D201126 PLC knowledge
D201126 In terms of structure, PLC is divided into two types: fixed type and combined type (modular). Fixed PLC includes CPU board, I/O board, display panel, memory block, power supply, etc. These elements are combined into a non-removable whole. Modular PLC includes CPU module, I/O module, memory, power module, baseboard or rack, these modules can be combined according to certain rules.
CPU components
CPU is the core of PLC, plays the role of the nerve center, each PLC has at least one CPU, which receives and stores user programs and data according to the function given by the PLC system program, collects the status or data sent by the field input device by scanning, and stores it in the specified register, at the same time, Diagnose the working state of power supply and PLC internal circuit and grammar errors in 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 an arithmetic device, a controller, a register and the data, control and state bus that realize the connection between them. The CPU unit also includes peripheral chips, bus interfaces and related circuits. Memory is mainly used to store programs and data, and is an indispensable component of PLC.
In the view of the user, it is not necessary to analyze the internal circuit of the CPU in detail, but there should be enough understanding of the working mechanism of each part. The controller of the CPU controls the work of the CPU, by which the instructions are read, interpreted and executed. But the pace of work is controlled by oscillating signals. The arithmetic unit is used to perform digital or logical operations, working under the command of the controller. The register participates in the operation and stores the intermediate result of the operation, which also works under the command of the controller.
CPU speed and memory capacity are important parameters of PLC, which determine the working speed of PLC, the number of IO and the software capacity, so the control scale is limited.
I/O module
D201126 The interface between the PLC and the electrical circuit is completed through the input and output part (I/O). I/O module integrates PLC I/O circuit, the input register reflects the input signal state, the output point reflects the output latch state. The input module converts the electrical signal into a digital signal into the PLC system, and the output module is the opposite. I/O is divided into switching input (DI), switching output (DO), analog input (AI), analog output (AO) and other modules.
Common I/O categories are as follows:
Switching quantity: according to voltage level, there are 220VAC, 110VAC, 24VDC, according to isolation mode, there are relay isolation and transistor isolation.
Analog: 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 and so on.
D201126 In addition to the above general IO, there are special IO modules, such as thermal resistance, thermocouple, pulse and other modules.
Module specifications and number are determined by the number of I/O points, but the maximum number of I/O modules is limited by the ability of the basic configuration that the CPU can manage, that is, by the maximum number of baseboards or rack slots.
Power module
D201126 The PLC power supply is used to provide working power for the integrated circuit of each PLC module. At the same time, some also provide 24V working power supply for the input circuit. Power input type: AC power supply (220VAC or 110VAC), DC power supply (commonly used 24VDC).
Bottom plate or frame
Most modular PLCS use a baseboard or frame, and its role is: electrically, to achieve the connection between the modules, so that the CPU can access all modules on the baseboard, and mechanically, to achieve the connection between the modules, so that the modules form a whole.
High reliability, strong anti-interference ability
D201126 High reliability is the key performance of electrical control equipment. Due to the use of modern large-scale integrated circuit technology, PLC adopts strict production process manufacturing, the internal circuit adopts advanced anti-interference technology, with high reliability. For example, the F-series PLC produced by Mitsubishi Corporation has an average trouble-free time of up to 300,000 hours. Some PLCS that use redundant cpus have a higher average trouble-free operating time. From the PLC external circuit, the use of PLC to form a control system, and the relay contactor system of the same size, electrical wiring and switching contacts have been reduced to hundreds or even thousands of one, the fault is greatly reduced. In addition, the PLC has a hardware fault self-detection function, which can issue alarm information in time when there is a failure. In the application software, the user can also program the fault self-diagnosis program of peripheral devices, so that the circuit and equipment other than PLC in the system can also obtain fault self-diagnosis protection. Therefore, it is not surprising that the whole system has a high reliability.