As is widely recognized, the progress of industrial production and the advancement of science and technology are closely linked to the use of Programmable Logic Controllers (PLCs) for automation.
PLC can be broadly understood as:
Centralized control cabinet for relay extensions.
In practical industrial applications, the use of a Programmable Logic Controller (PLC) significantly reduces control costs while improving centralized management and automation of equipment.
To gain a complete understanding of PLC, it is essential to first establish a solid foundation in the technology.
1.From the composition of the PLC, in addition to the CPU, memory and communication interface, which other interfaces are directly related to the industrial site? And explain its main functions.
1) Input interface:
Receive the signal from the controlled equipment and trigger the internal circuit to turn on or off through the optocoupler and input circuit.
2) Output interface:
The result of program execution is output through the optocoupler and output components (relay, thyristor and transistor) of the output interface to control the activation or deactivation of the external load.
2. What parts does the PLC basic unit consist of? What is the role of each one?
1）CPU:
The main component of the PLC, which commands the PLC to perform various works.
How to receive programs and data from the user, diagnosis, execution of programs, etc.;
2) Memory:
Storage system and user programs and data;
3) I/O Interface:
The connection part between the PLC and the controlled object at the industrial production site is used to receive the signal from the controlled equipment and output the result of program execution;
4) Communication interface:
Exchange information with monitor, printer and other equipment through a communication interface;
5) Power supply.
3.What are the types of PLC switch output interfaces? What are the characteristics of each one?
Thyristor output type:
Generally, it can only carry AC load, with fast response speed and high action frequency;
T resistor output type:
Generally, it can only carry DC load, with fast response speed and high action frequency;
Relay Output Type:
Generally, it can carry AC and DC loads, but its response time is long and its action frequency is low.
4. According to the structure type, what types can PLC be divided into? What are the characteristics of each one?
1) Integral:
CPU, power supply and I/O components are all concentrated in one chassis, which has a compact structure and low price. Generally, small PLC adopt this structure;
2) Modular:
Each part of the PLC is divided into several separate modules, and different modules can be selected according to needs to form a system. It has features of flexible configuration, convenient expansion and maintenance. Generally, this structure is adopted for medium and large PLC.
The modular PLC is composed of a frame or base plate and several modules, which are installed in the socket of the frame or base plate.
3) Battery type:
Combined with the characteristics of integral and modular, the CPU, power supply and I/O interface of the stacked PLC are also independent modules, but they are connected by cables, which makes the system not only flexible in configuration, but also small volume.
5. What is the PLC scanning cycle? What is your main influence?
The PLC digitization process includes five steps: internal processing, communication service, input processing, program execution and output processing.
The time required for scanning in these five steps is called the scanning cycle.
The scan cycle is related to the CPU execution speed, the PLC hardware configuration, and the user program duration.
6. How does the PLC execute the user program? What are the stages of user program execution?
The PLC executes the user program through cyclic scanning.
The user program execution process includes input sampling stage, program execution stage and output updating stage.
Button switch structure schematic diagram
1. Button Cover
2. Return spring
3. Mobile Contact
4. Normally closed static contact
5. Normally open static contact
7. What are the advantages of PLC control system compared with relay control system?
1) Control method:
The PLC adopts program mode to realize control, which is easy to change or increase control requirements, and the PLC contact is unlimited;
2) Working mode:
The PLC adopts serial working mode to improve the system's anti-interference ability;
3) Control speed:
The PLC contact is actually a trigger, and the instruction execution time is at the microsecond level;
4) Timing and counting:
PLC uses semiconductor integrated circuit as timer, clock pulse is provided by crystal oscillator, with high delay accuracy and wide range.
The PLC has a counting function that the relay system does not have;
5) Reliability and ease of maintenance:
The PLC adopts microelectronic technology with high reliability, and its self-inspection function can be discovered in time.
8. Why does PLC produce delay in output response? How to improve I/O response speed?
The PLC adopts a circular sweep mode with centered sampling and centered output. This means that the input state can only be read during the input sampling stage of each scan cycle, and the program execution result can only be output during the output update stage.
Additionally, the input and output delay of the PLC, along with the length of the user program, may cause a delay in the output response.
To improve I/O response, several options are available, including direct input sampling, output refresh, interrupt input and output, and intelligent I/O interface.
9. What are the internal soft relays of Siemens series PLC?
Input relay, output relay, auxiliary relay, status register, timer, counter and data register.
10. How to select PLC?
1) Model selection:
The structural form, installation mode, functional requirements, response speed, reliability requirements, model unification and other aspects must be considered;
2) Capacity selection:
I/O points and user storage capacity must be considered;
3) I/O module selection:
Including switching value selection and analog I/O module, as well as special function module selection;
4) Selection of power module, programmer and other equipment.
11. B briefly describes the characteristics of PLC centralized sampling and centralized output working mode, and what are the advantages and disadvantages of this working mode?
Centralized sampling:
In a scan cycle, sampling of the input state is performed only in the input sampling stage. When entering the program execution phase, the end of the input will be blocked.
Centralized output:
During a scan cycle, the output-related state in the output image register is only transferred to the output latch during the output update stage to update the output interface. The output state is always preserved in the output image record during the other stages.
Although this operating mode can improve the reliability and anti-interference capability of the system, it can also result in a delay in the input/output response of the PLC.
12. What kind of working mode does the PLC adopt? What are the characteristics?
The PLC adopts the working mode of centralized sampling, centralized output and cyclic scanning.
C feature:
Centralized sampling refers to the process in which, during a scan cycle, the PLC samples the input state only at the input sampling stage. When entering the program execution stage, the input end is blocked.
Centralized output, on the other hand, involves the PLC transferring the output-related state in the output image register to the output latch only at the output update stage. This updates the output interface and the output state is saved in the output image register during other stages.
Cyclic scanning refers to the process where the PLC needs to perform multiple operations in one scanning cycle. This is achieved through the time-sharing scanning method, where operations are executed one by one in order and repeated continuously.
13. What are the main parts of the electromagnetic contactor? B briefly describe the working principle of the electromagnetic contactor.
An electromagnetic contactor typically consists of several components, including an electromagnetic mechanism, contacts, an arc extinguishing device, a release spring mechanism, bracket, and base.
The contactor works according to the electromagnetic principle:
When an electromagnetic coil is energized, the current passing through it generates a magnetic field. This magnetic field causes the stationary iron core to generate electromagnetic attraction, which pulls the armature toward it. As a result, the contact action is triggered and the normally closed contact is opened while the normally open contact is closed.
These two actions are interdependent.
When the coil is no longer energized, the electromagnetic force ceases to exist and the release spring causes the armature to return to place, thus restoring the contacts. This means that the normally open contact becomes open again while the normally closed contact is closed.
14. Brief definition of programmable logic controller (PLC).
A programmable logic controller (PLC) is an electronic device designed for digital operations in an industrial environment. It uses programmable memory to store instructions for performing logical, sequential, timing, counting, and arithmetic operations. This allows you to control various types of machines or production processes through digital or analog inputs and outputs.
When designing a PLC and its related peripheral equipment, it is essential to follow the principle that it should be easy to integrate with the industrial control system and expand its functions to form a unified system.
15. Briefly answer the difference in working principle between PLC system and relay contactor system.
There are different components, various numbers of contacts and different control methods.
