People who are engaged in electric power operations know that the development of industrial production and technology is inseparable from the automatic control of PLC . PLC can be broadly understood as: centralized relay extension control cabinet. In actual production applications, PLC greatly saves industrial control. It strengthens 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 composition of PLC, besides the CPU , memory and communication interface , what 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 components (relay, thyristor , transistor) of the output interface to control the on or off of the external load.
2. What parts does the basic unit of PLC consist of? What is the function of each?
(1) CPU: The core component of PLC, which directs PLC to carry out various tasks. Such as accepting user programs and data, diagnosing, executing executive programs, etc.;
(2) Memory : stores the programs and data of the system and users;
(3) I/O interface: the connecting part between PLC and the controlled object in the industrial production site, 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 equipment through the communication interface ;
(5) Power supply.
3. What types of PLC switching output interfaces are there? 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 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 operating frequency is low.
4. According to the structure type, what types of PLCs are there? 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 adopt this structure;
(2) Modular type: Divide each part of the PLC into several separate 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. The modular PLC consists of a frame or base plate and various modules, and the modules are mounted on the sockets of the frame or base plate.
(3) Stacked type: Combining the features of integral type and modular type, the CPU, power supply and I/O interface of stackable PLC are also independent modules, but they are connected by cables, so that the system not only configures Flexible and compact.
5. What is the scan cycle of PLC? What is it mainly affected by?
The scanning process of PLC includes five stages: internal processing, communication service, input processing, program execution, and output processing. The time required to scan each of these five stages is called scanning cycle.
The scan cycle is related to the running speed of the CPU , the hardware configuration of the PLC and the length of the user program.
6. What method does the PLC use to execute the user program? What stages does the user program execution process include?
PLC executes the user program by cyclic scanning. The execution process of the user program includes the input sampling stage, the program execution stage and the output refresh stage.
7. What are the advantages of PLC control system compared with relay control system?
(1) In terms of control method: PLC adopts program method to realize control, which is easy to change or increase control requirements, and the contact points of PLC are infinite;
(2) Working mode: PLC adopts serial working mode to improve the anti-interference ability of the system;
(3) In terms of control speed: the contact of PLC is actually a trigger, and the execution time of the instruction is in the microsecond level;
(4) Timing and counting: PLC uses semiconductor integrated circuits as timers, and clock pulses are provided by crystal oscillators, with high delay accuracy and wide range. PLC has counting function that relay system does not have;
(5) In terms of reliability and maintainability: PLC adopts microelectronic technology, which has high reliability. Its self-checking function can detect its own fault in time, and the monitoring function is convenient for debugging and maintenance.
8. Why does PLC produce output response lag? How to improve 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 stage of each scan cycle, and the execution result of the program is only sent out in the output refresh stage; secondly, the PLC input , output delay delay, the length of user program, etc. can cause output response delay.
To improve I/O input 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 soft relays are there in FX0N series PLC?
Input relays, output relays, auxiliary relays , status registers , timers, counters, data registers.
10. How to choose PLC?
1) Model selection: should be considered from the aspects of structure, installation, functional requirements, response speed, reliability requirements, and model unity;
2) Capacity selection: should be considered from the two aspects of I/O points and user storage capacity;
3) I/O module selection: including switch and analog I/O module selection, as well as special function module selection;
4) Selection of other equipment such as power supply module and programmer.
11. Briefly describe the characteristics of PLC centralized sampling and centralized output working mode, what are the advantages and disadvantages of adopting this working mode?
Centralized sampling: In one scan cycle, the sampling of the input state is only carried out in the input sampling stage, and the input terminal will be blocked after entering the program execution stage.
Centralized output: in one scan cycle, only in the output refresh stage, the output-related state in the output image register is transferred to the output latch, and the output interface is refreshed, and the output state is kept in the output image in other stages. in the register. This way of working can improve the anti-interference ability of the system and enhance the reliability of the system, but it will cause the delay of PLC input/output response.
12. What kind of working method does PLC adopt? What are the characteristics?
PLC adopts the working mode of centralized sampling, centralized output and cyclic scanning.
Features: Centralized sampling means that in one scan cycle, the PLC samples the input state only in the input sampling stage, and the input terminal will be blocked after entering the program execution stage.
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 in the output refresh stage, refreshes the output interface, and keeps the output state in other stages. in the output image register .
Cyclic scan means that PLC needs to perform multiple operations in one scan cycle. It uses time-sharing scan method to execute one by one in sequence, and runs repeatedly.
13. What are the main parts of the electromagnetic contactor? Briefly describe the working principle of the electromagnetic contactor.
Electromagnetic contactor is generally composed of electromagnetic mechanism , contacts, 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 makes the static iron core generate electromagnetic attraction to attract the armature, and drives the contacts to act, so that the normally closed contacts are disconnected and the normally open contacts are closed. linked. When the coil is powered off, the electromagnetic force disappears, and the armature is lowered and released under the action of the release spring, so that the contact is restored, 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).
A programmable logic controller (PLC) is an electronic device specially designed for digital operation in industrial environments. It uses a programmable memory to store instructions for performing logical operations, sequential operations, timing, counting and arithmetic operations, and can control various types of digital or analog inputs and outputs. Mechanical or production process.
PLC and its related peripheral equipment should be designed according to the principle that it is easy to form a whole with the industrial control system, and it is easy to expand its functions.
15. Briefly answer the difference between the working principle of PLC system and relay contactor system.
The components are different;
The number of contacts is different;
Different methods of implementing controls;
Works differently.
16. Short answer What are the characteristics of the STL stepping ladder instruction of Mitsubishi FX 2N series PLC ?
(1) The transfer source is automatically reset;
(2) Double output is allowed;
(3) Main control function.
Having said so much, everyone remember to pay attention to the first comment below, there are dry goods~