It is universally acknowledged that the advancement of industrial production and technological development is inseparable from the automated control of PLCs, which can be broadly interpreted as centralized relay extension control cabinets. In practical production applications, PLCs significantly reduce the cost of industrial control, enhancing centralized management and automatic control of equipment. To excel in PLC programming, a solid foundation in PLC fundamentals is paramount.
- Regarding the composition of PLCs, aside from the CPU, memory, and communication interfaces, what other interfaces are directly related to industrial fields? Please elaborate on their primary functions.
- Input Interface: It receives signals from controlled equipment and drives the internal circuitry to connect or disconnect through photoelectric coupling devices and input circuits.
- Output Interface: The execution results of programs are output through photoelectric coupling devices and output components (relays, thyristors, transistors) of the output interface, controlling the connection or disconnection of external loads.
- What are the constituent parts of a PLC's basic unit, and what role does each play?
- CPU: The core component of the PLC, directing various operations such as receiving user programs and data, performing diagnostics, and executing programs.
- Memory: Stores system and user programs and data.
- I/O Interface: The connecting component between the PLC and controlled objects in industrial production sites, used to receive signals from controlled equipment and output the execution results of programs.
- Communication Interface: Enables information exchange with other devices such as monitors and printers through the communication interface.
- Power Supply.
- What are the types of PLC switching output interfaces, and what are their respective characteristics?
- Thyristor Output Type: Typically capable of driving AC loads only, with fast response speed and high operating frequency.
- Transistor Output Type: Typically limited to driving DC loads, also featuring fast response speed and high operating frequency.
- Relay Output Type: Can drive both AC and DC loads, but has a longer response time and lower operating frequency.
- What are the different types of PLCs based on their structural forms, and what are their respective characteristics?
- Integrated Type: The CPU, power supply, and I/O components are all concentrated within a single enclosure, resulting in a compact structure and low cost. This type is commonly used in small PLCs.
- Modular Type: The PLC is divided into several individual modules, allowing for flexible configuration and easy expansion and maintenance by selecting and combining different modules as needed. This type is typically used in medium to large PLCs. The modular PLC consists of a frame or baseboard with various modules mounted on sockets.
- Stackable Type: Combining the features of both integrated and modular types, the CPU, power supply, and I/O interfaces of stackable PLCs are independent modules connected by cables, offering both flexible configuration and a compact size.
- What is the scanning cycle of a PLC, and what factors primarily affect it?
The scanning process of a PLC comprises five stages: internal processing, cycle communication is services influenced, by input the processing CPU, operating program speed execution,, PLC and hardware output configuration execute processing,. and The the time required length for of one the full user scan program of.
- How does a PLC execute user programs, and what stages does the execution process comprise?
A 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.
- Compared to relay control systems, what are the advantages of PLC control systems?
- Control Method: PLCs use programming to achieve control, making it easy to change or add control requirements. PLCs have unlimited "contacts."
- Working Mode: PLCs operate in a serial manner, enhancing the system's anti-interference capability.
- Control Speed: PLC "contacts" are essentially triggers, with instruction execution times measured in microseconds.
- Timing and Counting: PLCs use semiconductor integrated circuits as timers, with clock pulses provided by crystal oscillators, offering high delay accuracy and a wide range. PLCs also possess counting functions not found in relay systems.
- Reliability and Maintainability: PLCs employ microelectronics, ensuring high reliability and self-diagnostic capabilities for timely troubleshooting.
- Why does PLC exhibit output response lag, and how can I/O response speed be improved?
PLC's centralized sampling and output cycle scanning method cause output response lag. Input states are only read during the input sampling stage of each scanning cycle, and program execution results are only output during the output refresh stage. Additionally, PLC input/output delays and user program length can contribute to response lag.
To improve I/O response speed, methods such as direct input sampling and output refresh, interrupt-driven I/O, and intelligent I/O interfaces can be employed.
- What types of internal soft relays are found in Siemens PLCs?
Input relays, output relays, auxiliary relays, status registers, timers, counters, and data registers.
- How do you select a suitable PLC?
- Model Selection: Consider factors such as structural form, installation method, functional requirements, response speed, reliability requirements, and model uniformity.
- Capacity Selection: Evaluate based on I/O points and user storage capacity.
- I/O Module Selection: Involves choosing switching and analog I/O modules, as well as special function modules.
- Selection of Power Modules, Programmers, and Other Devices.
- Briefly describe the characteristics of PLC's centralized sampling and output working mode, and what are its advantages and disadvantages?
Centralized Sampling: Within a scanning cycle, input states are sampled only during the input sampling stage. Once the program execution stage begins, the input terminals are blocked.
Centralized Output: During a scanning cycle, output-related states in the output image register are transferred to the output latch only during the output refresh stage, refreshing the output interface. During other stages, output states are retained in the output image register. This mode enhances system anti-interference capability and reliability but can cause input/output response lag in PLCs.
- What working mode does PLC adopt? And what are its characteristics?
PLC adopts a working mode of centralized sampling, centralized output, and cyclic scanning.
Characteristics: Centralized sampling refers to the fact that within a scanning cycle, PLC only samples the input status during the input sampling stage, and the input terminals are blocked once the program execution stage begins.
Centralized output means that within a scanning cycle, PLC only transfers the output-related status in the output image register to the output latch during the output refresh stage, refreshing the output interface. During other stages, the output status remains stored in the output image register.
Cyclic scanning indicates that PLC needs to execute multiple operations within a scanning cycle, which it accomplishes sequentially in a time-sharing manner, repeating the cycle continuously.
- What are the main components of an electromagnetic contactor? Briefly describe its working principle.
An electromagnetic contactor typically consists of an electromagnetic mechanism, contacts, an arc suppression device, a release spring mechanism, a bracket, a base, and other components. The contactor operates based on electromagnetic principles: When the electromagnetic coil is energized, the coil current generates a magnetic field that causes the stationary iron core to produce an electromagnetic force attracting the armature, which in turn actuates the contacts, opening the normally closed contacts and closing the normally open contacts simultaneously. These actions are interconnected. When the coil is de-energized, the electromagnetic force disappears, and the armature is released by the release spring, restoring the contacts to their original positions, i.e., the normally open contacts open, and the normally closed contacts close.
- Briefly define Programmable Logic Controller (PLC).
A Programmable Logic Controller (PLC) is an electronic device designed specifically for digital computational operations in industrial environments. It utilizes a programmable memory to store instructions for performing logical, sequential, timing, counting, and arithmetic operations internally. It can control various types of machinery or production processes through digital or analog inputs and outputs. Both the PLC and its related peripheral equipment are designed to easily integrate with industrial control systems and facilitate functional expansion.
- Briefly explain the differences in working principles between a PLC system and a relay contactor system.
- Different constituent components;
- Varying numbers of contacts;
- Different methods of implementing control.