A Programmable Logic Controller (PLC) is a type of programmable memory used to store programs and execute user-oriented instructions such as logic operations, sequential control, timing, counting, and arithmetic operations. It controls various types of machinery or production processes through digital or analog inputs/outputs. Its origins lie in 1968 when General Motors in the United States requested the replacement of relay control devices. Subsequently, in 1969, Digital Equipment Corporation (DEC) developed the first PLC, the PDP-14, which was successfully tested on General Motors' production line. This marked the first application of programmed methods to electrical control, and it is widely recognized as the world's first PLC. PLCs have now been in use for 46 years.
I. Overview
With the development of science and technology, PLCs (and related products) are increasingly widely used in industrial control. The reliability of PLC control systems directly affects the safe production and economic operation of industrial enterprises, and the system's anti-interference capability is crucial to the reliable operation of the entire system. Various types of PLCs used in automation systems are installed either centrally in the control room or on production sites and various motor equipment, mostly in harsh electromagnetic environments created by high-voltage circuits and equipment. To improve the reliability of PLC control systems, on the one hand, PLC manufacturers need to improve the anti-interference capability of their equipment; on the other hand, it requires high attention from engineering design, installation, construction, and maintenance, and multi-party cooperation is essential to effectively solve problems and enhance the system's anti-interference performance.
II . Sources of electromagnetic interference and their impact on the system
1. Interference sources and general classification of interference
The interference sources affecting PLC control systems are similar to those affecting general industrial control equipment; they mostly originate in areas where current or voltage changes drastically. These areas where charge moves drastically are the noise sources, or interference sources.
Interference types are typically classified according to their causes, noise interference modes, and waveform characteristics. Specifically: based on the cause, they are classified as discharge noise, surge noise, high-frequency oscillation noise, etc.; based on waveform and characteristics, they are classified as continuous noise, intermittent noise, etc.; and based on interference mode, they are classified as 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 the signal and ground, mainly formed by the superposition of common-mode (same-direction) voltages induced on the signal line by grid interference, ground potential difference, and spatial electromagnetic radiation. Common-mode voltage can sometimes be large, especially in power supply rooms using distribution equipment with poor isolation performance, where the common-mode voltage of the transmitter output signal is generally high, sometimes exceeding 130V. Common-mode voltage can be converted into differential-mode voltage through asymmetrical circuits, directly affecting the measurement and control signals and causing component damage (this is the main reason for the high failure 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 two poles of a signal. It is mainly formed by the coupling induction of spatial electromagnetic fields between signals and the voltage formed by the conversion of common-mode interference by unbalanced circuits. This voltage is directly superimposed on the signal and directly affects the measurement and control accuracy.
2. Main sources of electromagnetic interference in PLC control systems
(1) Radiative interference from space
Radiated electromagnetic interference (EMI) in space is mainly generated by power networks, transient processes of electrical equipment, lightning, radio broadcasts, television, radar, high-frequency induction heating equipment, etc., and is commonly referred to as radiated interference, the distribution of which is extremely complex. If a PLC system is placed within the radio frequency field, it will receive radiated interference, the effects of which are mainly through two paths: one is direct radiation into the PLC's internal circuitry, causing interference through circuit induction; the other is radiation into the PLC's communication network, introducing interference through communication line induction. Radiated interference is related to the layout of field equipment and the magnitude of the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by installing shielded cables and local shielding of the PLC, as well as high-voltage discharge components.
(2) Interference from external leads of the system
This interference is mainly introduced through power and signal lines and is commonly referred to as conducted interference. This type of interference is quite serious in industrial settings in my country.
Interference from the power supply
Practice has shown that PLC control system failures are often caused by interference introduced by the power supply. I encountered this during the commissioning of a certain project, and the problem was only solved after replacing the PLC power supply with one that had higher isolation performance.
PLC systems are normally powered by the power grid. Due to the grid's wide coverage, it is susceptible to electromagnetic interference from all directions, inducing voltage and circuits on the lines. In particular, changes within the power grid, such as surges from switch operations, the start-up and shutdown of large power equipment, harmonics from AC/DC drives, and transient impacts from grid short circuits, are all transmitted to the primary side of the power supply through the transmission lines. PLC power supplies typically employ isolated power supplies, but their isolation is not ideal due to their structure and manufacturing processes. In reality, absolute isolation is impossible due to the presence of distributed parameters, especially distributed capacitance.
