In the field of industrial automation, programmable logic controllers (PLCs) have become the foundation of most automation systems, bringing unprecedented and remarkable changes to industrial control. Industrial control systems using PLCs offer unparalleled advantages in operation, control, efficiency, and accuracy compared to traditional relay-based systems. While relay-based control devices in industrial control systems will not be completely eliminated, the advent of PLCs has fundamentally altered the design philosophy of industrial control engineers.
I. Disadvantages of Control Relays
Today, relays are used in various fields of home and industrial control. They offer higher reliability than previous products. However, this also brings some problems. For example, most control relays operate under conditions of long-term wear and fatigue, making them prone to damage. Furthermore, relay contacts are prone to arcing, and can even melt together, causing malfunctions and serious consequences. Moreover, for a device with hundreds of relays, the control box would be large and cumbersome. Under full load, large relays generate significant heat and noise, while also consuming substantial amounts of electrical energy. Additionally, relay control systems require manual wiring and installation; even simple modifications require considerable time, manpower, and resources for modification, installation, and debugging.
II. Advantages, Features, and Functions of Programmable Logic Controllers
Programmable logic controllers (PLCs) are renowned for their small size and powerful functionality. They can easily perform sequential logic, motion control, timing control, counting control, digital arithmetic, and data processing. Furthermore, they can establish connections with various production machinery via input/output interfaces, enabling automated control of the production process. Especially now, with the advent of the information and network age, the functionality of PLCs has been expanded, giving them strong networking and communication capabilities, leading to their wider application in numerous industries.
1. Sequence control
Sequential control is the most basic and widely used application of PLCs. Sequential control refers to the automatic sequential operation of various actuators in a production process under the influence of control signals, following the order of the technological process. Because it offers advantages such as flexible programming, high speed, high reliability, low cost, and ease of maintenance, it can completely replace traditional relay and contactor control systems in single-machine control, multi-machine group control, and production process control. It primarily controls mechanical moving parts to perform corresponding operations based on operation buttons, limit switches, and other command signals and sensor signals from the field, thereby achieving automated production line control. Typical applications include the control of automatic elevators, the automatic opening and closing of electromagnetic valves on pipelines, and the sequential start-up of belt conveyors. For example, the raw material mixing system in my branch plant utilizes the sequential control function of a PLC.
2. Closed-loop process control
Previously, analog signals for process control were implemented using hardware circuitry-based PID controllers for open and closed-loop control. Now, however, PLC control systems can be used, employing analog control modules whose functions are implemented in software. The system's accuracy is determined by the number of bits, unaffected by component limitations, resulting in higher reliability. This allows for the implementation of complex and advanced control methods, and the simultaneous control of multiple control loops and parameters, such as temperature, flow rate, pressure, and speed in a production process.
3. Motion position control
PLCs can support the control and management of CNC machine tools. In the machining industry, programmable controllers are integrated with computer numerical control (CNC) to complete the motion position control of machine tools. Their function is to receive machining information from input devices, process and calculate it, and send corresponding pulses to the drive device. Through stepper motors or servo motors, the machine tool moves along a predetermined trajectory, thus achieving multi-axis servo motor self-control. Currently, it is used to control centerless grinding, stamping, segmented blanking of complex parts, and hobbing/digging applications.
4. Monitoring and management of the production process
PLCs can connect to peripherals such as display terminals and printers via communication interfaces. A display, acting as a Human-Machine Interface (HMI), is an intelligent device containing a microprocessor chip. Combined with a PLC, it can replace numerous control buttons, selector switches, indicator lights, production process simulation screens, and a large number of intermediate relays and terminal blocks in the control cabinet. All operations can be performed on the operating elements displayed on the screen. PLCs can conveniently and quickly collect and process data during the production process, and can display parameters in binary, decimal, hexadecimal, and ASCII character formats. On the display screen, color changes of icons reflect the operating status of field equipment, such as valve opening and closing, motor starting and stopping, and the status of position switches. PID loop control uses a comprehensive method, including data and bar graphs, to reflect changes in quantities during the production process. Operators can adjust parameters through parameter settings, retrieve data records at any given time through data queries, and save relevant production data through printing, facilitating future production management and process parameter analysis.
5. Network characteristics
PLCs can enable communication and networking between multiple PLCs or between multiple PLCs and a computer, thereby forming a multi-level distributed control system and a factory automation network.
(1) Remote monitoring of the control system is achieved through communication modules, host computers and corresponding software.
(2) It connects to remote client computers via modem and public telephone network, enabling managers to remotely monitor the control system via telephone line.
Conclusion
Programmable logic controllers (PLCs), as a general-purpose industrial controller, can be used in all industrial fields. Currently, both domestically and internationally, PLCs have been successfully applied in various fields such as machinery, metallurgy, petroleum, chemical industry, textiles, transportation, power, and military, achieving considerable technological and economic benefits. Furthermore, it represents the world's advanced level of electrical control technology and is considered one of the four pillars of industrial automation technology, alongside numerical control (NC) technology, CAD/CAM technology, and industrial robotics technology.
