It is well known that industrial production and technological development rely heavily on the automation control provided by PLCs. A PLC can be broadly understood as a centralized relay extension control cabinet. In practical production applications, PLCs significantly reduce the costs of industrial control and enhance centralized management and automatic control of equipment. To master PLCs, a solid foundation in PLC basics is essential.
1. From the composition of a PLC, apart 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 and output components (relays, thyristors, transistors) of the output interface, controlling the connection or disconnection of external loads.
2. What are the basic components of a PLC? What roles do they play?
1) CPU: The core component of the PLC, directing it to perform various tasks such as accepting user programs and data, diagnostics, and executing programs;
2) Memory: Stores system and user programs and data;
3) I/O Interface: The connecting component between the PLC and the controlled objects in the industrial production site, used to accept signals from controlled devices and output the execution results of the program;
4) Communication Interface: Facilitates information exchange 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 drive AC loads, with fast response speed and high action frequency;
Transistor Output Type: Generally can only drive DC loads, with fast response speed and high action frequency;
Relay Output Type: Generally can drive both AC and DC loads, but has a longer response time and lower action frequency.
4. What types of PLCs are there based on structural form? What are their characteristics?
1) Integrated Type: Combines the CPU, power supply, and I/O components in one chassis, compact and low-cost, generally used in small PLCs;
2) Modular Type: Divides the PLC into several separate modules, allowing for flexible configuration and easy expansion and maintenance, generally used in medium to large PLCs. Modular PLCs consist of a frame or baseboard and various modules, with modules installed in sockets on the frame or baseboard.
3) Stacked Type: Combines the characteristics of both integrated and modular types, where the CPU, power supply, and I/O interfaces are independent modules connected by cables, making the system both flexible in configuration and compact in size.
5. What is the PLC scanning cycle? What factors influence it?
The PLC scanning process includes five stages: internal processing, communication services, input processing, program execution, and output processing. The time required to scan through these five stages is called the scanning cycle.
The scanning cycle is influenced by 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 involved in the execution process?
A PLC executes user programs using a cyclic scanning method. The execution process includes the input sampling stage, program execution stage, and output refresh stage.
7. What advantages does a PLC control system have over a relay control system?
1) Control Method: PLC uses a programmatic approach to implement control, making it easy to change or add control requirements, and PLC contacts are virtually unlimited;
2) Operating Mode: PLC operates in a serial manner, enhancing the system’s anti-interference capability;
3) Control Speed: PLC contacts are essentially triggers, with instruction execution times in the microsecond range;
4) Timing and Counting: PLC uses semiconductor integrated circuits as timers, with clock pulses provided by crystal oscillators, offering high delay accuracy and a wide range. PLC has counting functions that relay systems do not possess;
5) Reliability and Maintainability: PLC employs microelectronic technology, ensuring high reliability, and its self-diagnostic functions can promptly identify issues.
8. Why does a PLC experience output response lag? How can I/O response speed be improved?
Due to the PLC’s centralized sampling and centralized output cyclic scanning operation mode, the input status can only be read during the input sampling stage of each scanning cycle, while the execution results of the program are only sent out during the output refresh stage. Additionally, delays in PLC input and output, as well as the length of the user program, can cause output response lag.
To improve I/O sampling and output refresh, or to implement direct input sampling and output refresh, as well as interrupt input/output and intelligent I/O interfaces, among other methods.
9. What types of internal soft relays are available in Siemens series PLCs?
Input relays, output relays, auxiliary relays, status registers, timers, counters, data registers.
10. How to select a PLC?
1) Model Selection: Consider structural form, installation method, functional requirements, response speed, reliability requirements, and model uniformity;
2) Capacity Selection: Consider the number of I/O points and user storage capacity;
3) I/O Module Selection: Includes selection of digital and analog I/O modules, as well as special function modules;
4) Selection of power supply modules and programming devices, among other equipment.
11. Briefly describe the characteristics of the PLC centralized sampling and centralized output working method. What are the advantages and disadvantages of this working method?
Centralized Sampling: During a scanning cycle, input status sampling occurs only during the input sampling stage, and the input end will be blocked during the program execution stage.
Centralized Output: During a scanning cycle, the output image register’s states related to output are only transferred to the output latch during the output refresh stage, refreshing the output interface, while the output states remain in the output image register during other stages. This working method can enhance the system’s anti-interference capability and reliability, but it can cause lag in PLC input/output responses.
12. What working method does a PLC adopt? What are its characteristics?
A PLC adopts a centralized sampling, centralized output, and cyclic scanning working method.
Characteristics: Centralized sampling means that during a scanning cycle, the PLC samples the input status only during the input sampling stage, and the input end will be blocked during the program execution stage.
Centralized output means that during a scanning cycle, the PLC only transfers the states related to output from the output image register to the output latch during the output refresh stage, refreshing the output interface, while the output states remain in the output image register during other stages.
Cyclic scanning means that the PLC needs to perform multiple operations within a scanning cycle, executing them sequentially in a time-sharing manner, continuously repeating the process.
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 energized, the coil current generates a magnetic field, causing the static iron core to produce electromagnetic attraction, which moves the armature and actuates the contacts, opening the normally closed contacts and closing the normally open contacts, with both actions linked. When the coil is de-energized, the electromagnetic force disappears, and the armature falls back under the action of the release spring, restoring the contacts, i.e., opening the normally open contacts and closing the normally closed contacts.
14. Briefly define a Programmable Logic Controller (PLC).
A Programmable Logic Controller (PLC) is an electronic device 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.
PLCs and their related peripheral devices should be designed to easily integrate into an industrial control system and facilitate functional expansion.
15. Briefly explain the differences in working principles between PLC systems and relay contactor systems.
The components differ;
The number of contacts differs;
The methods of implementing control differ.
Source: This article is reproduced from the internet, and the copyright belongs to the original author. If there are any copyright issues, please contact us promptly for removal. Thank you!
Scan to Follow
WeChat ID｜13615417996
Follow the QR code on the left to get free access
[Siemens Data Collection]