What is a PLC control system? What does PLC mean? What is PLC

Since the 1960s, when the United States introduced Programmable Logic Controllers (PLCs) to replace traditional relay control devices, PLCs have experienced rapid development and have been widely used around the world. At the same time, the functions of PLC are constantly improving. With the continuous development of computer technology, signal processing technology, control technology, and network technology, as well as the increasing demand from users, PLC has added functions such as analog processing and motion control on the basis of switch processing. Today’s PLCs are no longer limited to logic control, but also play a very important role in fields such as motion control and process control.

As the preferred product for discrete control, PLC developed rapidly in the 1980s and 1990s, with an annual growth rate of 20% to 30% worldwide. With the continuous improvement of factory automation and the expansion of the PLC market capacity base, the growth rate of PLC in industrialized countries has slowed down in recent years. However, the growth of PLC is very rapid in developing countries such as China. Based on relevant information, the global sales revenue of PLCs in 2004 was around 10 billion US dollars, occupying a very important position in the field of automation.

PLC was developed based on the simulation of the original relay control principle. In the 1970s, PLC only had switch logic control and was first applied in the automotive manufacturing industry. It stores instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations; And control various types of machinery or production processes through digital input and output operations. The control program developed by the user expresses the process requirements of the production process and is pre stored in the user program memory of the PLC. Execute the stored program content one by one during runtime to complete the operations required by the process flow. The CPU of the PLC has a program counter that indicates the storage address of the program step. During the program running process, the counter automatically increments by 1 for each executed step. The program executes sequentially from the starting step (step number zero) to the final step (usually the END instruction), and then returns to the starting step for cyclic calculation. The time required for a PLC to complete each cycle operation is called a scanning cycle. Different models of PLCs have a cyclic scanning period ranging from 1 microsecond to several tens of microseconds. PLC uses ladder diagram programming, which shows the advantage of being fast in solving logic. At the microsecond level, it can solve 1K logic programs in less than 1 millisecond. It treats all inputs as switch values, with 16 bits (or 32 bits) being an analog signal. Large PLCs use another CPU to perform analog calculations. Send the calculation results to the PLC controller.
A system with the same number of I/O points has a lower cost (about 40% savings) when using PLC compared to DCS. PLC does not have a dedicated operation station, and its software and hardware are universal, so maintenance costs are much lower than DCS. A PLC controller can receive thousands of I/O points (up to over 8000 I/O points). If the controlled object is mainly equipment interlocking with few circuits, PLC is more suitable. Due to the use of universal monitoring software, PLC is easier to design management information systems for enterprises.
In the past decade, with the continuous decrease in PLC prices and the expansion of user demand, more and more small and medium-sized equipment have begun to use PLC for control, and the application of PLC in China has grown rapidly. With the rapid development of the Chinese economy and the continuous improvement of basic automation level, PLC will continue to maintain a high-speed growth momentum in China in the coming period.

When a general-purpose PLC is applied to specialized equipment, it can be considered as an embedded controller, but PLC has higher reliability and better stability compared to general embedded controllers. In practical work, some users who used embedded controllers are gradually replacing them with general-purpose PLCs or customized PLCs.

What is PLC?
It is a real-time system that is different from the traditional relay based motor control system of personal computers. Whenever the design is changed, the entire system needs to be remade, which is not only time-consuming but also laborious; At the same time, due to the disadvantages of poor contact, wear, and large size of relays, they cause problems such as increased cost, low reliability, and difficulty in maintenance. In order to improve these disadvantages, DEC in the United States first published the Programmable Controller in 1969

At the beginning of its publication, the programmable logic controller (PLC) was called the Programmable Logic Controller (PLC). Its original purpose was to replace relays and perform sequential control of relay logic and other timing or counting functions. Therefore, it is also known as a sequential controller. Its structure is similar to a microcomputer, so it can also be called a microcomputer programmable controller (MCPC). It was not until 1976 that the American Electrical Machinery Manufacturers Association officially named it the Programmable Controller, abbreviated as PC. Due to the widespread use of personal computers and their frequent use in conjunction with programmable controllers, in order to distinguish between the two, programmable controllers are generally referred to as PLCs At present, there are various types of PLCs on the market, which vary depending on the manufacturer and applicable location. However, each brand can be classified into large, medium, and small sizes according to the complexity of the unit; Most factories and schools usually use small PLCs, among which the Japanese MITSUBISHI F-series and the A-series PLCs produced by Shilin Electric in China are more popular among Chinese people And this CAI will mainly introduce Mitsubishi FX2 PLC, hoping that users can have a deeper understanding of PLC and be more proficient in using PLC The basic internal structure of a programmable controller can be represented by the following diagram. Its internal units include three major parts: CPU, input module, and output module. The CPU of the PLC will obtain the signals generated by the input components through the input module, and then retrieve the control instructions originally input from the programmer from the memory one by one. After logical calculation by the computing department, the results will be driven by the output module to drive the external output components

Internal structure diagram of PLC
Program input device: responsible for providing operator input, modification, and monitoring of the functions used by the program
Central processing unit (CPU): responsible for PLC management, execution, computation, control and other functions
Program memory: responsible for storing sequential program parameters and annotations designed by users
Data memory: responsible for storing the state of input and output devices and the conversion data of sequential programs
System memory: stores the system programs required for PLC to execute sequential control
Input circuit: responsible for receiving signals from external input components
Output circuit: responsible for receiving signals from external output components
Widely used in industrial applications, such as the control of various automation equipment in semiconductor wafer factories, elevators, mechanical parking equipment, roadside traffic light change control, and automated production lines

Basic structure of PLC

PLC is essentially a computer specialized for industrial control, and its hardware structure is basically the same as that of a microcomputer
A Central Processing Unit (CPU)
The central processing unit (CPU) is the control center of the PLC. It receives and stores user programs and data entered from the programmer according to the functions assigned by the PLC system program; Check the status of power supply, memory, I/O, and alert timer, and be able to diagnose syntax errors in user programs. When the PLC is put into operation, it first receives the status and data of various input devices on site in a scanning manner, and stores them separately in the I/O mapping area. Then, it reads the user program from the user program memory one by one, interprets the commands, and executes logical or arithmetic operations according to the instructions. The results are then sent to the I/O mapping area or data register. After all user programs have been executed, the output states or data in the output registers of the I/O mapping area are finally transferred to the corresponding output devices, and this process is repeated until it stops running.
In order to further improve the reliability of PLCs, in recent years, redundant systems with dual CPUs or voting systems with three CPUs have been adopted for large PLCs. In this way, even if a CPU fails, the entire system can still operate normally.
b. Memory storage
The memory that stores system software is called system program memory.
The memory that stores application software is called user program memory.
C. Power supply
The power supply of PLC plays a very important role in the entire system. Without a good and reliable power system, it cannot function properly, so PLC manufacturers attach great importance to the design and manufacturing of power supplies. Generally, if the AC voltage fluctuates within the range of+10% (+15%), the PLC can be directly connected to the AC power grid without taking any other measures.
The working principle of PLC
One scanning technique
After the PLC is put into operation, its working process is generally divided into three stages, namely input sampling, user program execution, and output refresh. Completing the above three stages is called a scanning cycle. During the entire operation, the CPU of the PLC repeatedly executes the above three stages at a certain scanning speed.
（1） Input sampling stage
In the input sampling stage, the PLC sequentially reads in all input states and data in a scanning manner, and stores them in the corresponding units in the I/O mapping area. After the input sampling is completed, it enters the user program execution and output refresh phase. In these two stages, even if the input state and data change, the state and data of the corresponding unit in the I/O mapping area will not change. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scanning period to ensure that the input can be read in under any circumstances.
（2） User program execution phase
During the execution phase of the user program, the PLC always scans the user program in a top-down order (ladder diagram). When scanning each ladder diagram, always scan the control circuit composed of each contact on the left side of the ladder diagram first, and perform logical operations on the control circuit composed of contacts in the order of left to right and top to bottom. Then, based on the result of the logical operation, refresh the corresponding bit status of the logical coil in the system RAM storage area; Or refresh the status of the corresponding bit of the output coil in the I/O mapping area; Or determine whether to execute the special functional instructions specified in the ladder diagram.
That is, during the execution of the user program, only the state and data of the input point in the I/O mapping area will not change, while the state and data of other output points and software devices in the I/O mapping area or system RAM storage area may change. Moreover, the ladder diagram at the top will have an effect on any ladder diagram below that uses these coils or data; On the contrary, in the ladder diagram below, the status or data of the refreshed logic coil can only affect the program above it in the next scanning cycle.
（3） Output refresh phase
After scanning the user program, the PLC enters the output refresh phase. During this period, the CPU refreshes all output latch circuits according to the corresponding states and data in the I/O image area, and then drives the corresponding peripherals through the output circuits. At this point, it is the true output of the PLC.
The same number of ladder diagrams, with different arrangement orders, result in different execution outcomes. In addition, there are differences between the results of scanning user programs and the hard logic parallel operation of relay control devices. Of course, if the time occupied by the scanning cycle can be ignored for the entire operation, then there is no difference between the two.
Generally speaking, the scanning cycle of PLC includes self diagnosis, communication, etc., as shown in the following figure, that is, one scanning cycle is equal to the sum of all the time for self diagnosis, communication, input sampling, user program execution, output refresh, etc.

Application Fields of PLC
At present, PLC has been widely used in various industries such as steel, petroleum, chemical, power, building materials, machinery manufacturing, automobiles, textiles, transportation, environmental protection, and cultural entertainment both domestically and internationally. The usage can be roughly classified into the following categories.

Logic control of switch quantity
This is the most basic and widely used application field of PLC, which replaces traditional relay circuits to achieve logic control and sequential control. It can be used for controlling a single device, as well as for multi machine group control and automated assembly lines. Such as injection molding machines, printing machines, bookbinding machines, modular machine tools, grinding machines, packaging production lines, electroplating assembly lines, etc.

Analog control
In industrial production processes, there are many continuously changing quantities, such as temperature, pressure, flow rate, liquid level, and speed, which are all analog quantities. In order for a programmable controller to process analog signals, it is necessary to implement A/D conversion and D/A conversion between analog and digital signals. PLC manufacturers produce matching A/D and D/A conversion modules to enable programmable controllers for analog control.

Motion control
PLC can be used for controlling circular or linear motion. In terms of control mechanism configuration, in the early days, it was directly used to connect position sensors and actuators to switch I/O modules, but now specialized motion control modules are generally used. A single axis or multi axis position control module that can drive stepper motors or servo motors. Almost all major PLC manufacturers in the world have motion control functions in their products, which are widely used in various machinery, machine tools, robots, elevators, and other applications.

process control
Process control refers to closed-loop control of analog quantities such as temperature, pressure, and flow rate. As an industrial control computer, PLC can develop various control algorithm programs to achieve closed-loop control. PID regulation is a commonly used adjustment method in general closed-loop control systems. Large and medium-sized PLCs all have PID modules, and currently many small PLCs also have this functional module. PID processing generally involves running specialized PID subroutines. Process control has a wide range of applications in metallurgy, chemical engineering, heat treatment, boiler control, and other fields.

data processing
Modern PLCs have functions such as mathematical operations (including matrix operations, function operations, and logical operations), data transmission, data conversion, sorting, table lookup, and bit operations, which can complete data collection, analysis, and processing. These data can be compared with reference values stored in memory to perform certain control operations, and can also be transmitted to other intelligent devices through communication functions or printed into tables. Data processing is generally used for large-scale control systems, such as unmanned flexible manufacturing systems; It can also be used in process control systems, such as some large-scale control systems in the paper, metallurgy, and food industries.

Communication and networking
PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of computer control, factory automation networks have developed rapidly, and various PLC manufacturers attach great importance to the communication function of PLCs and have launched their own network systems. The newly produced PLCs all have communication interfaces, making communication very convenient.

Future prospects of PLC
In the 21st century, PLC will have greater development. Technically speaking, new achievements in computer technology will be more applied to the design and manufacturing of programmable controllers, resulting in the emergence of varieties with faster computing speed, larger storage capacity, and stronger intelligence; From the perspective of product scale, it will further develop towards ultra small and ultra large sizes; From the perspective of product compatibility, the variety of products will be more diverse and the specifications will be more complete. A perfect human-machine interface and complete communication equipment will better adapt to the needs of various industrial control scenarios; From the market perspective, the situation where each country produces multiple varieties of products will break with the intensification of international competition, resulting in a situation where a few brands monopolize the international market and the emergence of internationally recognized programming languages; From the perspective of network development, the networking of programmable controllers and other industrial control computers to form large-scale control systems is the direction of programmable controller technology. There are already a large number of programmable controllers applied in the current computer distributed control system (DCS). With the development of computer networks, programmable controllers, as an important component of automation control networks and international general networks, will play an increasingly important role in many fields beyond industry.