PLC comes in various sizes, allowing for a wide range of control applications. Smaller PLCs may control a single device, a component, or a station, while larger PLCs can manage multiple devices, an entire production line, or even a whole factory. It can be said that PLCs are indispensable in both large and small industrial control scenarios.
Initially, PLCs were primarily used for logical control of binary signals. With technological advancements, the application scope of PLCs has continuously expanded. Today, they are used not only for binary control but also for controlling analog and digital signals, data collection and storage, and monitoring control systems. Additionally, they can connect to networks and communicate, enabling extensive cross-regional control and management.
PLCs have increasingly become an important member of the industrial control device family.
01. Used for Binary Control
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The capability of PLCs to control binary signals is robust. The number of input and output points they can control ranges from a few dozen to several thousand, even tens of thousands. Because they can be networked, the number of points is virtually unlimited. Regardless of the number of points, various logic issues can be addressed: combinations, sequences, instantaneous, delayed, counting, non-counting, fixed order, random operations, etc., can all be managed.
The hardware structure of PLCs is variable and the software programs are programmable, making them very flexible for control purposes. When necessary, multiple sets or groups of programs can be written and called as needed. They are well-suited for the diverse working conditions and state changes present in industrial settings.
There are numerous examples of using PLCs for binary control across various industries such as metallurgy, machinery, light industry, chemical engineering, and textiles. Almost all industrial sectors require their use. Currently, the primary advantage of PLCs is that they can easily and reliably control binary signals, a feat that other controllers cannot match.
02. Used for Analog Control
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Analog signals, such as current, voltage, temperature, and pressure, continuously vary in magnitude. In industrial production, especially in continuous processes, it is often necessary to control these physical quantities.
As an industrial control electronic device, a PLC that cannot control these quantities is significantly lacking. Consequently, various PLC manufacturers have invested heavily in this area. Currently, not only large and medium-sized PLCs can perform analog control, but even small PLCs can handle such tasks. To perform analog control, PLCs must be equipped with A/D and D/A units for converting between analog and digital signals. These units are also special I/O units.
The A/D unit converts external analog signals into digital signals, which are then sent to the PLC; the D/A unit converts the PLC’s digital signals back into analog signals for external circuits. As a special I/O unit, it still possesses characteristics such as anti-interference for I/O circuits, isolation between internal and external circuits, and information exchange with input/output relays (or internal relays, which are also a writable area of the PLC’s working memory).
In the A/D unit, ‘A’ typically refers to current or voltage, and sometimes temperature. In the D/A unit, ‘A’ usually refers to voltage or current. The voltage and current ranges are typically 0-5V, 0-10V, and 4-20mA, with some capable of handling both positive and negative values. In small PLCs, ‘D’ is often an 8-bit binary number, while medium and large PLCs typically use 12-bit binary numbers. A/D and D/A units can be single-channel or multi-channel, with multi-channel units requiring more input/output relays. Once the A/D and D/A units are in place, the remaining processing is purely digital, which is not difficult for PLCs with information processing capabilities. Medium and large PLCs have even greater processing capability, enabling not only basic arithmetic operations but also square roots, interpolation, and floating-point calculations. Some even include PID instructions to perform proportional, derivative, and integral calculations on deviations, leading to corresponding outputs. Essentially, anything a computer can compute, a PLC can compute as well.
Thus, achieving analog control with PLCs is entirely feasible.
PLCs can also integrate A/D and D/A units and utilize PID or fuzzy control algorithms to achieve high-quality control. The advantage of using PLCs for analog control is that they can simultaneously control both analog and binary signals. This is a capability that other controllers may lack or find inconvenient to implement. However, for systems that are purely analog, using PLCs may not be as cost-effective as using dedicated controllers.
03. Used for Motion Control
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In addition to binary and analog signals, motion control is another important physical quantity. For example, the displacement of machine tool components is often represented as a digital quantity. An effective method for motion control is NC, or numerical control technology, which originated in the 1950s in the United States and is based on computer technology. Today, it is widely adopted and highly refined.
Currently, in advanced countries, the ratio of numerically controlled metal-cutting machine tools exceeds 40%-80%, with some even higher. PLCs are also based on computer technology and are becoming increasingly sophisticated. PLCs can receive counting pulses at frequencies ranging from a few kHz to tens of kHz and can accept these pulses in various ways, including multi-channel reception. Some PLCs also have pulse output functions, with pulse frequencies reaching tens of kHz. With these two functionalities, combined with the PLC’s data processing and computation capabilities, and when equipped with appropriate sensors (such as rotary encoders) or pulse servos, it is entirely possible to implement various controls based on NC principles. High-end and mid-range PLCs have also developed NC units or motion units that can achieve point control. These motion units can also implement curve interpolation and control curved movements.
Therefore, if a PLC is equipped with such units, it can fully utilize NC methods for digital control. Newly developed motion units even feature NC programming languages to facilitate better digital control using PLCs.
04. Used for Data Collection
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With the development of PLC technology, their data storage capacity has significantly increased. For example, PLCs from Devison can have a data storage area (DM area) of up to 9999 words. Such a vast data storage area can hold a large amount of data. Data collection can utilize counters to accumulate and record the number of collected pulses, which can then be periodically transferred to the DM area. Data collection can also be performed using A/D units, where analog signals are converted into digital signals and then periodically transferred to the DM area. PLCs can also be equipped with small printers to regularly print the data from the DM area.
PLCs can communicate with computers, allowing the computer to read data from the DM area and process it further. In this scenario, the PLC acts as a data terminal for the computer.
Electricity users have previously utilized PLCs to record real-time electricity usage, implementing different pricing methods based on varying electricity consumption times, encouraging users to consume more electricity during low-demand periods, thereby achieving reasonable and energy-saving electricity usage.
05. Used for Signal Monitoring
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PLCs have numerous self-diagnostic signals and internal components, many users do not fully leverage their capabilities. In fact, they can be used to monitor the PLC’s own operations or the controlled objects. For a complex control system, especially an automated control system, monitoring and further self-diagnosis are essential. This can reduce system failures, facilitate troubleshooting when faults occur, increase the average time between failures, reduce repair times, and enhance system reliability.
06. Used for Networking and Communication
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PLCs possess strong networking and communication capabilities, and new networking structures are continually being introduced.
PLCs can connect to personal computers for communication, allowing computers to participate in programming and managing PLC control, making PLCs easier to use.
To fully utilize the computer’s capabilities, one computer can control and manage multiple PLCs, with the number reaching up to 32. A single PLC can communicate with two or more computers, exchanging information to achieve better monitoring of the PLC control system.
PLCs can also communicate with each other, enabling one-to-one PLC communication, several PLCs communicating together, and even hundreds of PLCs.
PLCs can connect with intelligent instruments and intelligent actuators (such as frequency converters) for networking and data exchange, allowing for remote control systems that can span up to 10 kilometers or more. They can form local networks where not only PLCs but also high-end computers and various intelligent devices can connect. Networking can be done via bus networks or ring networks, and networks can also be nested. Networks can bridge with each other, allowing thousands of PLCs, computers, and intelligent devices to be organized into a single network. Nodes between networks can communicate and exchange information directly or indirectly.
Networking and communication align with the current needs of Computer Integrated Manufacturing Systems (CIMS) and intelligent factories. This allows industrial control to evolve from point control to line control and ultimately to holistic factory management, creating greater benefits. This promising future is increasingly clear for our generation.
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