Basic knowledge of PLC ( Introduction to PLC will see)

1 PLC development course in the process of industrial production, a large number of switch quantity sequence control, it shall be carried out in accordance with the logical condition order movement, and according to the logic relation between chain protection action of control, and the amount of a large number of discrete data acquisition. Traditionally, these functions are realized through pneumatic or electric control system. In 1968 the United States GM ( General motors) Company instead of following the requirement of electric control device is put forward, in the second year, the United States digital company developed based on the integrated circuit and the control device of electronic technology, adopt programmatic approach is applied to the electrical control for the first time, this is the first generation of Programmable Controller, said Programmable Controller ( PC） 。 Personal computer ( Hereinafter referred to as PC) Development up, for the sake of convenience, also in order to reflect the functions and characteristics of the Programmable Controller, Programmable Controller called Programmable Logic Controller ( PLC） 。 In the 1980 s to mid - 90 - s, it is of the fastest growing period of PLC, the annual growth rate has maintained for 30 ~ 40%. PLC in this period, the ability to deal with analog and digital computing power, man-machine interface and network ability get improved, gradually into the field of process control of PLC, in some applications to replace the dominant in the field of process control in the DCS system. PLC has strong commonality, easy to use, adaptation of wide, high reliability, strong anti-interference ability, simple programming and so on. The status of PLC in industrial automation control especially sequence control, in the foreseeable future, cannot be replaced. 2 the composition of PLC points from the structure, PLC is divided into fixed and modular ( Modular) Two kinds. Fixed PLC including CPU boards, I/O board, display panel, memory blocks, power supply, etc. , these elements are combined into a sealed, as a whole. Modular PLC, including CPU module, I/O module, memory, or the chassis, power supply module, these modules can be according to certain rules of combination configuration. Three components of the CPU is the core of the PLC CPU, ACTS as the nerve center, each PLC has at least one CPU, which is according to the function of the PLC system program to receive and store the user program and data, collected by scanning way by the state or data from input devices, and deposited in the register of regulations, at the same time, the diagnosis of the power and the working state of the circuit inside the PLC controller and programming in the process of grammatical errors, etc. Enter after the operation, to read a detailed instructions from the user program memory, after analyzing the task as stipulated in the instructions again to produce the corresponding control signal, to command the control circuit. Mainly by arithmetic unit, controller, CPU registers and their connections between data bus, control and state, CPU units include peripheral chips, bus interface and circuitry. Memory is mainly used to store programs and data, it is the indispensable component unit of PLC. In consumer point of view, unnecessary details the internal circuit of CPU, but due to the working mechanism of each part or enough understanding. CPU controller control CPU, which read instructions, explanation and executes instructions. But working rhythm by the shock signal control. Arithmetic unit used for digital or logic operations, work under the controller command. Register to participate in the operation, and store operations in the middle of the results, it is also the work under the controller command. CPU speed and memory capacity is one of the important parameters of the PLC, they determine the working speed of PLC, I/o number and software capacity etc. , thereby limiting the control scale. 4 I/O module of PLC and the electric circuits of the interface, is through the input/output section ( I / O) The finished. I/O module integrates the PLC I/O circuit, its input register reflects the state of the input signal, the output points reflect the state of the output latch. Input module will be converted to digital signals into electrical signals, PLC system output module instead. I/O can be divided into switch input ( DI） , switch output ( 做) , analog input, AI） Analog output ( AO） Such as modules. Commonly used I/O classification is as follows: switch quantity: according to the voltage level, there are 220 vac, 110 vac, 24 VDC, according to isolation methods of points, a relay isolated and transistors. Analog: divide by signal type, current mode ( 4 - 20mA,0- 20mA） , voltage type ( 0 - 10 v, 0 - 5V,- 10 - 10 v) And so on, according to the precision of points, there are 12 bit, 14 bit, 16 bit, etc. In addition to the above general IO, there are special IO modules, such as thermal resistance, thermocouple, pulse module. According to the I/O points to determine the module specification and quantity, I/O modules may be less than, but its most by CPU to manage large Numbers of the ability of basic configuration, i. e. the biggest or the rack of the slot number. 5 power supply module of PLC power supply is used to work for PLC module integrated circuit to provide power supply. At the same time, some even work for input circuit provides 24 v power supply. Power input types are: ac power source ( 220 vac or 110 vac) , dc power supply ( Commonly used for 24 VDC) 。 6 floor frame most modular PLC use backplane or frame, its effect is: electrical, realize the relationship between each module, the CPU can access all the modules on the floor, on the mechanical, realize the connection between the various modules, each module as a whole.
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