In the field of industrial automation, Programmable Logic Controllers (PLC) have become the foundation of most automated systems, bringing unprecedented changes to industrial control. When compared to traditional industrial control systems that use relays, PLC-based industrial control systems offer unparalleled advantages in terms of operation, control, efficiency, and precision.
1. Disadvantages of Control Relays
Relays have long been applied in various fields of household and industrial control. Although modern relays are more reliable than before, they still bring a series of problems. Most control relays operate under long-term wear and fatigue conditions, making them prone to failure. Moreover, relay contacts can easily create electrical arcs or even fuse together, causing malfunction and leading to serious consequences. Additionally, for equipment with hundreds of relays, the control cabinet would be large and bulky. Under full load operation, large relays generate significant heat and noise while consuming substantial energy. Furthermore, relay control systems require manual wiring and installation, and even minor changes necessitate considerable time, manpower, and resources for modification, installation, and debugging.
2. Advantages, Characteristics, and Functions of PLC
PLC is known for its compact size and powerful functionality. It can easily perform sequential logic, motion control, timing control, counting, digital operations, and data processing. Additionally, PLC can establish digital and analog connections with various production machinery through input and output interfaces, thereby enabling automatic control of production processes. Especially today, with the advent of the information and network era, the functionality of PLCs has been expanded, providing strong network communication capabilities, which allows for wider applications across many industries.
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Sequential Control
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Sequential control is the most fundamental and widely used field for PLCs. It refers to controlling the actions of various actuators in the production process automatically in a defined sequence based on control signals. Thanks to its flexible programming design, fast speed, high reliability, low cost, and ease of maintenance, PLCs can completely replace traditional relay contactor control systems for single machine control, multi-machine group control, and production process control. PLCs control mechanical movement components according to signals from buttons, limit switches, and other commands or sensor inputs, achieving automated production line control. Common applications include automatic elevator control, automatic valve opening and closing in pipelines, and sequential start-up of conveyor belts. For example, in our factory’s raw material mixing system, we utilize the sequential control function of PLC.
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Closed-Loop Process Control
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In the past, analog process control used hardware circuits like PID regulators to achieve open or closed-loop control. Today, PLC control systems can fully handle this by using analog control modules, with the function being completed via software. The system’s accuracy is determined by its bit resolution and is not affected by component performance, making it more reliable. PLC can handle complex control and advanced control methods, simultaneously managing multiple control loops and parameters, such as temperature, flow, pressure, and speed in production processes.
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Motion and Position Control
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PLCs support control and management of CNC (Computer Numerical Control) machines. In the machinery manufacturing industry, PLCs are integrated with CNC systems to control motion and positioning. PLCs receive processing information from input devices, process and compute it, then send corresponding pulses to the drive devices. These pulses control stepper or servo motors, making the machine move along a predefined path, completing the control of multi-axis servo motors. It is commonly used in applications such as centerless grinding, stamping, segmental cutting of complex parts, and gear hobbing.
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Monitoring and Management of Production Processes
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PLCs can connect with display terminals and printers through communication interfaces. The display, serving as a Human-Machine Interface (HMI), is an intelligent device containing a microprocessor. When combined with PLCs, it can replace numerous control buttons, switches, signal indicators, production flow simulation panels, and large quantities of intermediate relays and terminal blocks found in control cabinets. All operations can be performed through the control elements on the display screen. PLCs facilitate quick and easy data collection and processing during production, and the parameters can be displayed in binary, decimal, hexadecimal, or ASCII formats. Changes in the operation status of field equipment, such as valve opening and closing, motor starting and stopping, and position switch status, can be reflected through color changes on the screen. PID loop control and data bar graphs can display changes in production variables, and operators can adjust parameters or query and print data records at any time, simplifying future production management and process parameter analysis.
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Networking Features
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PLCs can facilitate communication between multiple PLCs or between PLCs and computers, forming a multi-level distributed control system and establishing a factory automation network.
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- Through communication modules, supervisory systems, and the corresponding software, remote monitoring of control systems can be achieved.
- Via modems and public telephone networks, PLCs can connect to remote client computers, allowing management personnel to remotely monitor the control system through phone lines.