Those who work in the power industry know that industrial production and technological development are inseparable from PLC automation control. PLC can be broadly understood as a centralized relay extension control cabinet. In actual production applications, PLC greatly saves industrial control costs and strengthens centralized management and automatic control of equipment. To learn PLC well, you must first have a solid foundation in PLC.
1. From the perspective of PLC composition, besides CPU, memory, and communication interface, what other interfaces are directly related to the industrial field? Please explain their main functions.
(1) Input interface: It receives signals from the controlled device and drives the internal circuit to be turned on or off through optocouplers and input circuits.
(2) Output interface: The execution result of the program is output through the optocoupler and output components (relay, thyristor, transistor) of the output interface to control the connection or disconnection of the external load.
2. What are the basic components of a PLC unit? What is the function of each component?
(1) CPU: The core component of the PLC, which directs the PLC to perform various tasks, such as receiving user programs and data, diagnostics, and executing programs.
(2) Memory: Stores system and user programs and data;
(3) I/O interface: The connection component between the PLC and the controlled object in the industrial production site, used to receive signals from the controlled equipment 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, it can only drive AC loads, with fast response speed and high operating frequency;
Transistor output type: Generally, it can only drive DC loads, with fast response speed and high operating frequency;
Relay output type: Generally, it can carry AC and DC loads, but its response time is long and its operating frequency is low.
4. According to structural type, what are the different types of PLCs? What are the characteristics of each?
(1) Integrated type: The CPU, power supply and I/O components are all integrated into one chassis. It has a compact structure and low price. Small PLCs generally adopt this structure;
(2) Modular: The PLC is divided into several individual modules. Different modules can be selected and matched to form a system as needed. 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. The modules are installed in the sockets of the frame or base plate.
(3) Stacked type: Combining the features of integrated type and modular type, the CPU, power supply, I/O interface, etc. of stacked type PLC 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 PLC scan cycle? What are its main influencing factors?
The scanning process of a PLC consists of five stages: internal processing, communication services, input processing, program execution, and output processing. The time required for one scan in each of these five stages is called the scan cycle.
The scan cycle is related to the CPU speed, PLC hardware configuration, and the length of the user program.
6. How does the PLC execute user programs? What are the stages involved in the execution of a user program?
The PLC executes the user program using a cyclic scanning method. 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 a PLC control system compared to a relay control system?
(1) In terms of control method: PLC uses a program method to achieve control, which makes it easy to change or add control requirements, and PLC has an unlimited number of contacts;
(2) In terms of operation mode: The PLC adopts a serial operation mode, which improves the system's anti-interference capability;
(3) In terms of control speed: the contacts of the PLC are actually triggers, and the instruction execution time is in the microsecond range;
(4) Timing and counting: The PLC uses semiconductor integrated circuits as timers, with clock pulses provided by a crystal oscillator, resulting in high delay accuracy and a wide range. The PLC also has a counting function that relay systems do not possess;
(5) In terms of reliability and maintainability: PLC adopts microelectronic technology, has high reliability, and its self-test function can detect its own faults in a timely manner. Its monitoring function makes debugging and maintenance convenient.
8. Why does a PLC exhibit output response lag? How can I/O response speed be improved?
Because PLCs use a centralized sampling and centralized output cyclic scanning mode, the state of the input terminal can only be read in the input sampling phase of each scanning cycle, while the execution result of the program is only sent out in the output refresh phase; secondly, the input and output delays of the PLC, the length of the user program, etc., can all cause output response lag.
There are several ways to improve I/O response, such as input sampling and output refresh, or direct input sampling and output refresh, as well as interrupt input/output and intelligent I/O interfaces.
9. What types of internal soft relays are available in the FX0N series PLC?
Input relays, output relays, auxiliary relays, status registers, timers, counters, and data registers.
10. How to choose a PLC?
1) Model selection: Considerations should include structural form, installation method, functional requirements, response speed, reliability requirements, and model uniformity.
2) Capacity selection: Consideration should be given to both I/O points and user storage capacity;
3) I/O module selection: including selection of digital and analog I/O modules, as well as selection of special function modules;
4) Selection of power modules and programmers and other equipment.
11. Briefly describe the characteristics of a PLC's centralized sampling and centralized output working mode. What are the advantages and disadvantages of this working mode?
Centralized sampling: Within a scan cycle, the input state is sampled only during the input sampling phase. Once the program execution phase begins, the input terminal will be blocked.
Centralized output: Within a scan cycle, the output-related state in the output image register is transferred to the output latch only during the output refresh phase to refresh the output interface. In other phases, the output state remains in the output image register. This operating mode improves the system's anti-interference capability and enhances its reliability, but it causes a lag in the PLC's input/output response.
12. What is the operating mode of a PLC? What are its characteristics?
The PLC adopts a working mode of centralized sampling, centralized output, and cyclic scanning.
Features: Centralized sampling means that within a scan cycle, the PLC samples the input status only during the input sampling phase. Once the program execution phase begins, the input terminals will be blocked.
Centralized output means that within a scan cycle, the PLC only transfers the output-related status from the output image register to the output latch during the output refresh phase to refresh the output interface. In other phases, the output status is always stored in the output image register.
Cyclic scanning refers to a PLC performing multiple operations within a single scan cycle. It employs a time-division scanning method, executing each operation sequentially and 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 several parts, including an electromagnetic mechanism, contacts, an arc-extinguishing device, a release spring mechanism, a support, and a base. The contactor operates based on electromagnetic principles: when the electromagnetic coil is energized, the coil current generates a magnetic field, which causes the stationary iron core to generate an electromagnetic attraction that draws in the armature, thus actuating the contacts. This causes the normally closed contacts to open and the normally open contacts to close; the two actions are linked. When the coil is de-energized, the electromagnetic force disappears, and the armature, under the action of the release spring, descends, causing the contacts to return to their original position—that is, 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 a digital electronic device specifically designed for industrial applications. It uses a programmable memory to store instructions for performing logical, sequential, 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 peripherals should be designed according to the principle of easy integration with industrial control systems and easy expansion of their functions.
15. Briefly explain the differences in working principles between PLC systems and relay contactor systems.
Different components;
Different number of contacts;
The methods of implementing control differ;
Different working methods.
16. Briefly describe the characteristics of the STL step ladder instruction in the Mitsubishi FX 2N series PLC.
(1) Automatic reset of the transfer source;
(2) Allows dual output;
(3) Main control function.
