receive interference waves.

Common impedance coupling: Because the current flows through the common impedance between loops, the voltage drop caused by the current in one loop affects the other. Common coupling impedance exists in computer control systems, such as power leads, ground and common power lines on printed circuit boards, bus bars, etc. These buses all have certain impedance, which is common coupling impedance for multiple loops. When the current of digital signal flows through a larger current, it acts as an antenna, introducing interference into the circuits. At the same time, there is a capacitance between the reflux strips, through which the digital pulse can be coupled. The ground on the printed board is essentially a common return line. Because it still has a certain resistance, the signal coupling between the circuits is generated through it.

5. The mode of interference

According to the different modes of interference, interference can be divided into differential mode interference, common mode interference and long-line transmission interference.

Differential mode interference (DMI): Also known as series mode interference (SMI), is the interference in series and signal source circuit. It is connected in series in the signal source circuit and is added to the measured signal to input the system. Differential mode interference is mainly caused by electrostatic coupling of distributed capacitors, magnetic coupling in space, mutual inductance of long-line transmission, power frequency interference of 50HZ, and parameter variation of components in signal circuit.

Common-mode interference: The ground of the computer used for process control, the ground of the signal amplifier and the ground of the field signal source are usually separated by a distance of tens of meters or even hundreds of meters. There is often a potential difference between the two ground points. The interference of this potential difference to the amplifier is called common-mode interference, also known as common-mode interference or longitudinal interference. This kind of interference can be either direct current interference or alternating current interference, and its amplitude can be several volts or even higher, depending on the environmental conditions of the field interference and the grounding condition of the computer and other equipment.

Long-line transmission interference: In computer control system, long-line transmission is required for field signal to control computer and control computer to field executing mechanism. For high-speed signal transmission lines, that is, in high-frequency signal circuits, the length of the wire can be regarded as a long line, depending on the signal frequency of the circuit. In some cases, the line of about 1m should be regarded as a long line.

There are three problems in the long-line transmission of signals: first, the reflection of high-speed signals in the long-line transmission, second, the delay of signals, and third, the external interference in the long-line transmission.

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