With the development of computer technology, storage logic began to enter the industrial control field. Programmable Logic Controller (Programmable Logic Controller, PLC) as a general industrial control computer, is the representative results of storage logic in industrial applications.
Since 1969, the first PLC was successfully developed and applied to automobile manufacturing automatic assembly line, PLC has been constantly updated. Especially in the past two decades, the rapid development of programmable controller technology, increasingly powerful, widely used in the production process, as one of the three pillars of industrial automation technology plays an increasingly important role in the economic field.
PLC is based on the traditional sequence controller, synthesized computer technology, microelectronics technology, automatic control technology, digital technology and communication network technology and the formation of a new type of general industrial automatic control device, is an important pillar of modern industrial control. This section mainly introduces the use, characteristics, classification and performance indicators of PLC.
Applications of PLC
In the recent decade, with the microprocessor chip and related components prices have dropped dramatically, the price of PLC has also dropped, but the function has been greatly enhanced, can solve complex computing and communication problems, so PLC is more and more widely used.
At present, PLC has been widely used at home and abroad in steel, mining, cement, petroleum, chemical, electric power, machinery manufacturing, automotive, loading and unloading, paper, textile, environmental protection and entertainment industries.
The application scope of PLC can usually be divided into the following five categories
1) Sequence control
Sequence control is the most widely used field of PLC, and it is also the most suitable field for PLC to play its specialties.PLC sequence control is used to replace the traditional relay sequence control.
PLCs are used in single machine control, multi-level group control, and automatic production line control, such as injection molding machines, printing machines, stapling machines, packaging machines, paper cutting machines, combined machine tools, grinding machines, assembly lines, plating lines, and elevator control.
2) Motion Control
PLC manufacturers now offer single- or multi-axis position control modules for stepper motors or servo motors. In most cases, the PLC sends data describing the target position to the control module, and its output moves one or more axes to reach the target position. As each axis moves, the position control module maintains the proper speed and acceleration to ensure smooth motion.
Comparatively speaking, the position control module is smaller, less expensive, faster and easier to operate than a Computer Number Control (CNC) device.
3) process control
PLC can also monitor a large number of physical parameters, such as temperature, pressure, flow, level and speed. Proportional - integral - differential (Proportion Integration Differentiation, PID) module makes the PLC has a closed-loop control function, that is, a PLC with PID control capabilities can be used for process control. When a variable in the process control deviates, the PID control algorithm calculates the correct amount of control to keep the output at the set value.
4) Data Processing
In machining, PLCs are used in CNC systems as the main control and management system, and can perform a large amount of data processing.
5) Communication Network
PLC communication includes communication between the host and remote I/O, communication between multiple PLCs, and communication between PLCs and other intelligent control devices (such as computers, inverters, numerical control devices, etc.) PLCs, together with other intelligent control devices, can be used to form a distributed control system with "centralized management and decentralized control".
PLC features
1) high reliability, strong anti-interference ability
In order to meet the requirements of industrial production on the safety and reliability of control equipment, PLC uses microelectronics technology, a large number of switching actions are completed by the non-contact semiconductor circuits, in the structure of the industrial production environment to take full account of the temperature, humidity, dust, vibration and other aspects of the impact: in the hardware used in isolation, filtering, shielding, grounding and other anti-interference measures; in the software used in the fault diagnosis, Data protection and other measures. These technologies make the PLC has a high anti-interference ability.
At present, the PLC produced by various manufacturers, the average failure-free time are far more than the International Electrotechnical Commission (International Electrotechnical Commission, IEC) stipulated 100,000 hours, and some have even reached hundreds of thousands of hours.
2) Universal and Flexible
PLC products have been serialized production, a variety of structural forms, there is a lot of room for choice in the model. In addition, PLC and peripheral module varieties, the user can be based on the requirements of different tasks, choose different components to flexibly combine into a control device with different hardware structure.
More importantly, the main function of the PLC control system is realized through the program, so in the need to change the control function of the device, only need to modify the program and a small amount of wiring, the workload is very small, and this is a general relay control system is difficult to do.
3) Programming is simple and convenient
PLC application program is very easy to prepare. Programming can be used with the relay contactor control circuit is very similar to the ladder language, this programming language image intuitive, easy to grasp, even without computer knowledge is also very easy to master. The sequence function chart (Sequential Function Chart, SFC) is a structure block control flow chart, which can make programming more simple and convenient.
4) Functionality is perfect, strong expansion capabilities
PLC's input / output system is functionally perfect, reliable performance, can adapt to various forms and nature of the switching and analog input / output.
PLC function unit can easily realize D / A, A/D conversion and PID operation, process control, digital control and other functions. It can also be used with other computer systems and control equipment to form a distributed or decentralized control system, which can well meet the needs of various control. Public number "Mechanical Engineering Literature", the engineer's gas station!
5) Short cycle of design, construction and commissioning, easy maintenance
Relay contactor control system in the intermediate relay, time relay, counter and other electrical components in the PLC control system is a "soft component" form, and the program instead of hard wiring, so the installation and wiring workload is small; staff can also be in advance according to the specific control requirements in the PLC before the arrival of programming, greatly reducing the construction workload. Staff can also program the PLC in advance according to the specific control requirements before the arrival of the PLC, which greatly shortens the construction period.
PLC is small in size, light in weight, and easy to install; PLC has perfect self-diagnostic and monitoring functions, such as displaying its internal working status, communication status, I/O point status, abnormal status, and power supply status. The staff can find out the cause of the fault through it, so that it is easy to deal with it quickly.
Because of the above characteristics of PLC, PLC has a wide range of applications, it can be said that as long as there are factories, there will be PLC applications for control requirements.
Classification of PLC
PLC is produced in response to the needs of modernized production, and the classification of PLC must be in line with the needs of modernized production. Generally speaking, PLC can be categorized from three perspectives, namely, control scale, control performance, structural characteristics.
1, according to the PLC control scale classification
PLC according to the control scale can be divided into small PLC, medium-sized PLC and large PLC.
1) Small PLC
Small PLC generally refers to PLCs with less than 256 input/output points (I/O points), a single CPU (8-bit or 16-bit), and a user program memory of less than 4KB, mainly for switching control.
Due to the limitation of the number of control points, its control function has certain limitations. However, small PLC small, flexible, can be installed directly in the electrical control cabinet, very suitable for stand-alone control or small system control.
The S7-200 and S7-1200 series from Siemens, Germany, and the FX series from Mitsubishi, Japan, are small PLCs.
2) Medium-sized PLC
Medium-sized PLC generally refers to PLCs with 256~2048 I/O points, dual CPUs or multiple CPUs, and a user program memory of 2~8KB or larger, with switching and analog control functions and stronger digital calculation capabilities.
Because medium-sized PLCs have more control points and strong control functions, they can be used for direct control of equipment and monitoring of multiple lower-level PLCs, and are suitable for control of medium-sized or large control systems.
Siemens S7-300 series, Japan's OMRON C200H series, Japan's Mitsubishi Q series of some models belong to the medium-sized PLC.
3) Large PLC
Large PLC generally refers to PLCs with more than 2048 I/O points, dual CPUs or multiple CPUs (16-bit or 32-bit), and a user program memory of 8-16KB or larger, which can perform not only complex arithmetic operations but also complex matrix operations due to the large number of control points, strong control functions, and strong computational capabilities.
Large PLC can not only be used for direct control of equipment, but also for monitoring and control of multiple lower-level PLCs to form a centralized production process control system. Large PLCs are suitable for equipment automation processes, process automation control and process monitoring systems.
Siemens S7-400 series, Japan's OMRON CVM1 and CS1 series, Japan's Mitsubishi Q series of some of the models are large PLC.
2, according to the PLC control performance classification
PLCs can be categorized into low-grade, mid-grade and high-grade machines according to their control performance.
1) Low-grade machine
This type of PLC has basic control functions and general arithmetic ability, low operating speed, and supports fewer numbers and types of input and output modules.
These PLCs are suitable for simple control on a small scale, and are generally suitable for use as a slave station in a network. For example, Siemens S7-200 series belongs to this category.
2) Mid-range
This kind of PLC has strong control function and strong calculation ability, which can not only complete general logic operation, but also can complete more complex trigonometric operation, exponential operation and PID operation, with faster working speed, more input and output modules, and more types of input and output modules.
This kind of PLC can not only accomplish small-scale control tasks, but also larger-scale control tasks, and can be used as a slave station or master station in networking. For example, Siemens S7-300 series belongs to this category.
3) High-grade machine
This type of PLC has powerful control functions and strong arithmetic ability, not only can complete the logic operation, trigonometric function operation, exponential operation and PID operation, but also can carry out complex matrix calculation, working speed is very fast, and it can drive a large number of input and output modules, and the types are comprehensive. The public number "Mechanical Engineering Literature", the gas station of engineers!
This type of PLC can not only complete medium-sized control tasks, but also can complete a very large scale control tasks, in the networking is generally used as the master station. For example, Siemens S7-400 series belongs to this category.
PLC can be categorized into two types according to the structure: integral type and combined type.
3,Categorized by PLC structure
1) Integral type
Integral structure of the PLC power supply, CPU, memory, I / O system is compactly installed in a standard enclosure, as a whole, constituting the basic unit of the PLC.
A basic unit is a complete PLC that can realize various controls. When the number of control points does not meet the needs, it can be connected to an expansion unit, which does not have a CPU, and the basic unit and a number of expansion units can form a larger system.
The advantages of the overall structure is very compact, small size, low cost, easy to install, the disadvantage is that the number of input and output points have a limited ratio. Small PLC mostly for the overall structure. For example, Siemens S7-200 series and Japan's Mitsubishi FX series PLC is the overall structure.
2) Combination type
Combination PLC is a PLC system components are functionally divided into a number of modules, such as CPU modules, input modules, output modules, power supply modules, etc., and these modules can be inserted into the frame or substrate to form a complete control system. Although the function of each module is relatively single, but the types of modules are increasingly rich.
For example, some PLC in addition to the basic I/O modules, there are some special function modules, such as temperature detection module, position detection module, PID control module, communication module and so on. PLCs of the modular structure use a building block approach to form a system by inserting the required modules on a substrate.
The characteristics of PLC of combined structure are that CPU, input and output are independent modules, uniform module size, easy installation, free selection of I/O modules (according to the number of points), and convenient installation, debugging, expansion and maintenance.
Medium-sized and large-sized machines are mostly of combined structure, for example, S7-300 series and S7-400 series of Siemens and Q series PLC of Mitsubishi.
The composition of the combined PLC is shown in the figure below, and the modules are connected with each other through the bus on the base plate, and the distance between the CPU and the expansion modules should not exceed 10m if they are connected by cables.
Technical specifications of PLC
The technical indicators of PLC include hardware indicators and software indicators.
1, hardware indicators
Hardware indicators include general indicators, input characteristics and output characteristics.
General indicators are mainly reflected in the ambient temperature, ambient humidity, vibration, shock, noise, interference and voltage resistance and other performance.
Input characteristics are mainly reflected in the degree of isolation of the input circuit, input sensitivity, response time and the required power supply performance.
Output characteristics are mainly reflected in the circuit composition (here refers to the relay output, transistor output or thyristor output), circuit isolation, maximum load, minimum load, response time and external power supply performance.
2,Software index
Software indexes mainly include program capacity, programming language, communication function, running speed, instruction type, type and number of components. Public number "Mechanical Engineering Literature", the gas station for engineers!
Program capacity refers to the size of the PLC's memory and external memory, generally from several kilobytes to several megabytes. The types of memory are generally RAM, EPROM and EEPROM.
Programming language is the language used by the PLC to write the user program. there are many programming languages that can be used by the PLC, such as ladder diagrams, statement tables, sequential function charts and function block diagrams. Each additional programming language will make the preparation of user programs faster and more convenient.
Communication function refers to whether or not the PLC has communication capability and what kind of communication capability it has. Generally, it can be categorized into remote I/O communication, computer communication, point-to-point communication, high-speed bus, MAP network, and so on. Currently, communication capability is a major indicator of PLC performance.
Running speed refers to the length of the operation processing time, which can be measured by the execution time of the basic instruction, the shorter the better, generally below the microsecond level. The stronger the function of the instruction, the better the performance of the PLC.
The number of types and number of components not only reflects the performance of the PLC, but also indicates the size of the PLC. the number of I/O components indicates the input/output capability of the PLC; the number of types of I/O components (DC, AC, analog, high-speed counting, positioning, PID) indicates the performance of the PLC.
3, the main performance indicators
1) Memory capacity
The storage capacity refers to the capacity of the user program memory. The storage capacity determines the size of the user program that the PLC can accommodate, and is generally calculated in units of bytes. Every 1024 bytes is 1KB. The storage capacity of medium-sized and small PLCs is generally below 8KB, while the storage capacity of large PLCs can reach 256KB~2MB. Some PLCs use the number of user program commands to indicate the capacity, and the number of commands stored in a medium-sized or small PLC is generally 2,000 commands.
2) Number of Input/Output (I/O) Points
The number of I/O points refers to the sum of the number of input points and the number of output points, the more I/O points, the more external access to the input devices and output devices, the larger the scale of control, so the number of I/O points is a measure of the size of the PLC index. The international popularity of the total number of I/O points in 64 points and below the PLC is called micro PLC; 64 ~ 256 points is called a small PLC; 256 ~ 2048 points is called a medium-sized PLC; 2048 points or more is called a large PLC.
3) Scanning Speed
Scanning speed refers to the speed at which a PLC executes a program. Generally, the scanning speed is measured by the time it takes to execute 1KB. The speed of executing instructions for different functions varies greatly, and currently the speed of executing Boolean instructions is also used to characterize the speed of PLC operation. Some brands of PLCs give the time used to execute various instructions in the user's manual, and the speed of PLC operation can be measured by comparing the time used by various PLCs to perform similar operations.
4) Function and number of instructions
The strength of the function and the number of instructions reflect the strength of the PLC's capability. Generally speaking, the more types and number of programming instructions, the stronger the processing and control capabilities, and the easier it is to prepare user programs.
5) Types and number of internal components
When programming a program, a large number of internal components are required to store variables, intermediate results, timing and counting information, module setup parameters, and various flag bits. The greater the number and type of such components, the greater the information processing capability of the PLC.
6) Number of Intelligent Units
In order to accomplish some special control tasks, PLC manufacturers have designed special intelligent units for their products, such as analog control units, positioning control units, speed control units and communication units. The number of types of intelligent units and the strength of the function is an important indicator of the level of PLC products.
7) Expansion capability
The expansion capability of PLC includes the expansion of the number of I/O points, the expansion of storage capacity, the expansion of networking functions and the expansion of the connection of various modules. Most of the PLCs can expand the number of I/O points with I/O expansion units; some PLCs can be expanded with various function modules. However, there is always a limit to the expansion function of PLC.
After understanding the various indexes of PLC, you can select a suitable product from a large number of PLCs according to the requirements of specific control projects.