I. Core Views
1.1 PLC core advantages are prominent, essential for industrial control
Programmable logic controller (PLC) occupies a vital position in industrial control. Its high reliability is one of its key advantages. The use of modern large-scale integrated circuit technology, strict production process manufacturing and advanced anti-interference technology makes the reliability of the entire system extremely high. For example, in the automobile manufacturing process, PLC can run stably, control the movement of robots and the operation of automated assembly lines, detect and handle abnormal situations in a timely manner, and ensure the safety of the production process. At the same time, PLC also has a hardware fault self-detection function, which can issue alarm information in time when a fault occurs, greatly reducing the risk of production stagnation caused by faults.
1.2 Wide application scenarios, promoting industrial intelligence
PLC has a wide range of application scenarios and plays an important role in various industrial fields. From aquarium control systems to fitness equipment control, from restaurant automated kitchens to garbage disposal systems, to artificial rain systems, etc., PLCs are present. In terms of intelligent building control, PLCs can be applied to energy management, security monitoring, lighting control and other aspects to achieve energy-saving, safe and comfortable building environments. For example, using PLC to control building energy management systems can achieve energy consumption monitoring, ambient temperature control, and improve building energy efficiency. In industrial production, PLCs can be used in welding and processing, analog control, motion control and other fields to promote the development of industrial intelligence.
1.3 Technological development leads future industrial transformation
With the continuous advancement of technology, PLC technology is also developing, leading the profound transformation of future industrial production. On the one hand, PLC technology is more integrated into industrial, automotive, communication and other products, with increasing miniaturization and integration, and more refined process requirements. On the other hand, with the development of AI technology, intelligent PLC technology continues to emerge, such as the intelligent I/O module of PLC used in industrial control and the intelligent temperature control module in automobiles, which will have more application scenarios in the future. In addition, diversified PLC technology is also developing, including PLC-related programming languages and tools, customized PLC products will increase, and PLC modular technology will also increase. The development of these technologies will make industrial control systems more flexible, efficient and reliable, and promote industrial production to develop in a more intelligent, autonomous and green direction.
II. Analysis of working principle
2.1 Overview of work flow
2.1.1 Detailed explanation of the three-step working method
The work of PLC is mainly divided into three steps: information collection, program operation, and instruction output. First, in the input sampling stage, the PLC reads all input states and data in sequence in a scanning manner and stores them in the corresponding units in the I/O image area. For example, when a sensor signal is transmitted to the PLC, at this stage, the PLC will read the sensor state and store it in a specific input image register. Next, in the user program execution stage, the PLC always scans the user program in order from top to bottom. In this stage, the PLC will take data from the input image register and the internal component register, perform logical operations and arithmetic operations, and then store the results in related memories such as the output image register. For example, when the program executes a logical judgment instruction, the PLC will judge according to the input state and the data in the internal register, and store the result. Finally, in the output refresh stage, the CPU refreshes all output latch circuits according to the corresponding state and data in the I/O image area, and then drives the corresponding peripherals through the output circuit.
2.1.2 Cyclic Scanning Working Mode
PLC adopts cyclic scanning working mode, which has significant characteristics and advantages. This working mode has centralized input and centralized output, high reliability and strong anti-interference ability. For example, in a complex industrial environment, there may be various electromagnetic interference and noise, but the cyclic scanning working mode of PLC can ensure the stable operation of the system without being affected by external interference. However, this working mode also has certain disadvantages, namely, delayed response and slow speed. For example, for some application scenarios that require fast response, special measures may need to be taken, such as using high-speed counting modules, interrupt processing, etc., to improve the response speed of the system.
2.2 In-depth interpretation of each stage
2.2.1 Input sampling stage
In the input sampling stage, the PLC first scans all input terminals and stores each input state in the corresponding input image register. At this time, the input image register is refreshed. Generally speaking, the width of the input signal should be greater than one scan cycle, otherwise it is likely to cause signal loss. For example, if the input signal of a button lasts less than the scan cycle of the PLC, then this signal may not be correctly collected. This is because after the input sampling is completed, the program execution and output refresh phase begins. The input image register is isolated from the outside world. No matter how the input changes, its content remains unchanged until the input sampling phase of the next scan cycle, when the new content of the input terminal is rewritten.
2.2.2 User program execution phase
In the user program execution phase, the PLC scans and executes instructions one by one from top to bottom and from left to right in the order of the ladder diagram program. When the instruction involves input and output status, the PLC "reads" the corresponding input terminal status collected from the input image register and "reads" the current status of the corresponding component ("soft relay") from the component image register. Then the corresponding operation is performed, and the latest operation result is immediately stored in the corresponding component image register. For example, when the program executes an addition instruction, the PLC will read the corresponding value from the input image register, perform the addition operation, and store the result in the component image register. For the component image register, the state of each component ("soft relay") will be refreshed as the program executes.
2.2.3 Output Refresh Phase
In the output refresh phase, the states of all output relays in the component image register are transferred to the output latch together in the output refresh phase, and the output is concentrated in a certain way, and finally drives the external load through the output terminal. For example, when the state of an output relay in the output image register is "1", in the output refresh phase, this state will be transferred to the output latch, and then drive the corresponding external load, such as motor, indicator light, etc., through the output circuit. Before the next output refresh phase begins, the state of the output latch will not change, and thus the state of the corresponding output terminal will not change.
III. Composition structure analysis
3.1 Basic components
3.1.1 Power supply
PLC power supply can be 220VAC or 24VDC. Among them, the power supply for the PLC as a whole, and the power supply required by the CPU processing unit is 5VDC. Different power supply types meet the application requirements of PLC in different scenarios. For example, in some small control systems, 24VDC power supply is more common, while in some large industrial occasions, 220VAC power supply can provide more stable power support.
3.1.2 CPU
CPU, as the core part of PLC, includes controller, operator, and register. The controller is responsible for directing the work of the entire PLC and coordinating the operation of each part. The operator performs various logical operations and arithmetic operations to ensure the correct execution of the program. Registers are used to temporarily store data and instructions to increase the operation speed. For example, when performing complex industrial control tasks, the CPU's arithmetic unit will quickly process large amounts of data, while the controller ensures that each step is carried out in an orderly manner.
3.1.3 Memory
The memory is divided into ROM and RAM. The ROM storage area mainly stores system management programs, user program editing and instruction interpreters, subroutine calls and management programs. These programs are usually not modified during system operation, ensuring the stability of the system. The RAM storage area includes the user program storage area and the data storage area, which are used to store user programs and data during program execution.The ON/OFF state of each point. The data in the user storage area is readable, writable and executable, and the materials used are CMOS RAM or EPRAM and EEPRAM. The capacity is generally in "words", 16 bits are 1 word, and 8 bits are 1 byte.
3.1.4 Input unit
The input unit is a unit that receives signals. It can detect the information transmitted by the signal and convert it into a high- and low-level digital signal. The input unit is divided into switch quantity and analog quantity. The analog quantity usually uses A/D conversion circuit to convert the analog quantity into digital quantity. There are many interface circuits used for digital quantity, which are divided into internal DC input (12V or 24V), external AC input (100 - 120V, 200 - 240V), external AC/DC input, and input circuits connected to signal output. For example, in industrial production, the analog signal from the sensor is converted into a digital signal through the A/D conversion circuit of the input unit for processing by the PLC.
3.1.5 Output unit
The output unit converts the weak current signal processed by the CPU into a level signal. The output interface is divided into switch quantity and analog quantity. Analog interfaces usually convert digital quantities into analog quantities through D/A conversion circuits. There are many forms of digital output circuits, including relay output, transistor output (PNP, NPN), and crystal gate output. For example, when the PLC controls the motor to run, the output unit converts the digital signal into a corresponding level signal to drive the motor to run.
3.1.6 Communication interface and expansion interface
The communication interface can communicate with monitors, printers, other PLCs, computers and other devices. It enables PLCs to exchange data and work in collaboration with external devices. The expansion interface can add some special function modules to the PLC, such as high-speed counting modules, closed-loop control modules, motion control modules, interrupt control modules, etc. For example, through the communication interface, PLC can transmit production data to the computer for analysis and monitoring; and by adding a high-speed counting module through the expansion interface, the counting requirements for high-frequency signals can be met.
3.2 Synergy in the working process
In the working process of PLC, each component works closely together. The power supply provides stable power support for each part to ensure the normal operation of the system. As the core, the CPU directs the work of the entire system, reads instructions from the memory and performs calculations, and coordinates the work of the input unit and the output unit. The input unit receives external signals and converts them into digital signals, which are transmitted to the CPU. After the CPU performs calculations based on the input signals and programs, it transmits the results to the output unit. The output unit converts weak current signals into level signals to drive external loads. The communication interface and expansion interface play a role when it is necessary to communicate with external devices or expand functions. For example, on an automated production line, the input unit receives the signal from the sensor, the CPU calculates the signal, and controls the operation of the motor through the output unit. At the same time, the communication interface transmits the production data to the monitoring system, and the expansion interface can add functional modules as needed to meet different production needs. This synergy enables PLC to complete various industrial control tasks efficiently and stably.
IV. Application scenario display
4.1 Application cases in different industrial fields
In the manufacturing industry, PLC is widely used in automobile manufacturing, electronic product manufacturing and other fields. For example, in the automobile manufacturing process, Mitsubishi PLC programs play an important role in large-scale automated production lines. Mitsubishi Q series PLC is used to control 12 servo axes through Q01U PLC, and QD70P8 and QD70P4 modules are used to achieve precise control of the servo axes. At the same time, the height measurement between PLC and CCD camera is achieved through QJ71C24N-R2 module and Keyence DL-RS1A RS-232 communication module; the precise control of equipment position and movement is achieved through the connection of QD62 module and Omron encoder E6C2-CWZ6C; the measurement of equipment outer diameter is achieved through the connection of Q64AD module and Keyence CCD laser tester IG-1000. In addition, in the field of electronic product manufacturing, PLC can be used for the automation transformation of electronic production lines. The automation transformation of electronic production lines through PLC realizes the high-speed operation and precise control of production processes, improves production efficiency and product quality, and increases production efficiency by 40%.
In terms of automated production lines, PLC core development boards are widely used. In industrial automation control, PLC core development boards are responsible for controlling multiple actuators, sensors and robots, improving the automation level and production efficiency of production lines, reducing production costs and human error rates. In building automation management, PLC core development boards are used to control lighting, air conditioning, security and other systems, improving the intelligence level of buildings, improving the comfort of living and working, and reducing energy consumption and maintenance costs. In environmental monitoring and control, PLC core development boards are responsible for collecting and processing various environmental parameters and sending data to remote monitoring centers, improving the accuracy and real-time nature of environmental monitoring, and providing strong support for environmental protection and disaster warning. In energy management systems, PLC core development boards are used to monitor and control various energy equipment, realize refined management of energy use, improve the efficiency and accuracy of energy management, reduce energy waste and costs, and promote sustainable development.
4.2 Application Advantages and Benefit Analysis
PLC has many advantages in application scenarios. First, PLC has the characteristics of high reliability and strong anti-interference ability. The use of modern large-scale integrated circuit technology and advanced anti-interference technology enables PLC to operate stably in complex industrial environments. For example, in the automobile manufacturing process, PLC can stably control the movement of robots and the operation of automated assembly lines, timely detect and handle abnormal situations, and ensure the safety of the production process. Secondly, PLC programming is simple, easy to use, and easy to master. It supports multiple programming languages, such as ladder diagrams, instruction tables, etc., which is convenient for engineers to program according to actual needs and realize complex control logic. In addition, PLC is small in size, powerful in function, and cost-effective. It is simple to install, easy to debug and maintain, and has a small workload. Complete variety, powerful functions, complete hardware support, user-friendly and adaptable.
The application of PLC has brought significant economic benefits. On the one hand, the application of PLC can reduce manual operation and reduce labor costs. For example, in an automated production line, PLC can realize automated control of the production line, reduce manual intervention, improve production efficiency and reduce production costs. On the other hand, PLC can improve production efficiency and product quality and increase the economic benefits of the enterprise. For example, in the automobile manufacturing process, through the precise control of PLC, the production efficiency and product quality of the automobile are improved, and the market competitiveness of the enterprise is increased. In addition, PLC can also reduce energy waste and costs and promote sustainable development. For example, in building automation management, through the control of PLC, intelligent management of lighting, air conditioning and other systems is realized, reducing energy consumption and maintenance costs.
V. Technology Development Trends
5.1 Development in the direction of intelligence
With the continuous advancement of science and technology, PLC is rapidly developing in the direction of intelligence. The future PLC will have stronger learning and adaptive capabilities, and can automatically adjust the control strategy according to different working conditions. For example, through machine learning algorithms, PLC can analyze historical data, predict the operating status and possible failures of equipment, and perform maintenance and adjustments in advance, thereby reducing the risk of production interruptions.
Intelligent PLC will also pay more attention to the convenience of human-computer interaction. The use of more intuitive graphical interfaces and intelligent voice interaction technology will enable operators to program and monitor more easily. At the same time, PLC can also be connected to mobile devices such as smartphones and tablets to achieve remote monitoring and control and improve work efficiency.
In addition, intelligent PLC will be deeply integrated with other intelligent devices to achieve more efficient and flexible control. For example, combined with intelligent sensors, intelligent actuators and other equipment, it can achieve comprehensive perception and precise control of the production process. At the same time, PLC can also work with industrial robots, automated warehousing systems and other equipment to improve production efficiency and quality.
5.2 Integration with other technologies
The integration of PLC with technologies such as the Internet of Things and big data will bring new opportunities and challenges to industrial control.
In terms of integration with Internet of Things technology, future PLCs will have more powerful communication capabilities and can realize functions such as remote monitoring and remote debugging. Through the Internet of Things technology, PLCs can upload the operating data of equipment to the cloud in real time to realize remote management and maintenance of equipment. At the same time, the Internet of Things technology can also realize the interconnection between equipment, improve production efficiency and collaborative work ability. For example, in a smart factory, different equipment can be connected through the Internet of Things technology to realize the automation and intelligence of the production process.
In terms of integration with big data technology, PLC can collect and analyze a large amount of production data to provide decision support for enterprises. By analyzing production data, enterprises can understand the bottlenecks and problems in the production process, optimize the production process, and improve production efficiency and quality. At the same time, big data technologyIt can also achieve predictive maintenance of equipment, discover potential equipment failures in advance, reduce equipment downtime, and improve equipment reliability and availability.
In addition, PLC can also be integrated with artificial intelligence technology to achieve more intelligent control. For example, through artificial intelligence algorithms, PLC can optimize the production process and improve production efficiency and quality. At the same time, artificial intelligence technology can also realize equipment fault diagnosis and prediction, and improve equipment reliability and availability.
VI. Risk Analysis
6.1 Response Speed Risk
The slow response speed of PLC may bring a series of serious risks. In industrial production, it may lead to reduced production efficiency. For example, if the PLC responds too slowly to the sensor signal in an automated production line, it may delay the action of the equipment, thereby affecting the running speed of the entire production line and reducing the output per unit time. According to statistics, on some production lines with high response speed requirements, the production efficiency may drop by about 5% for every 0.1 second reduction in the PLC response speed.
In some control scenarios with extremely high real-time requirements, slow response speed may even cause safety accidents. For example, in the machining process, if the PLC does not respond to the emergency stop signal in time, it may cause equipment damage or casualties.
The main measures to deal with the slow response speed of PLC are as follows:
- Optimize the program: Optimize the PLC program, simplify complex logic, reduce the number of loops and conditional statements, and improve the execution efficiency of the program. For example, if there are a large number of nested loops in the PLC program, it may cause the execution time to be too long. By reasonably adjusting the program structure, the response speed can be significantly improved.
- Decompose tasks: Decompose large tasks into smaller subtasks, and improve response speed by parallel processing or using multiple PLCs to work together. For example, in a complex control system, different functional modules can be assigned to different PLCs to improve overall response speed through distributed control.
- Check input and output modules: Check whether the input and output modules of the PLC are working properly to ensure that there is no delay in the reading and output of signals. If there is a fault in the input and output modules, it may cause poor signal transmission and affect the response speed of the PLC.
- Optimize communication: Optimize the communication method between PLC and other devices to reduce communication delay, such as using a higher-speed communication protocol or reducing the communication load. For example, choosing Ethernet communication protocol instead of traditional serial communication protocol can greatly improve communication speed.
- Check network load: Check the network load of the PLC to ensure that the network bandwidth is sufficient and optimize the network settings to reduce network delay. If the network load is too high, it may cause slow data transmission and affect the response speed of the PLC.
- Check device status: Regularly check the status of PLC devices to ensure that the CPU, memory and other key components are working properly, and perform maintenance or replacement when necessary. If the PLC device itself fails, such as CPU performance degradation, memory failure, etc., it will also cause a slow response speed.
6.2 Anti-interference risk
The impact of external interference on PLC operation cannot be ignored. First, power supply interference may cause PLC system failure. For example, changes within the power grid, such as switching operation surges, start-up and stop of large power equipment, harmonics caused by AC and DC rotating devices, and transient impacts of power grid short circuits, may be transmitted to the power supply end of the PLC through the transmission line, affecting its normal operation. According to the survey, the proportion of PLC control system failures caused by power supply interference is relatively high, about 30%.
Secondly, the interference introduced by the signal line will cause the input and output signals of the PLC to be abnormal, which may greatly reduce the measurement accuracy and even cause damage to components in severe cases. For example, the signal line is interfered by the electromagnetic radiation induction in space, or the power grid interference is connected through the power supply of the transmitter or the common signal instrument, which may affect the normal operation of the PLC.
Furthermore, the interference when the grounding system is chaotic will affect the stability of the PLC. If the grounding is improper, there will be a difference in ground potential between different grounding points, which may cause ground loop current and affect the normal operation of the system. At the same time, the logic voltage interference tolerance of the PLC is low, and the distribution interference of the logic ground potential is easy to affect the logic operation and data storage of the PLC, causing data confusion, program runaway or crash.
Anti-interference measures mainly include the following aspects:
- Power supply interference suppression: Generally, protection is provided by setting shielded cables and PLC local shielding and high-voltage discharge components. Select equipment with better isolation performance, select excellent power supplies, and the routing of power lines and signal lines should be more reasonable. For the main components such as power transformers, central processing units, and programmers, use conductive and magnetic materials with good shielding to prevent the influence of external interference signals. In terms of power supply adjustment and protection, the +5V power supply required by the core components of the microprocessor is processed by multi-stage filtering and adjusted by an integrated voltage regulator to adapt to the fluctuations of the AC power grid and the influence of overvoltage and undervoltage. The grounding method of the shielding layer is different, and the interference suppression effect is also different. Generally, the secondary coil cannot be grounded. The input and output lines should use twisted pairs and the shielding layer should be reliably grounded to suppress common mode interference. In addition, you can install an isolation transformer with a shielding layer and a transformation ratio of 1:1 to reduce the interference between the equipment and the ground. You can also connect an LC filter circuit in series at the power input end.
- Anti-interference measures for signal line introduction: Power lines, control lines, and PLC power lines 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. Separate the I/O lines of the PLC and the high-power lines. If they must be in the same wire duct, bundle the AC lines and DC lines separately. If conditions permit, it is best to route them in separate ducts. This not only allows them to have as much space as possible, but also reduces interference to a minimum. In addition, using a signal isolator to solve the interference problem is also an ideal way. The principle is to first modulate and transform the signal received by the PLC through a semiconductor device, then isolate and transform it through an optical or magnetic induction device, and then demodulate and transform it back to the original signal or a different signal before isolation, and isolate the power supply of the isolated signal at the same time. Ensure that the converted signal, power supply, and ground are absolutely independent. As long as this isolator is added between the input and output ends where there is interference, the interference problem can be effectively solved.
- Correctly select the grounding point and improve the grounding system: Good grounding is an important condition to ensure the reliable operation of the PLC and can avoid accidental voltage shock hazards. There are usually two purposes for grounding, one is for safety, and the other is to suppress interference. A perfect grounding system is one of the important measures for PLC control systems to resist electromagnetic interference. In PLC control systems, there are many forms of grounding, mainly signal ground, shielding ground, protection ground, etc. Generally, the grounding method is related to the signal frequency. When the frequency is lower than 1MHz, one-point grounding can be used; when it is higher than 10MHz, multi-point grounding is used; between 1~10MHz, under normal circumstances, the PLC control system uses one-point grounding, connecting all ground terminals to the nearest grounding point to obtain the best anti-interference ability. The cross-sectional area of the grounding wire cannot be less than 2mm², and the grounding resistance cannot be greater than 100Ω; a dedicated grounding wire is used for the grounding wire.