The Role of PLCs in Industrial Production and Technology
The development of industrial production and technology is inseparable from the automation control provided by PLCs. Broadly speaking, a PLC can be understood as a centralized extension of a relay control cabinet. In practical applications, PLCs significantly reduce the cost of industrial control, enhance centralized management, and automate control. To master PLCs, one must first have a solid foundation in PLC basics.
- What interfaces are directly related to the industrial site, apart from the CPU, memory, and communication interfaces of a PLC? Explain their main functions.
- Input Interfaces: They receive signals from controlled devices, and through optocoupler devices and input circuits, they drive internal circuits to connect or disconnect.
- Output Interfaces: The results of program execution are output through optocouplers and output components (relays, thyristors, transistors), controlling the connection or disconnection of external loads.
- What are the basic components of a PLC, and what roles do they play?
- CPU: The core component of the PLC, it directs all operations including accepting user programs and data, diagnostics, and executing programs.
- Memory: Stores the system and user programs and data.
- I/O Interfaces: These connect the PLC to controlled objects at the industrial site, accepting signals from controlled devices and outputting program execution results.
- Communication Interfaces: Facilitate information exchange with monitors, printers, and other devices.
- Power Supply: Provides the necessary power for the PLC operations.
- What types of digital output interfaces does a PLC have? What are their characteristics?
- Thyristor Output: Generally used for AC loads, with fast response and high operation frequency.
- Transistor Output: Typically for DC loads, with quick response and high operation frequency.
- Relay Output: Can handle both AC and DC loads but has a longer response time and lower operation frequency.
- What are the different types of PLCs based on structure? What are their characteristics?
- Compact Type: Integrates CPU, power supply, and I/O components into one chassis, making it compact and cost-effective, usually used for small PLCs.
- Modular Type: Components are divided into separate modules, allowing flexible configuration, easy expansion, and maintenance, typical for medium to large PLCs. These consist of a frame or baseplate with various modules inserted into slots.
- Stackable Type: Combines features of compact and modular types; CPU, power supply, and I/O interfaces are separate modules connected by cables, offering flexibility with a smaller footprint.
- What is the scan cycle of a PLC? What mainly affects it?
- The scan cycle includes five stages: internal processing, communication service, input processing, program execution, and output processing. The time required for one complete cycle is called the scan cycle.
- The scan cycle is influenced by CPU processing speed, PLC hardware configuration, and the length of the user program.
- How does a PLC execute user programs? What stages are included in the execution process?
- PLCs execute user programs in a cyclic scan manner. The process includes the input sampling phase, program execution phase, and output refresh phase.
- What advantages does a PLC control system have over a relay control system?
- Control Method: PLCs control through programming, making it easy to modify or add control requirements, with virtually unlimited contacts.
- Operational Mode: PLCs work in a serial fashion, enhancing system immunity to interference.
- Control Speed: PLC contacts are essentially flip-flops, with execution times in microseconds.
- Timing and Counting: PLCs use semiconductor integrated circuits for timing, with clock pulses from crystals, offering high precision and a wide range. They also provide counting functions not available in relay systems.
- Reliability and Maintenance: With microelectronics, PLCs are highly reliable and include self-diagnostic features for quick fault detection, with monitoring functions aiding in debugging and maintenance.
