Programmable Logic Controllers ( PLCs ), as the mainstream control products in industrial automation, have been around for half a century. With the development of semiconductor, computer, and communication technologies, the field of industrial control has undergone tremendous changes. PLCs have experienced five generations of evolution in terms of performance, functionality, ease of use, and product form. Today, we'll discuss solutions to some of the challenges facing PLCs.
First, let's reiterate its definition:
It is a type of programmable memory used to store programs internally, execute user-oriented instructions such as logical operations, sequential control, timing, counting and arithmetic operations, and control various types of machinery or production processes through digital or analog input and output.
I. Anti-interference issues
With the development of science and technology, PLCs are increasingly widely used in industrial control. Their reliability 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. 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 to be paid to engineering design, installation, construction, and maintenance. Only through multi-party cooperation can problems be solved effectively and the system's anti-interference performance be enhanced.
[Interference Sources and General Classifications]
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 sources are usually classified according to the cause of interference, the noise interference mode, and the waveform properties of the noise.
1. According to the different causes of noise: discharge noise, surge noise, high-frequency oscillation noise
2. Based on the waveform and nature of the noise: continuous noise, intermittent noise.
3. According to different noise interference modes: common-mode interference, differential-mode interference
Common-mode interference and differential-mode interference are common classification methods. Common-mode interference is the potential difference between the transmitter and ground, mainly formed by the superposition of common-mode (same-direction) voltages induced on the signal lines by grid interference, ground potential difference, and electromagnetic radiation. Common-mode voltage can sometimes be quite large, especially in power supply rooms with poor isolation distribution equipment, 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 damage to components. 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.
Main sources of electromagnetic interference
1. Radiative interference from space
Radiated electromagnetic fields (EMI) in space are mainly generated by power networks, transient processes of electrical equipment, lightning, radio broadcasts, television, radar, high-frequency induction heating equipment, etc., and are commonly referred to as radiated interference.
The interference mainly occurs through two paths: 1) Direct radiation into the PLC's internal circuitry, which induces interference.
2) Radiation to the PLC communication network, interference introduced by induction in the communication lines.
Radiation interference is related to the layout of on-site equipment and the magnitude, especially the frequency, of the electromagnetic field generated by the equipment. It is generally protected by installing shielded cables, local shielding of PLCs, and high-voltage discharge components.
2. Interference from external system leads
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.
1) Interference from the power supply
Practice has shown that many PLC control system failures are caused by interference introduced by the power supply. The problem was only solved after replacing the PLC power supply with one that had higher isolation performance.
PLC power supplies typically employ isolated power supplies, but their isolation is not ideal due to their structure and manufacturing process. In reality, absolute isolation is impossible due to the presence of distributed parameters, especially distributed capacitance.
2) Interference introduced from signal lines
In addition to transmitting valid information, various signal transmission lines connected to the PLC control system are always subject to external interference signals.
There are two main ways this interference occurs: one is through grid interference introduced via the power supply of the transmitter or the power supply of the shared signal instrument, which is often overlooked.
Secondly, interference caused by electromagnetic radiation in space, i.e., external induced interference on the signal line, is very serious.
3) Interference from a disordered grounding system
Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Proper grounding can suppress the effects of electromagnetic interference and prevent the equipment from emitting interference; while incorrect grounding can introduce serious interference signals, making the PLC system unable to work properly.
The grounding wires of a PLC control system include system ground, shield ground, AC ground, and protective ground. A chaotic grounding system mainly interferes with the PLC system due to uneven potential distribution at various grounding points, creating potential differences between them, which causes ground loop currents and affects the normal operation of the system.
3. Interference from within the PLC system
The electromagnetic radiation is mainly generated by the mutual electromagnetic radiation between internal components and circuits, such as the mutual radiation between logic circuits and its impact on analog circuits, the mutual influence between analog ground and logic ground, and the mismatch between components. These are all part of the electromagnetic compatibility design of the PLC system by the manufacturer, which is quite complex and cannot be changed by the application department. Therefore, it is not advisable to consider them too much, but it is necessary to choose a system with a lot of application experience or that has been tested.
[Interference-resistant design]
1. Equipment Selection
When selecting equipment, the first priority should be to choose products with high anti-interference capabilities, including electromagnetic compatibility (EMC), especially the ability to resist external interference, such as PLC systems that use floating ground technology and have good isolation performance. Secondly, the anti-interference indicators provided by the manufacturer should be understood, such as common analog ratio, differential analog ratio, withstand voltage, and the maximum electric field strength and magnetic field strength allowed to be used in the environment. In addition, its application performance in similar work should be examined.
2. Comprehensive anti-interference design
The main considerations for suppressing electromagnetic interference from outside the system include: shielding the PLC system and external leads to prevent electromagnetic interference radiated from space; isolating and filtering external leads, especially the power cables, and layering them to prevent conducted electromagnetic interference from being introduced through external leads; properly designing grounding points and grounding devices, and improving the grounding system. In addition, software methods must be used to further improve the system's safety and reliability.
【Main Anti-interference Measures】
1. Use a high-performance power supply to suppress interference introduced by the power grid.
In PLC control systems, the power supply plays a crucial role. Interference from the power grid enters the PLC control system primarily through coupling with the PLC's power supply (such as the CPU power supply and I/O power supply), transmitter power supply, and the power supply of instruments directly electrically connected to the PLC. Currently, power supplies with good isolation performance are generally used for PLC systems. However, the power supplies for transmitters and instruments directly electrically connected to the PLC are not given sufficient attention. Although some isolation measures are implemented, they are generally inadequate, mainly due to the large distributed parameters of the isolation transformers used, resulting in poor interference suppression capabilities and the introduction of common-mode and differential-mode interference through power supply coupling. Therefore, for the power supplies of transmitters and shared signal instruments, power distribution units with small distributed capacitance and large suppression bands (such as those using multiple isolation and shielding techniques and leakage inductance technology) should be selected to reduce interference to the PLC system.
2. Cable selection considerations
Different types of signals are transmitted by different cables. Signal cables should be laid in layers according to the type of signal transmitted. It is strictly forbidden to transmit power and signals simultaneously by different conductors of the same cable. Signal lines and power cables should not be laid close to each other to reduce electromagnetic interference.
3. Hardware filtering and software anti-fault measures
Before the signal is connected to the computer, a capacitor is connected in parallel between the signal line and ground to reduce common-mode interference; a filter is installed between the two poles of the signal to reduce differential-mode interference.
4. Select the correct grounding point and improve the grounding system.
Grounding typically serves two purposes: safety and interference suppression. A robust grounding system is a crucial measure for PLC control systems to resist electromagnetic interference. There are three main grounding methods: floating ground, direct grounding, and capacitor grounding.
When the signal source is grounded, the shielding layer should be grounded on the signal side; when it is not grounded, it should be grounded on the PLC side; when there is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multiple grounding points must be avoided; when the shielded twisted pair of multiple measurement point signals is connected to the multi-core twisted pair shielded cable, each shielding layer should be connected to each other and insulated.
II. Improve operational efficiency
1. Based on the actual needs of the project, conduct functional block planning.
In PLCs, subroutines are relatively independent programs written for specific control purposes. They are executed using instructions like CALL. If the conditions for a subroutine call are not met , the program scan will only occur in the main program, avoiding scanning of the subroutine and thus reducing unnecessary scanning time.
2. Control output by sending single or double-word data to the DO point.
In PLC applications, there are usually a lot of output controls. Using word or double word data to transmit to DO points to control the output can improve speed. As long as the output addresses are reasonably allocated and the control words are changed according to the requirements of the actual application, the number of steps executed by the PLC program can be greatly reduced, thereby speeding up the PLC program running speed.
3. Pulse-triggered SET and RESET
In PLCs, the SET instruction only needs to be executed once, not every time the scan is performed, making it ideal for use with pulse output (PLS/PLF) instructions. Some engineers overlook this and use conventional methods to drive the SET instruction, unintentionally increasing the PLC program's scan execution time.
