What is PLC

In the field of automatic control, PLC is an important control equipment.  At present, more than 200 manufacturers in the world produce more than 300 varieties of PLC products, which are used in automobile (23%), grain processing (16.4%), chemical/pharmaceutical (14.6%), metal/mine (11.5%), pulp/paper (11.3%) and other industries.  In order to make it easier for beginners to understand PLC, this paper gives a brief introduction to the basic knowledge of PLC such as development, basic structure, configuration, application, etc., in order to be helpful to netizens.  　
First, the development process of PLC In the process of industrial production, a large number of switching values are controlled in sequence. It operates in sequence according to logical conditions, controls interlocking protection actions according to logical relationships, and collects a large number of discrete data.  Traditionally, these functions are realized through pneumatic or electrical control systems.  In 1968, the American GM (General Motors) Company proposed to replace the following electrical control device. The following year, the American Digital Company developed a control device based on integrated circuit and electronic technology. For the first time, it used programmed means for electrical control. This is the first generation of programmable controller, known as Programmable Controller(PC).  After the development of the personal computer (PC for short), for convenience and to reflect the functional characteristics of the programmable controller, the programmable controller is named programmable logic controller (PLC). now, PLC is still often referred to as PC for short.  There are many definitions of 　　PLC.  The definition of PLC by International Electrotechnical Commission (IEC) is: Programmable Controller is an electronic system operated by digital operation, specially designed for application in industrial environment.  It uses a programmable memory to store in it instructions for performing logical operations, sequence control, timing, counting and arithmetic operations, and controls various types of machinery or production processes through digital and analog inputs and outputs.  Programmable controllers and related equipment shall be designed according to the principle of easy integration with industrial control systems and easy expansion of their functions.  From the 1980s to the mid-1990s of the last century, PLC was the fastest growing period with an annual growth rate of 30~40%.  During this period, PLC's ability to handle analog quantity, digital operation, man-machine interface and network have been greatly improved. PLC has gradually entered the field of process control and replaced DCS system which is in the dominant position in the field of process control in some applications.  　　PLC has the characteristics of strong universality, convenient use, wide adaptability, high reliability, strong anti-interference ability, simple programming, etc.  PLC's position in industrial automation control, especially sequence control, cannot be replaced in the foreseeable future.  　　　　
Second, the composition of PLC
Structurally, PLC is divided into fixed type and combined type (modular type).  Fixed PLC includes CPU board, I/O board, display panel, memory block, power supply, etc. These elements are combined into an integral whole.  Modular PLC includes CPU module, I/O module, memory, power module, backplane or rack. These modules can be configured in combination according to certain rules.  　　　

III. Composition of CPU
The CPU is the core of PLC and acts as a nerve center. Each PLC has at least one CPU. It receives and stores user programs and data according to the functions given by the PLC's system programs. It uses scanning method to collect the states or data sent by the field input device and stores them into the specified registers. At the same time, it diagnoses the working states of power supply and PLC internal circuits and syntax errors in the programming process.  After entering the operation, the instructions are read one by one from the user program memory, and after analysis, the corresponding control signals are generated according to the tasks specified in the instructions to command the relevant control circuits.  　　　　
The CPU is mainly composed of arithmetic unit, controller, register and data, control and status bus to realize the connection between them. The CPU unit also includes peripheral chip, bus interface and related circuits.  Memory is mainly used to store programs and data and is an indispensable component of PLC.  　　　　
From the user's point of view, it is not necessary to analyze the internal circuit of CPU in detail, but the working mechanism of each part should be fully understood.  The controller of the CPU controls the CPU to work and reads, interprets and executes instructions.  However, the working rhythm is controlled by oscillation signals.  The arithmetic unit is used for digital or logical operation and works under the command of the controller.  Registers participate in the operation and store the intermediate results of the operation. They also work under the command of the controller.  　　CPU speed and memory capacity are important parameters of PLC, which determine the working speed, IO quantity and software capacity of PLC, thus limiting the control scale.  　　　　　　　　　　　　
I/O module
The interface between PLC and electrical circuit is completed through input/output part (I/O).  The I/O module integrates the I/O circuit of PLC, its input register reflects the input signal state, and its output point reflects the output latch state.  The input module converts electrical signals into digital signals and enters the PLC system, while the output module is opposite.  I/O is divided into switch input (DI), switch output (DO), analog input (AI), analog output (AO) and other modules.  　　　　
Switching value refers to signals with only on and off (or 1 and 0) states, and analog value refers to continuously changing quantity.  Common I/O classifications are as follows:
Switching value: divided by voltage level, 220VAC, 110VAC, 24VDC, divided by isolation mode, with relay isolation and transistor isolation.  　　　　
Analog quantity: divided by signal type, current type (4-20mA,0-20mA), voltage type (0-10V,0-5V,-10-10V), etc., divided by accuracy, 12bit,14bit,16bit, etc.  　
In addition to the above general IO, there are special IO modules, such as thermal resistance, thermocouple, pulse and other modules.  　　　　
According to the number of I/O points to determine module specifications and quantity, I/O modules can be more or less, but the maximum number is limited by the basic configuration capability that the CPU can manage, that is, the maximum number of chassis or rack slots.  　　　　
V. power supply module
PLC power supply is used to provide working power for integrated circuits of PLC modules.  At the same time, some also provide 24V working power for the input circuit.  The power input types are: AC power (220VAC or 110VAC) and DC power (24VAC is commonly used).  　　　　
Six, the bottom plate or frame
Most modular PLC uses a base plate or a rack, and its function is: electrically, to realize the connection between modules, so that the CPU can access all modules on the base plate, mechanically, to realize the connection between modules, so that each module forms a whole.  　　
Seven, other equipment of PLC system
1. Programming equipment: Programmer is an indispensable device for PLC development and application, monitoring and operation, and inspection and maintenance. It is used for programming, making some settings for the system, and monitoring the working conditions of PLC and PLC-controlled systems, but it does not directly participate in on-site control and operation.  Small Programmer PLC usually has a hand-held programmer. At present, a computer (running programming software) is generally used as the programmer.  　　　　
2. Man-machine interface: The simplest man-machine interface is indicator lights and buttons. At present, LCD (or touch screen) type integrated operator terminals are increasingly widely used, and computers (running configuration software) are widely used as man-machine interfaces.  3. Input and output equipment: used to permanently store user data, such as EPROM, EEPROM writer, bar code reader, potentiometer for inputting analog quantity, printer, etc.  　　　　　
Eight, PLC communication networking
Relying on advanced industrial network technology, production and management data can be quickly and effectively collected and transmitted.  Therefore, the importance of the network in the automation system integration engineering is more and more obvious, and even some people put forward the view that "the network is the controller".  　
PLC has the function of communication networking. It enables the exchange of information between PLC and PLC, between PLC and upper computer and other intelligent equipment, forming a unified whole and realizing decentralized centralized control.  Most PLC have RS-232 interface, and some have built-in interfaces supporting their respective communication protocols.  　　
Communication of PLC has not yet realized interoperability. IEC has stipulated a variety of field bus standards, which are adopted by PLC manufacturers.  For an automation project (especially for medium and large scale control systems), it is very important to choose a network.  First of all, the network must be open to facilitate the integration of different devices and the expansion of system scale in the future.  Secondly, according to the transmission performance requirements of different network levels, choose the network form, which must be carried out on the premise of a deeper understanding of the network standard protocols and mechanisms.  Thirdly, the network standards used at different levels are determined by comprehensively considering specific issues such as system cost, equipment compatibility, and applicability of field environment.