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16 PLC knowledge, understand these basic principles!

Last Updated on 2021-02-27 Hits:8724

Those engaged in electric power operations know that the development of industrial production and science and technology cannot be separated from the automatic control of PLC. PLC can be broadly understood as: a centralized relay extension control cabinet. In actual production applications, PLC greatly saves industrial control. The cost of this has strengthened the centralized management and automatic control of equipment. If you want to learn PLC well, first of all, the foundation of PLC needs to be solid.
1. From the perspective of the composition of the PLC, in addition to the CPU, memory and communication interfaces, what other interfaces are directly related to the industrial site? And explain its main functions.


(1) Input interface: accept the signal of the controlled device, and drive the internal circuit to turn on or off through the photoelectric coupling device and the input circuit.


(2) Output interface: The execution result of the program is output through the photoelectric coupling device and output component (relay, thyristor, transistor) of the output interface to control the on or off of the external load.


2. Which parts of the basic unit of a PLC are composed? What role does each play?


(1) CPU: The core component of the PLC, which instructs the PLC to perform various tasks. Such as accepting user programs and data, diagnosis, execution of execution programs, etc.;


(2) Memory: storage system and user programs and data;


(3) I/O interface: the connecting part between the PLC and the controlled object on the industrial production site, which is used to receive the signal of the controlled device and output the execution result of the program;


(4) Communication interface: exchange information with monitors, printers and other devices through the communication interface;


(5) Power supply.


3. What are the types of PLC switch output interfaces? What are the characteristics of each?


Thyristor output type: Under normal circumstances, it can only carry AC load, with fast response speed and high operating frequency;


Transistor output type: Under normal circumstances, it can only carry a DC load, with fast response speed and high operating frequency;


Relay output type: Under normal circumstances, it can carry AC and DC loads, but its response time is long and the action frequency is low.


4. According to the structure type, what are the types of PLC? What are the characteristics of each?


(1) Integral type: The CPU, power supply, and I/O components are all concentrated in one chassis, with compact structure and low price. Generally, small PLCs use this structure;


(2) Modular: Divide each part of the PLC into several individual modules. Different modules can be selected to form a system according to needs. It has the characteristics of flexible configuration, convenient expansion and maintenance. Generally, medium and large PLCs adopt this structure. Modular PLC is composed of a frame or base plate and various modules, and the modules are installed on the socket of the frame or base plate.


(3) Stacking type: Combining the characteristics of integral type and modular type, the CPU, power supply, I/O interface of the stacking type PLC are also independent modules, but they are connected by cables, making the system not only configuration Flexible and compact.


5. What is the scan cycle of the PLC? What are its main effects?


The PLC scan process includes five stages: internal processing, communication services, input processing, program execution, and output processing. The time required for these five stages to scan once is called the scan cycle.


The scan cycle is related to the CPU operating speed, PLC hardware configuration and the length of the user program.


6. In what way does the PLC execute the user program? What are the stages of the user program execution process?


The PLC executes the user program by cyclic scanning. The execution process of the user program includes the input sampling phase, the program execution phase and the output refresh phase.


7. Compared with the relay control system, what are the advantages of the PLC control system?


(1) In terms of control method: PLC adopts program method to realize control, it is easy to change or increase control requirements, and PLC has unlimited contacts;


(2) In terms of working mode: PLC adopts serial working mode to improve the anti-interference ability of the system;


(3) In terms of control speed: PLC's contacts are actually triggers, and the instruction execution time is in the microsecond level;


(4) Timing and counting: PLC uses semiconductor integrated circuits as timers, clock pulses are provided by crystal oscillators, with high delay accuracy and wide range. PLC has a counting function that the relay system does not have;


(5) Reliability and maintainability: PLC adopts microelectronics technology and has high reliability. The self-checking function can detect its own faults in time, and the monitoring function is convenient for debugging and maintenance.


8. Why does the PLC produce output response lag? How to improve the I/O response speed?


Because the PLC adopts the cyclic scanning mode of centralized sampling and centralized output, the state of the input terminal can only be read in the input sampling phase of each scan cycle, and the execution result of the program is only sent out during the output refresh phase; secondly, the input of the PLC , Output delay, the length of the user program, etc. can cause the output response to lag.


To improve the I/O response sampling, output refresh, or direct input sampling, output refresh, and interrupt input and output and intelligent I/O interface and other methods.


9. What kinds of internal soft relays are there in FX0N series PLC?


Input relay, output relay, auxiliary relay, status register, timer, counter, data register.


10. How to choose PLC?


1) Model selection: It should be considered from several aspects such as structural form, installation method, functional requirements, response speed, reliability requirements, and model uniformity;


2) Capacity selection: It should be considered from two aspects: the number of I/O points and user storage capacity;


3) I/O module selection: including the selection of switch and analog I/O modules, as well as the selection of special function modules;


4) Selection of other equipment such as power supply modules and programmers.


11. Briefly describe the characteristics of the PLC's centralized sampling and centralized output working method. What are the advantages and disadvantages of using this working method?


Centralized sampling: In a scan period, the sampling of the input state is only performed in the input sampling phase, and the input terminal will be blocked after entering the program execution phase.


Centralized output: in a scan cycle, only in the output refresh stage, the output-related state in the output image register is dumped to the output latch, and the output interface is refreshed. In other stages, the output state is always saved in the output image. In the register. Adopting this working method can improve the anti-interference ability of the system and enhance the reliability of the system, but it will cause the lag of PLC input/output response.


12. What kind of working method does PLC use? What are the characteristics?


PLC adopts the working mode of centralized sampling, centralized output, and cyclic scanning.


Features: Centralized sampling means that in a scan cycle, the PLC's sampling of the input status is only performed during the input sampling phase, and the input terminal will be blocked when the program enters the execution phase.


Centralized output means that in a scan cycle, the PLC only transfers the output-related state in the output image register to the output latch during the output refresh stage, refreshes the output interface, and keeps the output state in other stages. In the output image register.


Cyclic scanning means that the PLC needs to perform multiple operations in one scanning period. It uses time-sharing scanning to execute them one by one in order, and it runs repeatedly over and over again. 13. What are the main components of the electromagnetic contactor? Briefly describe the working principle of the electromagnetic contactor.


Electromagnetic contactor is generally composed of electromagnetic mechanism, contact, arc extinguishing device, release spring mechanism, bracket and base. The contactor works according to the electromagnetic principle: when the electromagnetic coil is energized, the coil current generates a magnetic field, which causes the static iron core to generate electromagnetic attraction to attract the armature, and drive the contact action to make the normally closed contact open and the normally open contact closed. The two are Linkage. When the coil is de-energized, the electromagnetic force disappears, and the armature is lowered and released under the action of the release spring to restore the contact, that is, the normally open contact is disconnected and the normally closed contact is closed.


14. Briefly describe the definition of programmable logic controller (PLC).


Programmable controller (PLC) is an electronic device designed for digital operation and operation specially designed for applications in an industrial environment. It uses a memory that can be programmed to store instructions for performing logical operations, sequential operations, timing, counting, and arithmetic operations, and can control various types of operations through digital or analog input and output. Machinery or production process.


PLC and its related peripheral equipment should be designed according to the principle of being easy to form a whole with industrial control system and easy to expand its functions.


15. Short answer: The difference between the working principle of the PLC system and the relay contactor system.


The components are different;


The number of contacts is different;


Different methods for implementing control;


It works differently.


16. Short answer What are the characteristics of the STL step ladder instruction of Mitsubishi FX 2N series PLC?


(1) The transfer source is automatically reset;


(2) Allow dual output;


(3) Main control function.

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