Everyone knows that industrial production and technological development are inseparable from the automation control of PLCs. PLC can be broadly understood as: a centralized relay extension control cabinet. In practical production applications, PLC greatly reduces the cost of industrial control and enhances the centralized management and automatic control of equipment. To learn PLC well, it is essential to have a solid foundation in PLC.
1. From the composition of PLC, aside from the CPU, memory, and communication interfaces, what other interfaces are directly related to the industrial site? Please explain their main functions.
1) Input Interface: Accepts signals from controlled devices and drives the internal circuit to connect or disconnect through optocoupler devices and input circuits.
2) Output Interface: The execution results of the program are output through the optocoupler devices of the output interface and output components (relays, thyristors, transistors), controlling the connection or disconnection of external loads.
2. What are the basic units of PLC composed of? What are their functions?
1) CPU: The core component of the PLC, directing it to perform various tasks such as receiving user programs and data, diagnostics, and executing programs;
2) Memory: Stores the system and user programs and data;
3) I/O Interface: The connection component between the PLC and the controlled objects in the industrial production site, used to accept signals from controlled devices and output the results of program execution;
4) Communication Interface: Exchanges information with other devices such as monitors and printers through the communication interface;
5) Power Supply.
3. What types of PLC digital output interfaces are there? What are their characteristics?
Thyristor Output Type: Generally can only carry AC loads, fast response speed, high action frequency;
Transistor Output Type: Generally can only carry DC loads, fast response speed, high action frequency;
Relay Output Type: Generally can carry both AC and DC loads, but has a longer response time and lower action frequency.
4. What types of PLC are there based on structural types? What are their characteristics?
1) Integrated Type: The CPU, power supply, and I/O components are all concentrated in one chassis, compact structure, low price, generally used for small PLCs;
2) Modular Type: Divides the various parts of the PLC into several separate modules, allowing for flexible configuration and easy expansion and maintenance, generally used for medium and large PLCs. Modular PLCs consist of a frame or baseboard and various modules, with modules inserted into the sockets on the frame or baseboard.
3) Stacked Type: Combines the characteristics of both integrated and modular types. The CPU, power supply, and I/O interfaces of the stacked PLC are also independent modules but are connected by cables, allowing for flexible configuration and compact size.
5. What is the scanning cycle of a PLC? What mainly affects it?
The scanning process of a PLC includes five stages: internal processing, communication services, input processing, program execution, and output processing. The time required to scan once through these five stages is called the scanning cycle.
The scanning cycle is related to the CPU operating speed, PLC hardware configuration, and the length of the user program.
6. How does a PLC execute user programs? What stages are included in the user program execution process?
A PLC executes user programs in a cyclic scanning manner. The execution process of the user program includes the input sampling stage, program execution stage, and output refresh stage.
7. What advantages does a PLC control system have compared to a relay control system?
1) In terms of control methods: PLC uses a programmatic approach to achieve control, making it easy to change or add control requirements, and PLC contacts are virtually unlimited;
2) In terms of working methods: PLC uses a serial working method, improving the system’s anti-interference capability;
3) In terms of control speed: PLC contacts are essentially triggers, with instruction execution times in the microsecond range;
4) In terms of timing and counting: PLC uses semiconductor integrated circuits as timers, with clock pulses provided by crystal oscillators, offering high delay accuracy and wide range. PLC has counting functions that relay systems do not possess;
5) In terms of reliability and maintainability: PLC uses microelectronic technology, offering high reliability, and its self-checking functions can identify issues promptly.
8. Why does a PLC produce output response lag? How can the I/O response speed be improved?
Because PLC uses a centralized sampling and centralized output cyclic scanning working method, the input status can only be read at the input sampling stage of each scanning cycle, and the results of program execution are only sent out during the output refresh stage; moreover, the input and output delays of the PLC, the length of the user program, etc., can all cause output response lag.
To improve I/O response speed, methods include direct input sampling and output refresh, or interrupt input/output, and intelligent I/O interfaces among various approaches.
9. What types of internal soft relays are there in Siemens series PLCs?
Input relays, output relays, auxiliary relays, status registers, timers, counters, data registers.
10. How to choose a PLC?
1) Model selection: Consider aspects such as structural form, installation method, functional requirements, response speed, reliability requirements, and model uniformity;
2) Capacity selection: Consider I/O point numbers and user storage capacity;
3) I/O module selection: Includes the selection of digital and analog I/O modules, as well as special function modules;
4) Selection of power supply modules and programming devices, etc.
11. Briefly describe the characteristics of PLC centralized sampling and centralized output working methods. What are the advantages and disadvantages of adopting this working method?
Centralized Sampling: In one scanning cycle, input status sampling only occurs in the input sampling stage, and the input end will be locked during the program execution stage.
Centralized Output: In one scanning cycle, the output image register’s status related to output will only be transferred to the output latch during the output refresh stage, refreshing the output interface, while in other stages, the output status remains in the output image register. This working method can enhance the system’s anti-interference ability and reliability but can cause lag in PLC input/output response.
12. What working methods does a PLC adopt? What are their characteristics?
A PLC adopts a centralized sampling, centralized output, and cyclic scanning working method.
Characteristics: Centralized sampling means that in one scanning cycle, the PLC samples the input status only during the input sampling stage, and the input end will be locked during the program execution stage.
Centralized output means that in one scanning cycle, the PLC only transfers the status related to output from the output image register to the output latch during the output refresh stage, refreshing the output interface while maintaining the output status in the output image register during other stages.
Cyclic scanning means that the PLC executes multiple operations within one scanning cycle, adopting a time-sharing scanning method to sequentially execute them, repeating the process continuously.
13. What are the main components of an electromagnetic contactor? Briefly describe the working principle of an electromagnetic contactor.
An electromagnetic contactor generally consists of an electromagnetic mechanism, contacts, an arc extinguishing device, a release spring mechanism, a bracket, and a base. The contactor operates based on electromagnetic principles: when the electromagnetic coil is powered, the current in the coil generates a magnetic field, causing the static iron core to generate electromagnetic attraction, pulling the armature and moving the contacts, opening the normally closed contacts and closing the normally open contacts, which are interlinked. When the coil is powered off, the electromagnetic force disappears, and the armature drops under the action of the release spring, restoring the contacts, meaning the normally open contacts open and the normally closed contacts close.
14. Briefly describe the definition of a Programmable Logic Controller (PLC).
A Programmable Logic Controller (PLC) is an electronic device specifically designed for digital operation in industrial environments. It uses programmable memory to store instructions for executing logical operations, sequential operations, timing, counting, and arithmetic operations, and can control various types of machinery or production processes through digital or analog inputs and outputs.
PLC and its related peripheral devices should be designed according to the principle of being easy to form a whole with industrial control systems and easy to expand their functions.
15. Briefly describe the differences in working principles between PLC systems and relay contactor systems.
Different component devices;
Different numbers of contacts;
Different methods of implementing control.
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