Those engaged in industrial control work know that the development of industrial production and technology relies heavily on the automation control of PLCs. PLC can be broadly understood as: a centralized relay extension control cabinet. In practical production applications, PLCs greatly save the cost of industrial control and enhance the centralized management and automatic control of equipment. To master PLC, a solid foundation in PLC is essential.
1. From the composition of 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 optocouplers and input circuits.
(2) Output Interface: The execution results of the program are output through the optocouplers of the output interface and output components (relays, thyristors, transistors) to control the connection or disconnection of external loads.
2. What are the basic units of PLC composed of? What roles do they play?
(1) CPU: The core component of the PLC, directing the PLC to perform various tasks, such as receiving user programs and data, diagnosing, executing programs, etc.;
(2) Memory: Stores the 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: 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 adopted by small PLCs;
(2) Modular Type: Divides the PLC into several individual modules, allowing for flexible configuration of different modules to form a system, characterized by flexible configuration, easy expansion, and maintenance, generally adopted by medium and large PLCs. Modular PLCs consist of a frame or baseboard and various modules, with modules installed in the sockets of 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 stacked PLCs are also independent modules, but they are connected by cables, making the system not only flexible in configuration but also compact in size.
5. What is the scanning cycle of a PLC? What primarily 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 through these five stages once 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 PLC execute user programs? What stages does the user program execution process include?
PLC executes user programs using a cyclic scanning method. 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 program-based method for control, making it easy to change or add control requirements, and PLC contacts are virtually unlimited;
(2) In terms of working methods: PLC uses serial working methods, 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, resulting in high delay accuracy and wide range. PLC has counting functions that relay systems do not possess;
(5) In terms of reliability and maintainability: PLC employs microelectronics technology, offering high reliability, and its self-diagnostic functions can promptly identify faults, while monitoring functions facilitate debugging and maintenance.
8. Why does PLC produce output response lag? How can I/O response speed be improved?
Because PLC adopts a centralized sampling and centralized output cyclic scanning working method, the input status can only be read during the input sampling stage of each scanning cycle, and the program execution results are only sent out during the output refresh stage. Additionally, 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, output refreshing, interrupting input and output, and intelligent I/O interfaces.
9. What types of internal soft relays are there in the FX0N series PLC?
Input relays, output relays, auxiliary relays, status registers, timers, counters, data registers.
10. How to select a PLC?
1) Model selection: Should consider structural form, installation method, functional requirements, response speed, reliability requirements, and model uniformity;
2) Capacity selection: Should consider I/O point count and user storage capacity;
3) I/O module selection: Includes selecting switch and analog I/O modules, as well as special function modules;
4) Selection of power supply modules, programmers, and other devices.
11. Briefly describe the characteristics of the PLC centralized sampling and centralized output working method. What are the advantages and disadvantages of adopting this working method?
Centralized Sampling: In a scanning cycle, input status sampling is only performed during the input sampling stage, and the input end will be locked when entering the program execution stage.
Centralized Output: In a scanning cycle, the output status is only transferred from the output image register to the output latch during the output refresh stage, refreshing the output interface, while the output status is maintained in the output image register during other stages. Adopting this working method can enhance the system’s anti-interference capability and reliability, but may cause response lag in PLC input/output.
12. What kind of working method does PLC adopt? What are its characteristics?
PLC adopts a working method of centralized sampling, centralized output, and cyclic scanning.
Characteristics:Centralized Sampling means that within a scanning cycle, PLC samples the input status only during the input sampling stage, and the input end will be locked when entering the program execution stage.
Centralized Output means that within a scanning cycle, PLC only transfers the output status related to the 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 PLC needs to perform multiple operations within a scanning cycle, adopting a time-sharing scanning method to execute them sequentially, repeating 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 the electromagnetic mechanism, contacts, arc extinguishing device, releasing spring mechanism, bracket, and 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 generate electromagnetic attraction to draw the armature, which drives the contacts to act, opening the normally closed contacts and closing the normally open contacts, with both being interlinked. When the coil is de-energized, the electromagnetic force disappears, and the armature is lowered by the releasing 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 designed for digital computation operations 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 based on the principle of ease of integration with industrial control systems and ease of expansion of their functions.
15. Briefly explain the differences in working principles between PLC systems and relay contactor systems.
Different component devices;
Different numbers of contacts;
Different methods of implementing control;
Different working methods.
16. Briefly describe the characteristics of the STL step ladder instructions of the Mitsubishi FX2N series PLC.
(1) The transfer source automatically resets;
(2) Allows dual outputs;
(3) Main control function.
Source: Coal Chemical Knowledge Base
China Chemical Safety Association
Editor: An An