PLC controller in dust removal system
Programmable logic controller (PLC) is a new generation of industrial control device based on microprocessor, which integrates computer, communication and networking and automatic control technology. It uses a programmable memory in the structure, which can store operation instructions such as performing logical operations, sequence control, timing, counting, and arithmetic operations, and control various equipment and devices through digital and analog input and output. production process. It has a series of advantages such as high reliability, strong anti-interference ability (MTBF up to 200,000h), good adaptability to harsh environments, good real-time performance, convenient programming, and simple learning.
Programmable logic controllers (PLCs) first entered China from the mid-1970s to the early 1980s. In the first phase of Baosteel, many projects introduced more than 200 PLCs with more than a dozen models. These PLCs are used in the entire steel smelting and processing production line from the raw material terminal to the steel pipe plant, blast furnace, and steel rolling, replacing the traditional relays. Logic system, and partially replaced analog control and small DDC system. Applications in various fields.
1.Programmable controller hardware composition
The basic components of a programmable controller include: a processing unit (CPU), memory expansion, power supply, input and output units, communication interfaces, intelligent input and output, and expansion interfaces, as well as external devices such as programmers, process control stations, and printers. The CPU is the central nerve of the PLC. Its role is to connect the memory, interface and I / O unit through the data bus, address bus, control bus and auxiliary circuits, diagnose the hardware status of the PLC, and accept user programs and data entered by means of a programmer. Read the interpretation and execute the user program, etc. Memory is used to store programs and data. It includes system program memory and user system program memory. The input and output unit is the connection part between the PLC and the field I / O equipment or other external equipment. The programmer is a common external device of PLC *. It is also a peripheral in the PLC that does not need to be connected to the bus directly through function modules. It is directly connected to the host through the programmer interface on the host.
2.Software system and programming language of programmable controller
2.1 Software system
The software includes system programs and user programs. The system program is the basis on which the PLC relies. It is written in assembly language and is fixed in the ROM-type system program memory without user intervention. System programs are divided into system monitoring programs and interpreters. The system monitoring program is used to monitor the work of the PLC, and the interpreter is used to interpret the user's program as a program that the microprocessor can execute. The user program is a program prepared by the user using a PLC programming language in order to complete a specific control task. It is entered into the PLC user program memory through a programmer.
2.2 Programming languages
Each type of PLC has its own programming language. Ladder diagrams, statement lists, logic symbol diagrams, sequential function diagrams, and programming languages are commonly used, among which the former two are used more often.
Ladder diagram language is a programming language similar to relay control circuit diagrams. It is oriented to the control process, intuitive and easy to understand. It is the most widely used language in the PLC programming language. Statement list is a mnemonic programming language similar to assembly language. Sequential function chart is a programming language that uses function chart to describe the program. It can decompose a complex control system into several subsystems. Starting from the function, the operation of the system has a clear meaning, which is convenient for designers and operators. Communication, which facilitates the design of program division of labor and inspection and debugging.
3. Working process
PLC adopts the cyclic scanning mode of operation. Each time before scanning the user program, the fault self-diagnosis program is executed. At the same time, before scanning the user program, it is also necessary to check whether there is a communication request with the programmer, computer, etc., and if necessary, perform corresponding processing.
The working process of PLC is generally divided into three main phases: 1) In the input sampling phase, the PLC reads the status of all input terminals into the input status register in a scanning manner in order. At this stage, the content of the sampling result will not change, and this sampling result will be used when the PLC executes the program in the next stage; 2) During the program execution stage, the PLC scans the program in order, generally from top to bottom and from left To the right. The input and original output states are processed, and then the output result is sent to the output status register, but this result will not be sent to the output port before the entire program is completed; 3) In the driver equipment phase, all user programs are executed After that, the PLC converts all the states in the output status register into voltage or current signals that the controlled device can accept through the output components to drive the controlled device. The work process of the above three phases is called a scan cycle.