Programmable Logic Controller (PLC) uses a type of programmable memory to store programs internally and perform logical operations, sequence control, timing, counting and arithmetic operations, etc. User-oriented instructions and control of various types of machinery or production processes through digital or analog input/output
The basic structure of a programmable logic controller is essentially a computer dedicated to industrial control.
p>Programmable logic controller
The hardware structure of the programmable logic controller is basically the same as that of a microcomputer. The basic composition is: 1. Power supply. The power supply of the programmable logic controller plays a very important role in the entire system. effect. Without a good and reliable power supply system, it cannot work properly. Therefore, manufacturers of programmable logic controllers also attach great importance to the design and manufacturing of power supplies. Generally, the AC voltage fluctuation is within the range of +10% (+15%), and the PLC can be directly connected to the AC power grid without taking other measures. 2. Central Processing Unit (CPU) The Central Processing Unit (CPU) is a programmable logic controller control center. It receives and stores the user program and data entered from the programmer according to the functions assigned by the programmable logic controller system program; checks the status of power supply, memory, I/O and alert timers, and can diagnose syntax errors in the user program. When the programmable logic controller is put into operation, it first receives the status and data of each input device on site in a scanning manner, and stores them in the I/O image area respectively, and then reads the user program one by one from the user program memory. After the command is interpreted, the result of logical or arithmetic operation is performed according to the instructions and sent to the I/O image area or data register. After all user programs are executed, each output status of the I/O image area or the data in the output register is finally transferred to the corresponding output device, and this cycle continues until the operation stops. In order to further improve the reliability of programmable logic controllers, in recent years, dual CPUs have been used to form a redundant system for large programmable logic controllers, or a three-CPU voting system has been adopted. In this way, even if a certain CPU fails, the entire system can still run normally. 3. Memory The memory that stores system software is called system program memory. The memory that stores application software is called user program memory. 4. Input and output interface circuit 1. The field input interface circuit consists of an optical coupling circuit and a microcomputer input interface circuit, and functions as an input channel for the interface between the programmable logic controller and field control. 2． The field output interface circuit is integrated by an output data register, a strobe circuit and an interrupt request circuit, and functions as a programmable logic controller to output corresponding control signals to the on-site execution components through the field output interface circuit. 5. Functional modules such as counting, positioning and other functional modules. 6. Working principle of communication module [1] When the programmable logic controller is put into operation, its working process is generally divided into three stages,
Programmable logic controller
i.e. input There are three stages: sampling, user program execution and output refreshing. Completing the above three stages is called a scan cycle. During the entire operation, the CPU of the programmable logic controller repeatedly executes the above three stages at a certain scanning speed. 1. Input sampling stage In the input sampling stage, the programmable logic controller sequentially reads in all input status and data in a scanning manner, and stores them in the corresponding units in the I/O image area. After the input sampling is completed, it enters the user program execution and output refresh phase. In these two phases, even if the input status and data change, the status and data of the corresponding unit in the I/O image area will not change. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scan period to ensure that the input can be read under any circumstances. 2. User program execution stage In the user program execution stage, the programmable logic controller always scans the user program (ladder diagram) in order from top to bottom. When scanning each ladder diagram, the control circuit composed of contacts on the left side of the ladder diagram is always scanned first, and logical operations are performed on the control circuit composed of contacts in the order of first left, then right, first up, then down. , and then according to the result of the logic operation, refresh the status of the corresponding bit of the logic coil in the system RAM storage area; or refresh the status of the corresponding bit of the output coil in the I/O image area; or determine whether to execute the ladder diagram. Specified special function instructions.
That is, during the execution of the user program, only the status and data of the input points in the I/O image area will not change, while the status and data of other output points and software devices in the I/O image area or system RAM storage area will not change. The status and data may change, and the program execution results of the ladder diagram listed above will affect any ladder diagram listed below that uses these coils or data; on the contrary, the ladder diagram listed below will have its program execution results affected. The status or data of the refreshed logic coil can only take effect on the program above it until the next scan cycle. If you use the immediate I/O instruction during program execution, you can directly access the I/O point. Even if I/O instructions are used, the value of the input process image register will not be updated. The program directly obtains the value from the I/O module, and the output process image register will be updated immediately. This is somewhat different from immediate input. 3. Output refresh stage After scanning the user program, the programmable logic controller enters the output refresh stage. During this period, the CPU refreshes all output latch circuits according to the corresponding status and data in the I/O image area, and then drives the corresponding peripherals through the output circuits. At this time, it is the real output of the programmable logic controller. Functional Features The programmable logic controller has the following distinctive features. 1. Easy to use and simple programming uses concise ladder diagram, logic diagram or statement list and other programming languages ??without computer knowledge, so the system development cycle is short and on-site debugging is easy. In addition, the program can be modified online and the control scheme can be changed without dismantling the hardware. 2. Strong functions and high performance-price ratio. A small PLC has hundreds or thousands of programming components available for users. It has strong functions and can realize very complex control functions. Compared with relay systems with the same functions, it has a high performance-price ratio. PLC can be networked through communication to achieve decentralized control and centralized management. 3. The hardware is complete, user-friendly, and highly adaptable. PLC products have been standardized, serialized, and modularized. They are equipped with a full range of hardware devices for users to choose. Users can flexibly and conveniently configure the system to form different functions. Systems of different sizes. The installation and wiring of PLC is also very convenient. Generally, terminal blocks are used to connect external wiring. PLC has strong load capacity and can directly drive general solenoid valves and small AC contactors. After the hardware configuration is determined, the user program can be modified to easily and quickly adapt to changes in process conditions. 4. High reliability and strong anti-interference ability. Traditional relay control systems use a large number of intermediate relays and time relays, which are prone to failure due to poor contact. PLC uses software to replace a large number of intermediate relays and time relays, leaving only a small number of hardware components related to input and output. The wiring can be reduced to 1/10-1/100 of the relay control system. Failures caused by poor contact contact are large. for reduction. PLC has adopted a series of hardware and software anti-interference measures. It has strong anti-interference ability and the average time between failures reaches tens of thousands of hours. It can be directly used in industrial production sites with strong interference. PLC has been recognized by the majority of users as the most One of the most reliable industrial control equipment. 5. Less workload in system design, installation and debugging. PLC uses software functions to replace a large number of intermediate relays, time relays, counters and other devices in the relay control system, greatly reducing the workload of design, installation and wiring of the control cabinet. PLC ladder diagram programs are generally designed using the sequential control design method. This programming method is very regular and easy to master. For complex control systems, it takes much less time to design a ladder diagram than it does to design a relay system circuit diagram with the same functionality. The user program of the PLC can be simulated and debugged in the laboratory. The input signal is simulated with a small switch, and the status of the output signal can be observed through the light-emitting diode on the PLC. After the installation and wiring of the system are completed, problems discovered during on-site unified debugging can generally be solved by modifying the program. The debugging time of the system is much shorter than that of the relay system. 6. The maintenance workload is small and the maintenance is convenient. The PLC has a very low failure rate and has complete self-diagnosis and display functions.
When a fault occurs in the PLC or external input device or actuator, the cause of the fault can be quickly identified based on the information provided by the light-emitting diodes on the PLC or the programmer, and the fault can be quickly eliminated by replacing the module. Development History Originated in the United States in 1968 General Motors put forward the requirement to replace the relay control device; in 1969, the American Digital Equipment Corporation developed the first programmable logic controller PDP-14, which was successfully tried on the production line of General Motors in the United States, and used programmed methods for the first time. Applied to electrical control, this is the first generation of programmable logic controller, called Programmable Logic Controller, or PLC for short, and is recognized as the first PLC in the world. In 1969, the United States developed the world's first PDP-14; in 1971, Japan developed the first DCS-8; in 1973, Germany's Siemens developed Europe's first PLC, model SIMATIC S4; in 1974 In 1977, China developed the first PLC and began industrial application in 1977. Development Microprocessors appeared in the early 1970s. People quickly introduced it into programmable logic controllers, which added computing, data transmission and processing functions to the programmable logic controller, completing an industrial control device that truly has computer characteristics. The programmable logic controller at this time is a product of the combination of microcomputer technology and conventional relay control concepts. After the development of personal computers, in order to facilitate and reflect the functional characteristics of programmable controllers, the programmable logic controller was named Programmable Logic Controller (PLC). In the mid-to-late 1970s, programmable logic controllers entered the stage of practical development. Computer technology has been fully introduced into programmable controllers, which has made a leap in their functions. Higher computing speed, ultra-small size, more reliable industrial anti-interference design, analog calculation, PID function and extremely high cost performance have established its position in modern industry. In the early 1980s, programmable logic controllers have been widely used in advanced industrial countries. There are more and more countries producing programmable controllers in the world, and their output is rising day by day. This marks that programmable controllers have entered a mature stage. From the 1980s to the mid-1990s, programmable logic controllers developed fastest, with an annual growth rate of 30 to 40%. During this period, PLC's analog processing capabilities, digital computing capabilities, human-machine interface capabilities and network capabilities have been greatly improved. Programmable logic controllers have gradually entered the field of process control, replacing their predecessors in the field of process control in some applications. Dominant DCS system. At the end of the 20th century, the development of programmable logic controllers was characterized by being more adaptable to the needs of modern industry. During this period, mainframes and ultra-small computers were developed, various special function units were born, and various human-machine interface units and communication units were produced, making it easier to match industrial control equipment using programmable logic controllers. System integration In the manufacturing industry, there are a large number of open-loop sequential controls based on switching values, which perform sequential action numbers according to logical conditions and act in sequence; in addition, there are also controls for chain protection actions based on logical relationships that have nothing to do with sequence and timing. ; and a large number of discrete quantity data collection and monitoring based on status quantities such as switching quantities, pulse quantities, timing, counters, and analog quantity over-limit alarms. Due to these control and monitoring requirements, PLC has developed into a product that replaces relay circuits and performs sequence control. In recent years, PLC manufacturers have gradually added various communication interfaces to the original CPU template, and fieldbus technology and Ethernet technology have also developed simultaneously, making PLC's application scope more and more extensive. PLC has the advantages of stability, reliability, low price, complete functions, flexible and convenient application, and convenient operation and maintenance. This is the fundamental reason why it can occupy the market for a long time.
Programmable logic controller [2] The hardware of the PLC controller itself adopts a building block structure, including a motherboard, digital I/O template, analog I/O template, special positioning template, and barcode By identifying modules such as templates, users can obtain the desired number of I/Os by expanding on the motherboard or using bus technology to equip remote I/O slaves as needed.
While realizing various numbers of I/O controls, PLC also has the ability to output analog voltages and digital pulses, allowing it to control various servo motors, stepper motors, variable frequency motors, etc. that can receive these signals, plus a touch screen With human-machine interface support, Schneider's PLC can meet your needs at any level in process control. Selection rules When designing a programmable logic controller system, the control scheme should be determined first, and the next step is to select the programmable logic controller engineering design. The characteristics and application requirements of the process flow are the main basis for design selection. Programmable logic controllers and related equipment should be integrated and standard. The selected programmable logic controllers should be in operation in relevant industrial fields in accordance with the principle of being easy to form an integral whole with the industrial control system and easy to expand their functions. For a proven, mature and reliable system, the system hardware, software configuration and functions of the programmable logic controller should be adapted to the device scale and control requirements. Familiarity with programmable controllers, function chart diagrams and related programming languages ??will help shorten programming time. Therefore, when selecting and estimating engineering design, the characteristics and control requirements of the process should be analyzed in detail, and the control tasks and scope determination requirements should be clearly understood. Then, according to the control requirements, estimate the number of input and output points, required memory capacity, determine the functions of the programmable logic controller, external device characteristics, etc., and finally select a programmable logic controller and design with higher performance-price ratio Corresponding control system. 1. Estimation of the number of input and output (I/O) points. Appropriate margin should be considered when estimating the number of I/O points. Usually, based on the statistical number of input and output points, an expandable margin of 10% to 20% is added as input and output. Points estimate data. When actually ordering, the number of input and output points needs to be rounded according to the product characteristics of the manufacturer's programmable logic controller. 2. Estimation of memory capacity Memory capacity is the size of the hardware storage unit that the programmable controller itself can provide. Program capacity is the size of the storage unit used by user application projects in the memory, so the program capacity is smaller than the memory capacity. In the design stage, since the user application program has not yet been compiled, the program capacity is unknown at the design stage and needs to be known after program debugging. In order to have a certain estimate of program capacity when designing and selecting, an estimate of memory capacity is usually used instead. There is no fixed formula for estimating memory capacity. Different formulas are given in many literatures. Generally speaking, they are based on 10 to 15 times the number of digital I/O points, plus 100 times the number of analog I/O points. The number is the total number of words in the memory (16 bits is one word), and a margin of 25% of this number is considered. 3. Selection of control functions This selection includes selection of features such as computing functions, control functions, communication functions, programming functions, diagnostic functions, and processing speed. 1. The computing functions of simple programmable logic controllers include logical operations, timing and counting functions; the computing functions of ordinary programmable logic controllers also include data shifting, comparison and other computing functions; more complex computing functions include algebraic operations, Data transmission, etc.; large programmable logic controllers also have analog PID calculations and other advanced calculation functions. With the emergence of open systems, programmable logic controllers currently have communication functions. Some products have communication with lower computers, some products have communication with peer computers or host computers, and some products also have communication with factories or enterprises. The function of network for data communication. When designing and selecting, the required computing functions should be reasonably selected based on the requirements of the actual application. Most applications only require logical operations and timing and counting functions. Some applications require data transmission and comparison. When used for analog detection and control, algebraic operations, numerical conversion and PID operations are used. To display data, operations such as decoding and encoding are required. 2. Control function The control function includes PID control operation, feedforward compensation control operation, ratio control operation, etc., which should be determined according to the control requirements. Programmable logic controllers are mainly used for sequential logic control. Therefore, single-loop or multi-loop controllers are often used in most situations to solve the control of analog quantities. Sometimes dedicated intelligent input and output units are also used to complete the required control functions and improve reliability. Program logic controllers for processing speed and memory capacity savings. For example, PID control unit, high-speed counter, analog unit with speed compensation, ASC code conversion unit, etc. are used. 3. Communication function Large and medium-sized programmable logic controller systems should support a variety of field buses and standard communication protocols (such as TCP/IP), and should be able to connect to the factory management network (TCP/IP) when necessary.
The communication protocol should comply with ISO/IEEE communication standards and should be an open communication network. The communication interface of the programmable logic controller system should include serial and parallel communication interfaces, RIO communication ports, commonly used DCS interfaces, etc.; the communication bus (including interface equipment and cables) of large and medium-sized programmable logic controllers should be configured with a 1:1 redundancy , the communication bus should comply with international standards, and the communication distance should meet the actual requirements of the device. In the communication network of the programmable logic controller system, the superior network communication rate should be greater than 1Mbps, and the communication load should not be greater than 60%. The main forms of the communication network of the programmable logic controller system are as follows: 1), PC is the master station, and multiple programmable logic controllers of the same model are the slave stations, forming a simple programmable logic controller network; 2), One programmable logic controller is the master station, and other programmable logic controllers of the same model are slave stations, forming a master-slave programmable logic controller network; 3), the programmable logic controller network is connected to a large-scale network through a specific network interface As a subnet of DCS in DCS; 4), dedicated programmable logic controller network (dedicated programmable logic controller communication network of each manufacturer). In order to alleviate the CPU communication tasks, communication processors with different communication functions (such as point-to-point, fieldbus, etc.) should be selected according to the actual needs of the network composition. 4. Offline programming method of programming function: The programmable logic controller and the programmer share a CPU. When the programmer is in programming mode, the CPU only provides services for the programmer and does not control the field equipment. After completing programming, the programmer switches to running mode, and the CPU controls the field device and cannot program. Offline programming can reduce system costs, but is inconvenient to use and debug. Online programming method: The CPU and the programmer have their own CPUs. The host CPU is responsible for on-site control and exchanges data with the programmer within a scan cycle. The programmer sends the program or data compiled online to the host. In the next scan cycle, The host runs according to the newly received program. This method is more expensive, but it is easy to debug and operate the system. It is often used in large and medium-sized programmable logic controllers. Five standardized programming languages: three graphical languages, Sequential Function Chart (SFC), Ladder Diagram (LD), and Function Block Diagram (FBD), and two text languages, Statement List (IL) and Structured Text (ST). The selected programming language should comply with its standard (IEC6113123). At the same time, it should also support multiple language programming forms, such as C, Basic, etc., to meet the control requirements of special control occasions. 5. Diagnostic function The diagnostic function of the programmable logic controller includes hardware and software diagnosis. Hardware diagnosis determines the fault location of the hardware through logical judgment of the hardware, and software diagnosis is divided into internal diagnosis and external diagnosis. Diagnosing the internal performance and functions of the PLC through software is internal diagnosis, and using software to diagnose the information exchange functions of the CPU and external input and output components of the programmable logic controller is external diagnosis. The strength of the diagnostic function of the programmable logic controller directly affects the technical ability requirements of operation and maintenance personnel, and affects the average maintenance time. 6. Processing speed The programmable logic controller works in scanning mode. From the perspective of real-time requirements, the processing speed should be as fast as possible. If the signal duration is less than the scanning time, the programmable logic controller will not be able to scan the signal, resulting in the loss of signal data. The processing speed is related to the length of the user program, CPU processing speed, software quality, etc. At present, programmable logic controller contacts have fast response and high speed, and the execution time of each binary instruction is about 0.2 to 0.4Ls. Therefore, it can adapt to the needs of applications with high control requirements and fast response requirements. The scan cycle (processor scan cycle) should meet: the scan time of small programmable logic controllers is no more than 0.5ms/K; the scan time of large and medium-sized programmable logic controllers is no more than 0.2ms/K. 4. Types of programmable logic controllers Programmable logic controllers are divided into two types: integral type and modular type according to the structure. They are divided into two types: on-site installation and control room installation according to the application environment; according to the CPU word length, they are divided into 1-bit, 4-bit bit, 8-bit, 16-bit, 32-bit, 64-bit, etc. From an application perspective, selection can usually be made based on control function or number of input and output points. The integral programmable logic controller has a fixed number of I/O points, so users have less room for choice and is used for small control systems; the modular programmable logic controller provides a variety of I/O cards or plug-in cards, so users can The I/O points of the control system are reasonably selected and configured, and the function expansion is convenient and flexible. It is generally used in large and medium-sized control systems.
5. PLC input/output type switching value. The switching value mainly refers to the input value and the output value. It refers to the auxiliary point of a device, such as the auxiliary point of the relay of the thermostat of the transformer (the transformer changes position after overheating. ), the auxiliary point of the valve cam switch (displacement after the valve switches), the auxiliary point of the contactor (displacement after the contactor acts), the thermal relay (displacement after the thermal relay acts), these points are generally transmitted For PLC or comprehensive protection device, the power is generally provided by the PLC or comprehensive protection device. It does not have power itself, so it is called a passive contact, also called the input of PLC or comprehensive protection device. 1. Digital quantities Physical quantities that are discrete in time and quantity are called digital quantities. Signals representing digital quantities are called digital signals. Electronic circuits that work under digital signals are called digital circuits. For example: When an electronic circuit is used to record the number of parts output from an automatic production line, a signal is given to the electronic circuit every time a part is sent out, so that it is recorded as 1. However, when no parts are sent out, the signal added to the electronic circuit is 0, and the signal is recorded as 0. number. It can be seen that the signal of the number of parts is discontinuous in both time and quantity, so it is a digital signal. The smallest quantity unit is 1. 2. Analog quantity: A physical quantity that is continuous in time or numerical value is called an analog quantity. The signal representing the analog quantity is called an analog signal. Electronic circuits that work under analog signals are called analog circuits. For example: The voltage signal output by a thermocouple during operation is an analog signal, because the measured temperature is unlikely to jump under any circumstances, so the measured voltage signal is continuous both in time and quantity. Moreover, any value of this voltage signal during the continuous change process has a specific physical meaning, that is, it represents a corresponding temperature. 6. Conversion Principle 1. Digital-to-analog converter is a system that converts digital signals into analog signals. This can generally be achieved by low-pass filtering. The digital signal is first decoded, that is, the digital code is converted into a corresponding level to form a ladder-like signal, and then low-pass filtered. According to the theory of signals and systems, the digital staircase signal can be regarded as the convolution of the ideal impulse sampling signal and the rectangular pulse signal. Then according to the convolution theorem, the spectrum of the digital signal is the spectrum of the impulse sampling signal and the rectangular pulse spectrum (i.e. Sa function) product. In this way, the reciprocal of the Sa function is used as the spectrum characteristic compensation, and the digital signal can be restored to the sampled signal. According to the sampling theorem, the spectrum of the sampled signal can be obtained by ideal low-pass filtering of the spectrum of the original analog signal. Generally, these principles are not directly used in implementation, because sharp sampling signals are difficult to obtain. Therefore, the two filters (Sa function and ideal low pass) can be combined (cascaded), and since the filtering characteristics of each system are physical It is not achievable, so it can only be approximately completed in a real system. 2. Analog-to-digital converter is a system that converts analog signals into digital signals. It is a process of filtering, sampling and holding, and encoding. The analog signal is band-limited filtered and sampled and held by a circuit to turn it into a ladder-shaped signal, and then passes through the encoder to convert each level in the ladder-shaped signal into a binary code. Choose from a wide variety of model PLC products. Different PLC models have different structural forms, performance, capacity, instruction systems, programming methods, prices, etc., and their applicable occasions also have different emphasis. Therefore, the reasonable selection of PLC is of great significance for improving the technical and economic indicators of the PLC control system.