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1. Overview
For many years, Programmable Logic Controllers (PLC) have evolved from wired logic to stored logic; their functionality has advanced from basic to sophisticated, achieving progress from logical control to digital control. The application fields have expanded from simple control of individual devices to handling complex tasks such as motion control, process control, and distributed control. Today’s PLCs have significantly improved their capabilities in processing analog signals, digital computations, human-machine interfaces, and networking, becoming mainstream control devices in industrial control, playing an increasingly vital role across various industries.
2. Application Fields of PLC
Currently, PLCs are widely used in various industries both domestically and internationally, including steel, petroleum, chemical, electric power, building materials, machinery manufacturing, automotive, light textiles, transportation, environmental protection, and cultural entertainment. Their usage can be categorized into the following types:
1. Discrete Logic Control
Replacing traditional relay circuits, PLCs achieve logical control and sequential control, applicable for both single device control and multi-machine group control as well as automated assembly lines, such as injection molding machines, printing machines, stapling machines, CNC machines, grinding machines, packaging production lines, and electroplating lines.
2. Industrial Process Control
In industrial production processes, there are continuously varying quantities such as temperature, pressure, flow, liquid level, and speed (i.e., analog quantities). PLCs use corresponding A/D and D/A conversion modules along with various control algorithm programs to process these analog quantities and complete closed-loop control. PID regulation is a commonly used method in general closed-loop control systems. Process control has extensive applications in metallurgy, chemical engineering, heat treatment, and boiler control.
3. Motion Control
PLCs can be used for controlling circular or linear motion. They typically use specialized motion control modules that can drive stepper motors or servo motors, widely applied in various machinery, machine tools, robots, and elevators.
4. Data Processing
PLCs have capabilities for mathematical operations (including matrix operations, function operations, and logical operations), data transmission, data conversion, sorting, table lookup, and bit manipulation, enabling data collection, analysis, and processing. Data processing is commonly used in large control systems in industries such as papermaking, metallurgy, and food processing.
5. Communication and Networking
PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of factory automation networks, modern PLCs come equipped with communication interfaces, making communication very convenient.
3. Characteristics of PLC Applications
1. High Reliability and Strong Anti-Interference Capability
High reliability is a key performance characteristic of electrical control devices. Due to the use of modern large-scale integrated circuit technology and strict manufacturing processes, PLCs incorporate advanced anti-interference technologies in their internal circuits, resulting in high reliability. When using PLCs to form control systems, the electrical wiring and switch contacts have been reduced to hundreds or even thousands of times less than those in equivalent relay contactor systems, thus greatly reducing failures. Additionally, PLCs have built-in hardware fault self-detection capabilities, issuing timely alerts when faults occur. In application software, users can also incorporate fault self-diagnosis programs for peripheral devices, providing fault self-diagnosis protection for circuits and devices other than the PLC itself. This results in an overall system with extremely high reliability.
2. Comprehensive Support, Complete Functions, Strong Applicability
PLCs have developed into a series of products of various scales suitable for various industrial control scenarios. In addition to logic processing functions, most PLCs possess complete data computation capabilities, applicable in various digital control fields. A wide variety of functional units have emerged, allowing PLCs to penetrate into position control, temperature control, CNC, and various industrial controls. Coupled with enhanced PLC communication capabilities and the development of human-machine interface technology, it has become very easy to use PLCs to form various control systems.
3. Easy to Learn and Use, Well Received by Engineering Technicians
PLCs are control devices designed for industrial enterprises. They feature easy interfaces and programming languages that are readily accepted by engineering technicians. The graphical symbols and expressions of ladder diagram language are very similar to relay circuit diagrams, making it easier for individuals unfamiliar with electronic circuits or computer principles to engage in industrial control.
4. System Design, Less Workload, Easy Maintenance, Easy Modification
PLCs replace wired logic with stored logic, significantly reducing external wiring for control devices, thus greatly shortening the design and construction cycle of control systems. Routine maintenance also becomes easier, and more importantly, it allows the same device to change its production process by altering the program. This is particularly suitable for production scenarios with multiple varieties and small batches.
4. Issues to Consider in PLC Applications
PLCs are devices used for industrial production automation control, generally requiring no special measures to be used directly in industrial environments. However, despite their high reliability and strong anti-interference capabilities as mentioned above, when the production environment is excessively harsh, with particularly strong electromagnetic interference, or if installation and use are improper, it may lead to program errors or calculation errors, resulting in erroneous inputs and outputs, which can cause equipment loss of control and erroneous actions, thus failing to ensure the normal operation of the PLC. To enhance the reliability of PLC control systems, manufacturers must improve the anti-interference capabilities of the devices, while also requiring design, installation, and maintenance to be given high attention, with multi-party cooperation to effectively solve problems and enhance the system’s anti-interference performance. Therefore, the following issues should be noted during use:
1. Working Environment
(1) Temperature
PLCs require an ambient temperature between 0~55°C, and should not be placed under components that generate significant heat, with adequate ventilation space around them.
(2) Humidity
To ensure the insulation performance of the PLC, the relative humidity of the air should be less than 85% (no condensation).
(3) Vibration
PLCs should be kept away from strong vibration sources to prevent frequent or continuous vibrations at frequencies of 10~55Hz. If unavoidable, damping measures must be taken, such as using damping rubber.
(4) Air Quality
Avoid corrosive and flammable gases, such as hydrogen chloride and hydrogen sulfide. For environments with high dust or corrosive gases, PLCs should be installed in well-sealed control rooms or cabinets.
(5) Power Supply
PLCs have a certain resistance to interference from power lines. In environments requiring high reliability or with severe power interference, an isolation transformer with a shielding layer can be installed to reduce interference between the equipment and ground. Generally, PLCs have a 24V DC output for the input terminals, and when using an external DC power supply for the input terminals, a stabilized DC power supply should be selected. Ordinary rectified and filtered power supplies can easily lead to erroneous information being received by the PLC due to ripple effects.
2. Interference and Its Sources in Control Systems
Electromagnetic interference in the field is one of the most common and easily influential factors affecting the reliability of PLC control systems. To address the problem effectively, it is essential to identify the source of interference.
(1) Sources of Interference and General Classification
Interference sources affecting PLC control systems mostly arise in areas with rapid changes in current or voltage, as the change in current generates a magnetic field that produces electromagnetic radiation; changes in magnetic fields generate currents, leading to electromagnetic waves. Generally, electromagnetic interference can be classified into common-mode interference and differential-mode interference based on different interference patterns. Common-mode interference refers to the potential difference of the signal to ground, mainly formed by the superimposed common-mode voltage induced by the power grid, ground potential differences, and spatial electromagnetic radiation on signal lines. Common-mode voltage can be converted into differential-mode voltage through unbalanced circuits, directly affecting measurement and control signals, causing component damage (this is a major reason for the high damage rates of some system I/O modules). This common-mode interference can be either DC or AC. Differential-mode interference refers to the interference voltage acting between the two poles of the signal, mainly formed by the coupling induction of spatial electromagnetic fields on the signals and the conversion of common-mode interference by unbalanced circuits. This interference superimposes on the signal and directly affects measurement and control accuracy.
(2) Main Sources and Pathways of Interference in PLC Systems
Strong Electrical Interference
The normal power supply for PLC systems is provided by the power grid. Due to the wide coverage of the power grid, it is susceptible to all spatial electromagnetic interference, inducing voltage on the lines. Especially changes within the power grid, such as surge from knife switches, start-stop of large power equipment, harmonics caused by AC and DC drive devices, and transient impacts from power grid short circuits, are transmitted to the power source through transmission lines.
Cabinet Interference
High-voltage electrical appliances, large inductive loads, and disorganized wiring within control cabinets can cause a certain degree of interference to PLCs.
Interference from Signal Lines
Various signal transmission lines connected to the PLC control system, besides transmitting valid information, will inevitably have external interference signals intruding. This interference mainly comes through two pathways: one is the grid interference induced through the power supply of transmitters or shared signal instruments, which is often overlooked; the second is interference induced on signal lines by spatial electromagnetic radiation, which is very serious. Interference introduced by signals can lead to abnormal I/O signals and significantly reduced measurement accuracy, and in severe cases, can damage components.
Interference from Confused Grounding Systems
Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Proper grounding can suppress the impact of electromagnetic interference and prevent equipment from emitting interference; incorrect grounding can introduce serious interference signals, causing the PLC system to malfunction.
Internal Interference from PLC Systems
This mainly arises from electromagnetic radiation between internal components and circuits, such as mutual radiation of logical circuits and their effects on analog circuits, the mutual influence of analog ground and logical ground, and the improper matching of components.
Interference from Frequency Converters
First, the harmonics generated during the startup and operation of frequency converters cause conductive interference to the power grid, leading to voltage distortion and affecting the quality of power supply; second, the output of frequency converters generates strong electromagnetic radiation interference, affecting the normal operation of surrounding equipment.
3. Major Anti-Interference Measures
(1) Reasonable Handling of Power Supply to Suppress Interference from the Power Grid
To address power grid interference introduced through the power supply, an isolation transformer with a shielding layer and a transformation ratio of 1:1 can be installed to reduce interference between the equipment and ground. Additionally, an LC filter circuit can be connected in series at the power supply input. As shown in Figure 1.
(2) Installation and Wiring
● Power lines, control lines, and the PLC’s power supply and I/O lines should be wired separately. The isolation transformer and the PLC and I/O should be connected using double-insulated wires. PLC I/O lines and high-power lines should be routed separately; if they must be in the same conduit, AC and DC lines should be bundled separately, and if conditions permit, routing them in separate conduits is best. This not only provides maximum spatial distance but also minimizes interference.
● PLCs should be kept away from strong interference sources such as welding machines, high-power silicon rectifier devices, and large power equipment; they should not be installed in the same switch cabinet as high-voltage electrical appliances. Inside the cabinet, PLCs should be kept away from power lines (the distance between them should be greater than 200mm). Inductive loads, such as large relays and contactor coils, installed in the same cabinet as the PLC should be connected in parallel with RC snubber circuits.
● It is best to route the input and output lines of the PLC separately, and switch signals should also be laid separately from analog signals. Shielded cables should be used for transmitting analog signals, with the shield grounded at one end or both ends, and the ground resistance should be less than 1/10 of the shield resistance.
● AC output lines and DC output lines should not be in the same cable; output lines should be kept as far away from high-voltage lines and power lines as possible to avoid parallel routing.
(3) Wiring of I/O Terminals
Input Wiring
● Input wiring should generally not be too long. However, if the environmental interference is low and the voltage drop is minimal, the input wiring can be somewhat longer.
● Input/output lines should not use the same cable; input/output lines should be separated.
● Use normally open contact forms to connect to the input terminals as much as possible to make the ladder diagram consistent with the relay schematic for easy reading.
Output Connection
● Output terminal wiring can be classified into independent outputs and common outputs. Different groups can use different types and voltage levels of output voltage. However, outputs within the same group can only use the same type and voltage level power supply.
● Since the output components of the PLC are encapsulated on the printed circuit board and connected to the terminal block, short-circuiting the load connected to the output components will burn the printed circuit board.
● When using relay outputs, the size of the inductive load will affect the service life of the relay; therefore, inductive loads should be chosen carefully, or isolation relays should be added.
● The output load of the PLC may generate interference; therefore, measures should be taken to control it, such as using flyback diodes for DC outputs, RC snubber circuits for AC outputs, and bypass resistors for transistor and bidirectional thyristor outputs.
(4) Correctly Selecting Ground Points and Improving Grounding Systems
Good grounding is an important condition to ensure the reliable operation of PLCs, avoiding occasional voltage spikes. The purpose of grounding typically has two aspects: one for safety and the other to suppress interference. A well-designed grounding system is one of the key measures to enhance the PLC control system’s resistance to electromagnetic interference.
The grounding system of the PLC control system includes system ground, shield ground, AC ground, and protective ground. Disorganized grounding can interfere with the PLC system primarily due to uneven potential distribution at various grounding points, resulting in ground loop currents that affect normal operation. For example, the shielding layer of cables must be grounded at one point; if both ends A and B of the cable shielding layer are grounded, there will be a potential difference, and current will flow through the shielding layer. When abnormal conditions such as lightning strikes occur, the ground current will be even larger.
Furthermore, the shielding layer, grounding wire, and earth may form a closed loop, and under the influence of changing magnetic fields, induced currents may appear within the shielding layer. Through the coupling between the shielding layer and the core wire, interference signals can form. If the system ground is confused with other grounding treatments, the resulting ground loop currents may create uneven potential distributions along the ground wire, affecting the normal operation of the logical and analog circuits within the PLC. The tolerance for logical voltage interference in PLC operation is relatively low, and interference in the distribution of logical ground potential can easily affect the logical operations and data storage of the PLC, resulting in data confusion, program runaway, or system crashes. The distribution of analog ground potential can lead to reduced measurement accuracy, causing severe distortion and erroneous actions in signal measurement and control.
● Safety ground or power supply grounding
The grounding terminal of the power supply line and the grounding connection of the cabinet body should be connected for safety grounding. In case of power leakage or if the cabinet body is live, the safety ground will direct current into the ground, preventing harm to individuals.
● System Grounding
PLC controllers are grounded to maintain the same potential with the controlled devices, referred to as system grounding. The grounding resistance should not exceed 4Ω, and generally, the PLC system ground should be connected with the negative terminal of the switching power supply inside the control cabinet to serve as the control system ground.
● Signal and Shield Grounding
Signal lines should have a unique reference ground, and shielded cables in environments prone to conductive interference should also be grounded at a single point in the control room to avoid forming ground loops. When grounding the signal source, the shielding layer should be grounded on the signal side; if not grounded, it should be grounded on the PLC side. When there are connections in the signal line, the shielding layer should be securely connected and insulated, avoiding multiple grounding points. When connecting multiple measurement point signals with a twisted shielded cable to a multi-core twisted shield cable, the shielding layers should be properly interconnected and insulated, with appropriate single-point grounding selected.
(5) Suppression of Interference from Frequency Converters
Interference from frequency converters can generally be addressed in the following ways:
Install an isolation transformer, mainly targeting conductive interference from the power supply, which can block most of the conductive interference before reaching the isolation transformer.
Use filters, which have strong anti-interference capabilities and can prevent the interference generated by the equipment itself from being conducted to the power supply, and some even have surge voltage absorption functions.
Use output reactors, adding AC reactors between the frequency converter and the motor primarily reduces electromagnetic radiation generated during energy transmission, affecting the normal operation of other equipment.
Interference in PLC control systems is a complex issue, so comprehensive consideration of various factors in anti-interference design is necessary to suppress interference effectively, ensuring the normal operation of PLC control systems. As the application fields of PLCs continue to expand, how to use PLCs efficiently and reliably has become a significant factor in their development.
5. Conclusion
In the future, PLCs will experience greater development, with a richer variety of products and more complete specifications. Through perfect human-machine interfaces and comprehensive communication devices, they will better adapt to the demands of various industrial control scenarios. As an important component of automation control networks and internationally accepted networks, PLCs will play an increasingly significant role in the field of industrial control.
Source: Instrumentation Engineering Network
China Chemical Safety Association
Editor: Chen Guofang
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