Introduction
PLC comes in various sizes, hence its control range can also vary. Smaller PLCs might only control a single device, or even a component at a site; larger ones can control multiple devices, an entire production line, or even an entire factory. It can be said that PLCs are indispensable in both large and small industrial control scenarios.
Initially, PLCs were mainly used for logic control of binary signals. With advancements in technology, the application fields of PLCs have continuously expanded. Nowadays, they are not only used for binary control but also for controlling analog and digital signals, capable of data collection and storage, and monitoring control systems; they can also be networked and communicate, achieving control and management over large geographical areas.
PLCs have increasingly become an important member of the industrial control device family.
Used for Binary Control
PLCs have a strong capability for controlling binary signals. The number of input and output points they can control ranges from a few to several thousand points, and even up to tens of thousands. Because they can be networked, the number of points is almost unlimited; regardless of how many points there are, they can be controlled. The logic issues they can handle are diverse: combinational, sequential, instantaneous, delayed, non-counting, counting, fixed sequence, random operations, etc.
The hardware structure of a PLC is variable, and the software program is programmable, making it very flexible for control. If necessary, multiple sets or groups of programs can be written and called as needed. It is well-suited for the requirements of various working conditions and state changes in industrial sites.
There are many instances of using PLCs for binary control in metallurgy, machinery, light industry, chemical engineering, textiles, etc. Almost all industrial sectors require it. Currently, the primary advantage of PLCs, unmatched by other controllers, is their ease and reliability in binary control.
Used for Analog Control
Analog signals, such as current, voltage, temperature, pressure, etc., vary continuously. In industrial production, especially in continuous production processes, it is often necessary to control these physical quantities.
As an industrial control electronic device, if a PLC cannot control these quantities, it is a significant shortcoming. Therefore, 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 also achieve such control. For PLCs to perform analog control, they must be equipped with A/D and D/A units for converting between analog and digital signals. These are special I/O units.
The A/D unit converts the analog signal from the external circuit into a digital signal, which is then sent to the PLC; the D/A unit converts the digital signal from the PLC back into an analog signal, which is sent to the external circuit. As a special I/O unit, it still possesses characteristics such as anti-interference of the I/O circuit, isolation of internal and external circuits, and exchanging information with input/output relays (or internal relays, which are also a memory area of the PLC, capable of read/write operations).
In this context, the ‘A’ in A/D usually refers to current or voltage, and sometimes temperature. The ‘A’ in D/A usually refers to voltage or current. The voltage and current ranges are typically 0-5V, 0-10V, or 4-20mA, and some can handle both positive and negative values. The ‘D’ in small PLCs is often an 8-bit binary number, while medium and large PLCs are often 12-bit binary numbers. A/D and D/A can be single-channel or multi-channel, with multi-channel occupying more input/output relays. With A/D and D/A units, the remaining processing is all digital, which is not difficult for PLCs with information processing capabilities. Medium and large PLCs have stronger processing capabilities, allowing not only addition, subtraction, multiplication, and division of numbers but also square roots, interpolation, and floating-point operations. Some even have PID instructions for proportional, differential, and integral operations on deviations, thereby generating corresponding outputs; they can perform almost all calculations that computers can do.
Thus, it is entirely feasible to achieve analog control using PLCs.
PLCs can also perform analog control using combined A/D and D/A units, and control can be achieved using PID or fuzzy control algorithms to achieve high control quality. The advantage of using PLCs for analog control is that while performing analog control, binary signals can also be controlled simultaneously. This advantage is not possessed by other controllers or is not as convenient as that achieved with PLCs. However, if the system is purely for analog control, PLCs may not be as cost-effective as using dedicated regulators.
Used for Motion Control
In addition to binary and analog signals, there is also motion control, such as the displacement of machine tool components, which is often represented as digital signals. An effective method for motion control is NC, or numerical control technology. This technology, based on computers, originated in the United States in the 1950s and has become widely used and well-developed today.
Currently, in advanced countries, the rate of numerical control in metal cutting machine tools exceeds 40%-80%, with some even higher. PLCs are also based on computer technology and are continually improving. PLCs can receive counting pulses, with frequencies reaching several kHz to tens of kHz, and can receive these pulses in various ways, including multi-channel reception. Some PLCs also have pulse output functions, with pulse frequencies also reaching tens of kHz. With these two functions, combined with the data processing and calculation capabilities of PLCs, and equipped with appropriate sensors (such as rotary encoders) or pulse servo devices, various controls can be achieved based on the principles of NC. High-end and mid-range PLCs have also developed NC units or motion units that can achieve point control. Motion units can also achieve curve interpolation and control curved movements.
Therefore, if PLCs are equipped with such units, they can fully utilize NC methods for digital control. Newly developed motion units even have released programming languages for NC technology, providing convenience for better digital control using PLCs.
Used for Data Acquisition
With the development of PLC technology, their data storage capacity has become increasingly large. For instance, the PLC from Devison company has a data storage area (DM area) that can reach up to 9999 words. Such a large data storage area can hold a substantial amount of data. Data acquisition can use counters to accumulate and record the number of pulses collected and periodically transfer them to the DM area. Data acquisition can also use 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 periodically print out the data from the DM area.
PLCs can also communicate with computers, allowing computers to read data from the DM area and process it further. In this case, the PLC acts as a data terminal for the computer.
Electricity users have previously used PLCs to record real-time electricity consumption, implementing different billing methods based on varying electricity usage times, encouraging users to consume more electricity during low-demand periods, thereby achieving reasonable and energy-saving electricity usage.
Used for Signal Monitoring
PLCs have many self-check signals and numerous internal components, yet most users do not fully exploit their capabilities. In fact, PLCs can be utilized to monitor their own operation or the controlled objects. For a complex control system, especially an automatic control system, monitoring and even self-diagnosis are essential, as they can reduce system failures, facilitate troubleshooting, improve the accumulated average time of trouble-free operation, decrease repair time, and enhance system reliability.
Used for Networking and Communication
PLCs have strong networking and communication capabilities, with new networking structures continually being introduced.
PLCs can connect to personal computers for communication, allowing computers to participate in programming and managing PLCs, making PLCs more convenient to use.
To fully leverage the capabilities of computers, one computer can control and manage multiple PLCs, with the number reaching up to 32. Additionally, a PLC can communicate with two or more computers to exchange information, achieving more comprehensive monitoring of the PLC control system.
PLCs can also communicate with one another, allowing one-to-one PLC communication, several PLCs communicating together, and even hundreds of PLCs.
PLCs can also be networked with smart instruments and intelligent actuators (such as frequency converters), exchanging data and interacting with each other. They can be connected to form remote control systems, with a range extending to 10 kilometers or more. They can form local networks, where not only PLCs but also high-end computers and various intelligent devices can join the network. Bus networks and ring networks can be used, and networks can be bridged. Networking can organize thousands of PLCs, computers, and intelligent devices into one network. Nodes within the network can communicate and exchange information directly or indirectly.
Networking and communication meet the needs of today’s computer-integrated manufacturing systems (CIMS) and the development of intelligent factories. They enable industrial control to evolve from point (Point) to line (Line) and then to surface (Aero), integrating equipment-level control, production line control, and factory management control into a cohesive whole, thereby creating higher efficiency. This limitless and promising future is becoming increasingly clear to our generation.
The applications mentioned above focus on qualitative aspects. Quantitatively, PLCs come in various sizes, so their control range can also vary. Smaller PLCs might only control a single device, or even a component at a site; larger ones can control multiple devices, an entire production line, or even an entire factory. It can be said that PLCs are essential in both large and small industrial control scenarios.
(Source: Electric Power Partner Image Source: Electric Power Partner)
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