Essential PLC Knowledge for Automation
Essential PLC Knowledge for Automation
In the realm of industrial production and technological advancement, PLCs (Programmable Logic Controllers) play a crucial role in automation control. A PLC can be broadly understood as a centralized relay extension control panel. In practical applications, PLCs significantly reduce industrial control costs and enhance equipment management and automation. To master PLCs, one must first grasp the foundational knowledge.
PLC Components and Their Functions
In addition to the CPU, memory, and communication interfaces, PLCs have input and output interfaces directly related to industrial sites.
Input Interface: Receives signals from controlled devices and drives internal circuits via optocouplers and input circuits.
Output Interface: Transmits program execution results through optocouplers and output components (relays, thyristors, transistors) to control external loads.
Basic PLC Unit and Its Components
The basic PLC unit consists of several key parts:
CPU: The core of the PLC, directing various operations such as receiving user programs and data, diagnostics, and program execution.
Memory: Stores system and user programs and data.
I/O Interface: Connects the PLC to industrial equipment, receiving signals and outputting program results.
Communication Interface: Enables information exchange with other devices like monitors and printers.
Power Supply: Provides power to the PLC system.
PLC Switching Output Interfaces and Their Characteristics
PLC switching output interfaces：
Thyristor Output Type: Typically used with AC loads, featuring fast response and high operating frequency.
Transistor Output Type: Usually used with DC loads, also offering fast response and high operating frequency.
Relay Output Type: Compatible with both AC and DC loads, but with longer response time and lower operating frequency.
PLC Structural Types and Their Features
PLCs can be categorized into three structural types:
Integral Type: With CPU, power supply, and I/O components housed in a single case, this type is compact and cost - effective, commonly used in small - scale PLCs.
Modular Type: Features separate modules for different functions, offering flexible configuration and easy expansion and maintenance. It is typically used in medium - and large - scale PLCs and consists of a frame or base plate and various modules.
Stackable Type: Combines the features of integral and modular types. The CPU, power supply, and I/O interfaces are independent modules connected by cables, ensuring flexible configuration and a compact size.
PLC Scan Cycle and Its Influencing Factors
The PLC scan cycle encompasses five stages: internal processing, communication service, input processing, program execution, and output processing. The time required to complete these five stages once is termed the scan cycle. It is influenced by the CPU's operating speed, PLC hardware configuration, and the length of the user program.
PLC Program Execution Method and Process
PLCs execute user programs using a cyclic scanning method. The execution process includes three stages: input sampling, program execution, and output refresh.
Advantages of PLC Control Systems Over Relay Control Systems
Control Method: PLCs use programmable control, allowing easy modification or enhancement of control requirements, with unlimited contacts.
Working Mode: PLCs operate in a serial mode, 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 timing precision and wide - ranging timing capabilities. They also possess counting functions unavailable in relay systems.
Reliability and Maintainability: PLCs utilize microelectronics technology and feature self - diagnostic functions for timely fault detection.
Causes of PLC Output Response Lag and Solutions
PLCs employ centralized sampling and output cyclic scanning. Input statuses are only read during the input sampling phase of each scan cycle, and program execution results are only sent out during the output refresh phase. Additionally, input and output delays and user program length can cause output response lag. To enhance I/O response speed, one can increase the frequency of input sampling and output refresh, adopt direct input sampling and output refresh, utilize interrupt input and output, or implement intelligent I/O interfaces.
Internal Soft Relays in Siemens PLC Series
Siemens PLCs feature various internal soft relays, including input relays, output relays, auxiliary relays, status registers, timers, counters, and data registers.
PLC Selection Considerations
Model Selection: Consider factors such as structure, installation method, functional requirements, response speed, reliability, and model uniformity.
Capacity Selection: Based on I/O points and user memory capacity.
I/O Module Selection: Covers switching and analog I/O modules as well as special - function modules.
Power Supply Module and Other Device Selection: Such as programming devices.
Characteristics of PLC Centralized Sampling and Output Working Mode
In centralized sampling, input status is sampled only during the input sampling phase of a scan cycle, and the input end is blocked during the program execution phase. In centralized output, the output refresh phase is the only time when the status in the output image register is transferred to the output latch to refresh the output interface. This working mode improves the system's anti - interference ability and reliability but may cause input/output response lag in PLCs.
PLC Working Mode and Features
PLCs operate using centralized sampling, centralized output, and cyclic scanning. Centralized sampling means input status is sampled only during the input sampling phase of a scan cycle, with the input end blocked during program execution. Centralized output refers to the transfer of output - related status from the output image register to the output latch only during the output refresh phase to refresh the output interface. Cyclic scanning involves executing multiple operations in a scan cycle through time - division scanning in sequence.
Composition and Working Principle of Electromagnetic Contactors
Electromagnetic contactors consist of electromagnetic mechanisms, contacts, arc - extinguishing devices, release spring mechanisms, and mounting components. When the electromagnetic coil is energized, the current generates a magnetic field, causing the stationary iron core to produce electromagnetic suction that attracts the armature and actuates the contacts. This causes normally closed contacts to open and normally open contacts to close. When the coil is de - energized, the electromagnetic force disappears, and the armature is released by the spring, restoring the contacts to their original state.
Definition of Programmable Logic Controllers (PLCs)
A PLC is a digital electronic device designed for industrial environments. It uses a programmable memory to store instructions for performing logical, sequential, timing, counting, and arithmetic operations. It controls various mechanical or production processes through digital or analog input/output.
PLCs and related peripheral devices are designed to easily integrate with industrial control systems and facilitate function expansion.
Differences Between PLC and Relay - Contactor Systems
The differences between PLC and relay - contactor systems lie in their compositional devices, number of contacts, and control implementation methods.