I. PLC
A programmable logic controller (PLC) is a digital electronic system specifically designed for industrial applications. It employs a programmable memory that stores instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations, controlling various types of machinery or production processes through digital or analog inputs and outputs.
By the early 1980s, programmable logic controllers (PLCs) had been widely used in advanced industrialized countries. The number of countries producing PLCs worldwide was increasing, and output was rising steadily. This marked the entry of PLCs into a mature stage.
The period from the 1980s to the mid-1990s was the fastest period of development for programmable logic controllers (PLCs), with an annual growth rate consistently between 30% and 40%. During this period, the PLC's ability to process analog signals, perform digital calculations, provide human-machine interfaces, and network was greatly improved. PLCs gradually entered the process control field and, in some applications, replaced the DCS systems that had dominated the process control field.
In the late 20th century, the development of programmable logic controllers (PLCs) was characterized by their greater adaptability to the needs of modern industry. This period saw the development of mainframe and ultra-miniature PLCs, the emergence of various special function units, and the production of various human-machine interface units and communication units, making it easier to integrate PLCs into industrial control equipment.
II. Application Areas of PLC
Currently, PLCs are widely used in various industries both domestically and internationally, including steel, petroleum, chemical, power, building materials, machinery manufacturing, automobiles, light textiles, transportation, environmental protection, and cultural entertainment. Their usage can be broadly categorized as follows.
1. Logic control of switch quantities
This is the most basic and widespread application area of PLCs. They replace traditional relay circuits to achieve logic control and sequential control. They can be used for controlling single devices, as well as for multi-machine group control and automated production lines. Examples include injection molding machines, printing presses, stapling machines, combination machine tools, grinding machines, packaging production lines, and electroplating production lines.
2. Analog control
In industrial production processes, many continuously changing quantities, such as temperature, pressure, flow rate, liquid level, and speed, are analog quantities. To enable programmable logic controllers (PLCs) to process analog quantities, analog-to-digital (A/D) and digital-to-analog (D/A) conversions must be implemented. PLC manufacturers produce compatible A/D and D/A conversion modules to enable PLCs for analog control.
3. Motion control
PLCs can be used to control circular or linear motion. In terms of control mechanism configuration, early PLCs directly connected position sensors and actuators using digital I/O modules. Now, dedicated motion control modules are generally used, such as single-axis or multi-axis position control modules that can drive stepper motors or servo motors. Almost all major PLC manufacturers worldwide offer products with motion control capabilities, and these are widely used in various machinery, machine tools, robots, elevators, and other applications.
4. Process Control
Process control refers to the closed-loop control of analog quantities such as temperature, pressure, and flow rate. As an industrial control computer, a PLC can program various control algorithms to complete closed-loop control. PID control is a commonly used method in general closed-loop control systems. Large and medium-sized PLCs all have PID modules, and many small PLCs now also have this function. PID processing typically involves running dedicated PID subroutines. Process control has very wide applications in metallurgy, chemical engineering, heat treatment, boiler control, and other fields.
5. Data Processing
Modern PLCs possess mathematical operations (including matrix operations, function operations, and logical operations), data transmission, data conversion, sorting, table lookup, and bit manipulation functions, enabling them to acquire, analyze, and process data. This data can be compared with reference values stored in memory to perform specific control operations, or it can be transmitted to other intelligent devices via communication functions, or printed into tables. Data processing is generally used in large-scale control systems, such as unmanned flexible manufacturing systems; it can also be used in process control systems, such as some large-scale control systems in the paper, metallurgical, and food industries.
6. Communication and networking
PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of computer control, factory automation networks are developing rapidly, and PLC manufacturers are paying close attention to the communication functions of their PLCs, launching their own network systems. Newly manufactured PLCs all have communication interfaces, making communication very convenient.
