Application areas of PLC
Currently, PLCs are widely used in various industries both domestically and internationally, including steel, petroleum, chemical, power, building materials, machinery manufacturing, automotive, light textile, transportation, environmental protection, and cultural entertainment. At the Yuncheng Power Plant, they are mainly used for chemical water treatment, domestic sewage treatment, industrial wastewater treatment, and condensate polishing. The applications of PLCs can be broadly categorized as follows.
Switch logic control
It replaces traditional relay circuits to achieve logic control and sequential control, and can be used for controlling single devices or multiple devices in a group. Examples include starting and stopping water pumps, opening and closing valves, sequential control of water production systems, and dry ash removal systems.
Industrial process control
In industrial production processes, there are continuously changing quantities (i.e., analog quantities) such as temperature, pressure, flow rate, liquid level, and speed. PLCs use corresponding A/D and D/A conversion modules and various control algorithms to process these analog quantities and complete closed-loop control. PID control is a commonly used method in general closed-loop control systems. Process control has wide applications in metallurgy, chemical engineering, and other fields; in Yuncheng Power Plant, it is mainly used in central air conditioning and heating systems.
Motion control
PLCs can be used to control circular or linear motion. They typically use dedicated motion control modules, such as single-axis or multi-axis position control modules that can drive stepper motors or servo motors, and are widely used in various machinery, machine tools, robots, elevators, and other applications.
Data processing
PLCs have functions such as mathematical operations (including matrix operations, function operations, and logical operations), data transmission, data conversion, sorting, and table lookup, and can complete data acquisition , analysis, and processing.
Communications 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 all have communication interfaces, making communication very convenient.
PLC application characteristics
1. High reliability and strong anti-interference ability
High reliability is a key performance characteristic of electrical control equipment. PLCs, employing modern large-scale integrated circuit technology and manufactured with stringent production processes, utilize advanced anti-interference technologies in their internal circuits, resulting in extremely high reliability. Compared to relay contactor systems of similar scale, control systems using PLCs reduce electrical wiring and switching contacts to hundreds or even thousands of times less, significantly decreasing the risk of failure. Furthermore, PLCs have built-in hardware fault self-detection functions, promptly issuing alarm messages when faults occur. In application software, users can also program fault self-diagnosis programs for peripheral devices, providing fault self-diagnosis protection for circuits and equipment other than the PLC. Thus, the overall system reliability is extremely high.
2. Complete accessories, comprehensive functions, and strong applicability.
Today, PLCs have evolved into a series of products of various sizes, suitable for industrial control applications of all scales. In addition to logic processing functions, most PLCs possess comprehensive data processing capabilities, enabling their use in various digital control fields. The proliferation of diverse functional units has allowed PLCs to penetrate various industrial control applications, including position control, temperature control, and CNC. Furthermore, the enhanced communication capabilities of PLCs and the development of human-machine interface technology have made it remarkably easy to construct various control systems using PLCs.
3. Easy to learn and use, and widely welcomed by engineering and technical personnel.
PLCs are industrial control devices designed for industrial and mining enterprises. They have easy-to-use interfaces, and their programming languages are readily accepted by engineering technicians. The graphical symbols and representations of ladder diagram language are quite similar to those of relay circuit diagrams, making it convenient for people unfamiliar with electronic circuits, computer principles, and assembly language to engage in industrial control.
4. The system design requires minimal work, is easy to maintain, and is readily adaptable.
(1) Design and Maintenance
PLCs replace wired logic with stored logic, greatly reducing external wiring of control equipment, significantly shortening the design and construction cycle of control systems, and making daily maintenance easier. More importantly, it makes it possible to change the production process by modifying the program on the same equipment. It is particularly suitable for multi-variety, small-batch production applications.
(2) Installation and wiring
Power lines, control lines, and PLC power and I/O lines should be wired separately. Twisted-pair cables should be used to connect the isolation transformer to the PLC and I/O lines. PLC I/O lines and high-power lines should be routed separately. If they must be in the same cable tray, AC and DC lines should be bundled separately. If conditions permit, separate cable trays are best, as this maximizes spatial distance and minimizes interference.
PLCs should be kept away from strong interference sources such as welding machines, high-power silicon rectifiers, and large power equipment, and should not be installed in the same switch cabinet as high-voltage electrical appliances. Inside the cabinet, the PLC should be kept away from power lines (the distance between them should be greater than 200mm). Inductive loads installed in the same cabinet as the PLC, such as the coils of high-power relays or contactors, should be connected in parallel with an RC arc suppression circuit.
PLC inputs and outputs should ideally be routed separately, and digital and analog signals should also be laid separately. Shielded cables should be used for analog signal transmission, and the shield should be grounded at one or both ends, with the grounding resistance less than 1/10 of the shield resistance.
Do not use the same cable for AC output lines and DC output lines. Output lines should be kept as far away as possible from high-voltage lines and power lines, and should not run in parallel.
(3) Wiring of I/O terminals
Input wiring: Input wiring should generally not be too long. However, if environmental interference is low and voltage drop is not significant, input wiring can be appropriately longer; input/output lines should not use the same cable and should be separate; normally open contacts should be used to connect to the input terminals whenever possible to ensure that the ladder diagram is consistent with the relay schematic diagram for easy reading.
Output Connections: Output wiring is divided into independent outputs and common outputs. Different types and voltage levels of output voltage can be used in different groups, but outputs in the same group can only use the same type and voltage level of power supply. Since the PLC's output components are encapsulated on a printed circuit board and connected to a terminal block, short-circuiting the load connected to the output component will burn out the printed circuit board. When using relay outputs, the size of the inductive load will affect the relay's lifespan; therefore, inductive loads should be selected appropriately or an isolation relay should be added. PLC output loads may generate interference, so measures must be taken to control it, such as freewheeling diode protection for DC outputs, RC snubber circuits for AC outputs, and bypass resistor protection for transistor and bidirectional thyristor outputs.
Issues to be aware of in PLC applications
A PLC is a device used for automated control in industrial production. Generally, it can be used directly in industrial environments without any special precautions. However, despite its high reliability and strong anti-interference capabilities, as mentioned above, harsh production environments with strong electromagnetic interference, or improper installation and use, can lead to program errors or calculation errors, resulting in erroneous inputs and outputs. This can cause equipment malfunctions and misoperations, thus compromising the normal operation of the PLC. To improve the reliability of a PLC control system, PLC manufacturers need to enhance the anti-interference capabilities of their equipment. Furthermore, careful attention must be paid to design, installation, and maintenance; multi-party cooperation is essential to effectively address problems and enhance the system's anti-interference performance. Therefore, the following issues should be noted during use.
Work environment
(1) Temperature: PLC requires an ambient temperature of 0 to 55℃. When installing, it should not be placed under components that generate a lot of heat, and the surrounding space for ventilation and heat dissipation should be large enough.
(2) Humidity: In order to ensure the insulation performance of PLC, the relative humidity of the air should be less than 85% (no condensation).
(3) Vibration: The PLC should be kept away from strong vibration sources to prevent frequent or continuous vibration with a frequency of 10-55Hz. When vibration is unavoidable in the operating environment, vibration reduction measures must be taken, such as using vibration damping rubber.
(4) Air: Avoid corrosive and flammable gases, such as chemical acids and alkalis. For environments with a lot of dust or corrosive gases in the air, the PLC can be installed in a well-sealed control room or control cabinet. For example, in power plants, dry slag discharge and dry ash removal, enclosed cabins were added in the later stages of infrastructure construction.
(5) Power Supply: PLCs have a certain degree of resistance to interference from power lines. In environments with high reliability requirements or particularly severe power interference, a shielded isolation transformer can be installed to reduce interference between the equipment and ground. Generally, PLCs are supplied with 24V DC output to the input terminals. When using an external DC power supply for the input terminals, a DC regulated power supply should be selected. This is because ordinary rectified and filtered power supplies, due to ripple, can easily cause the PLC to receive incorrect information.
Interference and its sources in control systems
Electromagnetic interference in the field is one of the most common and most likely factors to affect the reliability of PLC control systems.
(1) Interference sources and general classification
Interference sources affecting PLC control systems mostly originate from areas with drastic changes in current or voltage. This is because changes in current generate magnetic fields, which in turn produce electromagnetic radiation on the equipment; changes in magnetic fields generate current, and electromagnetic waves are produced at high speeds. Electromagnetic interference is typically classified into common-mode interference and differential-mode interference based on its interference mode. 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 lines by grid interference, ground potential difference, and spatial electromagnetic radiation. Common-mode voltage can be converted into differential-mode voltage through asymmetrical circuits, directly affecting 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 the two poles of the signal, 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 interference, superimposed on the signal, directly affects the measurement and control accuracy.
(2) Main sources and pathways of interference in PLC systems
High-voltage interference: The normal power supply of the PLC system is provided by the power grid. Due to the wide coverage of the power grid, it is subject to electromagnetic interference from all directions, which can induce voltage on the lines. In particular, changes within the power grid, such as surges from knife switch operations, starting and stopping of large power equipment, harmonics caused by AC/DC drives, and transient impacts from power grid short circuits, are all transmitted to the primary side of the power source through the transmission lines.
Internal interference: High-voltage electrical appliances, large inductive loads, and messy wiring inside the control cabinet can all easily cause a certain degree of interference to the PLC.
Interference introduced by signal lines: Various signal transmission lines connected to the PLC control system, besides transmitting valid information, are always subject to external interference signals. This interference mainly occurs through two pathways: first, interference from the power grid introduced through the transmitter's power supply or the power supply of shared signal instruments , which is often overlooked; second, interference induced by electromagnetic radiation from space, i.e., external induced interference on the signal lines, which is very serious. Interference introduced by signals can cause abnormal I/O signal operation and a significant reduction in measurement accuracy, and in severe cases, can damage components.
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 improper grounding can introduce serious interference signals, making the PLC system unable to work properly.
Interference from within the PLC system: mainly generated by mutual electromagnetic radiation between internal components and circuits, such as mutual radiation between logic circuits and its impact on analog circuits, mutual influence between analog ground and logic ground, and mismatch between components, etc.
Inverter interference: First, the harmonics generated during the startup and operation of the inverter cause conducted interference to the power grid, resulting in voltage distortion and affecting the power supply quality of the power grid; second, the output of the inverter generates strong electromagnetic radiation interference, which affects the normal operation of surrounding equipment.
Main anti-interference measures
(1) Reasonable handling of power supply to suppress interference introduced by the power grid. To reduce interference from the power grid introduced by the power supply, an isolation transformer with a shielded layer and a turns ratio of 1:1 can be installed to reduce interference between the equipment and the ground. An LC filter circuit can also be connected in series at the power input terminal.
(2) Correctly select the grounding point and improve the grounding system. Good grounding is an important condition for ensuring the reliable operation of PLC and can avoid the damage caused by accidental voltage surges. The purpose of grounding is usually to...
There are two reasons: firstly, for safety, and secondly, to suppress interference. A proper grounding system is one of the important measures for PLC control systems to resist electromagnetic interference.
The grounding wires of a PLC control system include system ground, shield ground, AC ground, and protective ground. A chaotic grounding system interferes with the PLC system mainly because of uneven potential distribution at various grounding points. Potential differences exist between different grounding points, causing ground loop currents and affecting normal system operation. For example, a cable shield must be grounded at one point. If both ends A and B of the cable shield are grounded, a potential difference exists, and current flows through the shield. In abnormal conditions such as lightning strikes, the ground current will be even greater.
Furthermore, the shielding layer, grounding wire, and earth can form a closed loop. Under the influence of a changing magnetic field, induced currents will appear within the shielding layer, interfering with the signal loop through the coupling between the shielding layer and the core wire. If the system grounding is confused with other grounding methods, the resulting ground loop current may create unequal potential distributions on the ground wire, affecting the normal operation of the logic and analog circuits within the PLC. PLCs have low logic voltage interference tolerance; interference from logic ground potential distributions can easily affect PLC logic operations and data storage, causing data corruption, program crashes, or system freezes. Analog ground potential distributions will lead to decreased measurement accuracy, causing serious distortion and malfunctions in signal measurement and control.
Safety grounding or power supply grounding: Connect the grounding terminal of the power cord to the cabinet and ground it for safety grounding. In case of power leakage or cabinet energization, the current can be conducted to the ground through the safety grounding, which will not cause harm to people.
System grounding: The PLC controller is grounded to ensure it is at the same potential as all the devices it controls; this is called system grounding. The grounding resistance value must not exceed 4Ω. Generally, the PLC device system ground and the negative terminal of the switching power supply in the control cabinet should be connected together as the control system ground.
Signal and shielding grounding: Generally, signal lines must have a unique reference ground, i.e., "single-point grounding". Shielded cables should also be grounded locally or in the control room in situations where conducted interference may occur, to prevent the formation of "ground loops".
When the signal source is grounded, the shielding layer should be grounded on the signal side; if it is not grounded, it should be grounded on the PLC side; when there is a connector in the middle of the signal line, the shielding layer should be grounded.
The layers should be firmly connected and insulated, and multiple grounding points must be avoided. When the shielded twisted-pair cable for 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, and a suitable single-point grounding point should be selected.
(3) Suppression of inverter interference
Interference handling for frequency converters generally involves the following methods: adding isolation transformers, primarily targeting conducted interference from the power supply, which can block most conducted interference before reaching the isolation transformer; using filters, which have strong anti-interference capabilities and prevent interference from the equipment itself from being conducted to the power supply, and some also have peak voltage absorption functions; and using output reactors, adding AC reactors between the frequency converter and the motor to reduce electromagnetic radiation generated by the frequency converter output during energy transmission, which could affect the normal operation of other equipment.
prospect
As the application fields of PLCs continue to expand, how to use PLCs efficiently and reliably has become an important factor in their development. In the future, PLCs will see even greater development, with a wider variety of products and more complete specifications. Through perfect human-machine interfaces and comprehensive communication equipment, they will better adapt to the needs of various industrial control applications. As an important component of automation control networks and internationally used networks, PLCs will play an increasingly important role in the field of industrial control.
