In the field of automation control, PLC is an important control device. At present, there are more than 200 manufacturers in the world producing more than 300 varieties of PLC products, applied in automobiles (23%), grain processing (16.4%), chemical/pharmaceutical (14.6%), metals/mines (11.5%), pulp/paper ( 11.3%) and other industries. In order to make it easier for beginners to understand PLC, this article gives an introduction to the basic knowledge of PLC development, basic structure, configuration, and application, so as to help the netizens.
First, the development process of PLC In the industrial production process, a large number of switch-level control, it is in accordance with the logical conditions for sequential actions, and in accordance with the logic of the chain of protection movement control, and a large number of discrete data collection. Traditionally, these functions are implemented by pneumatic or electrical control systems. In 1968, the United States GM (General Motors) proposed replacement of electrical control devices. In the following year, U.S. digital companies developed control devices based on integrated circuits and electronic technologies. The first use of procedural means was applied to electrical control. Is the first generation of programmable controller, called Programmable Controller (PC).
After the personal computer (PC) was developed, the programmable controller was named as Programmable Logic Controller (PLC) for the sake of convenience and to reflect the features of the programmable controller. Now, the PLC is often referred to as PC.
There are many kinds of PLC definitions. The definition of PLC by the International Electrotechnical Commission (IEC) is: The programmable controller is an electronic system for digital operation and is designed for application in industrial environments. It uses a programmable memory to store instructions for performing logical operations, sequence control, timing, counting, and arithmetic operations, and controls various types through digital and analog inputs and outputs. Mechanical or production process. Programmable logic controllers and related equipment should be designed in such a way that they can be easily integrated with industrial control systems and can easily be extended.
From the 1980s to the mid-1990s, PLC was the fastest growing period, and its annual growth rate remained at 30 to 40%. During this period, PLC has been greatly improved in handling analog capacity, digital computing capability, human-machine interface capability, and network capability. PLC has gradually entered the field of process control, replacing DCS, which is dominant in process control in some applications. system.
PLC has the characteristics of strong versatility, ease of use, wide adaptation, high reliability, strong anti-interference ability, and simple programming. PLC's position in industrial automation control, especially sequential control, cannot be replaced in the foreseeable future.
Second, the composition of the PLC from the structure points, PLC is divided into fixed and modular (module type) two. Fixed PLC includes CPU board, I/O board, display panel, memory block, power supply, etc. These elements are combined into a non-detachable whole. Modular PLCs include CPU modules, I/O modules, memory, power modules, backplanes, or racks. These modules can be combined and configured according to certain rules.
Third, the composition of the CPU CPU is the core of PLC, play the role of nerve center, each set of PLC has at least one CPU, it receives and stores the user program and data according to the function given by the PLC system program, and is collected by the scanning method from the scene. The status or data sent by the input device is stored in the specified register. At the same time, the power status and the PLC internal circuit's working status and syntax errors in the programming process are diagnosed. After entering the operation, the instructions are read one by one from the user program memory. After the analysis, the corresponding control signals are generated according to the tasks specified in the instruction and the relevant control circuit is commanded.
The CPU is mainly composed of an operator, a controller, a register, and data, control, and status buses that implement the connection between them. The CPU unit also includes a peripheral chip, a bus interface, and related circuits. Memory is mainly used to store programs and data and is an indispensable component of the PLC.
From the user's point of view, it is not necessary to analyze the internal circuits of the CPU in detail, but there should be sufficient understanding of the working mechanism of each part. The controller of the CPU controls the operation of the CPU, and it reads instructions, interprets instructions, and executes instructions. But the working rhythm is controlled by the oscillating signal. The calculator is used to perform digital or logical operations and work under the direction of the controller. The register participates in the operation and stores the intermediate result of the operation. It also works under the direction of the controller.
CPU speed and memory capacity are important parameters of PLC. They determine the working speed, IO quantity, and software capacity of PLC, thus limiting the control scale.
Fourth, I/O module PLC and electric circuit interface, through input and output part (I/O) Complete. The I/O module integrates the PLC's I/O circuit. Its input register reflects the status of the input signal, and the output point reflects the output latch status. The input module converts the electrical signal into a digital signal into the PLC system, and the output module is reversed. I/O is divided into digital input (DI), digital output (DO), analog input (AI), and analog output (AO).
Switching quantity refers to the signal of only two states of opening and closing (or 1 and 0). Analog quantity refers to the amount of continuous change. Commonly used I/Os are classified as follows:
Switching quantity: According to the voltage level, there are 220VAC, 110VAC, 24VDC, divided by isolation mode, there is relay isolation and transistor isolation.
Analog quantity: According to the signal type, there are current type (4-20mA, 0-20mA), voltage type (0-10V, 0-5V,-10-10V), etc. According to the accuracy, there are 12bit, 14bit, 16bit, etc. .
In addition to the general-purpose IO described above, there are special IO modules such as thermal resistance, thermocouple, and pulse modules.
According to the number of I/O points to determine the module specifications and quantity, I/O modules can be more or less, but the maximum number of CPUs can be managed by the basic configuration of the ability, that is subject to the maximum number of backplane or rack slots.
Fifth, the power module PLC power supply is used to provide the working power for the integrated circuits of each module of the PLC. At the same time, some also provide 24V operating power for the input circuit. Power input types are: AC power (220VAC or 110VAC), DC power (24VAC is commonly used).
6. Backplanes or Racks Most modular PLCs use backplanes or racks. Their role is: Electrically, to achieve the connection between the various modules, so that the CPU can access all the modules on the backplane, mechanically, to achieve the connection between the modules , Make all modules form a whole.
VII. Other devices of PLC system 1. Programming device: The programmer is an indispensable device for PLC development and application, monitoring and operation, inspection and maintenance. It is used for programming, making some settings for the system, and monitoring the work of PLC and PLC-controlled systems. Status, but it is not directly involved in the field control operation. The small programmer PLC generally has a handheld programmer. At present, generally, a computer (run programming software) acts as a programmer.
2. Human-Machine Interface: The simplest man-machine interface is the indicator light and the button. Currently, the integrated operator terminal with LCD screen (or touch screen) is more and more widely used. The computer (run configuration software) acts as the human-machine interface. Very popular.
3, input and output devices: for the permanent storage of user data, such as EPROM, EEPROM writer, bar code reader, analog input potentiometers, printers and so on.
VIII. PLC's communication networking relying on advanced industrial network technology can quickly and effectively collect and transmit production and management data. Therefore, the importance of the network in the integration of automation systems is becoming more and more significant. Some people even put forward the idea that "the network is the controller."
PLC has the function of communication networking. It can exchange information between PLC and PLC, between PLC and host computer and other intelligent devices, forming a unified whole and achieving decentralized centralized control. Most PLCs have an RS-232 interface, and some have built-in interfaces that support their respective communication protocols.
The communication of PLC has not realized the interoperability yet, IEC has stipulated many kinds of on-the-spot bus standards, all PLC manufacturers have adopted.
For an automation project (especially for medium and large-scale control systems), choosing a network is very important. First of all, the network must be open to facilitate the integration of different devices and the scale of future systems. Secondly, for the transmission performance requirements of different network layers, the form of the network must be selected. This must be a deeper understanding of the protocol of the network standard, Under the premise of the mechanism, once again, considering the specific issues such as system cost, equipment compatibility, and site environmental applicability, determine the network standards used at different levels.